Of course, I can’t really do a winter wrap-up or declare a winner in the Iditasnow until winter is a little more over, can I? Look at what has been happening around the state:

In the Arctic, no one expects anything like spring weather for some time to come. In fact, the weather there has been pretty average for this time of year: Temperatures in the 20s F (around -5C) with some wind, a little snow and blowing snow lately. However, just a few days ago it was below zero on the North Slope and Bering Strait area. After a very snowy winter in Kotzebue, the 27 inches (69 cm) of snow on the ground is holding steady with well below freezing temperatures.

The plows are still needed in Shishmaref. Click on the image to see it full size.

On the west coast, Alaska Peninsula and the Aleutian Islands winter is holding on very tightly. After a moderate April in Nome, temperatures are showing little if ay upward trend, running about 10 F degrees (5 C degrees) below average, with new snow over the last couple days. The Pribilof Islands have have also been about 10 degrees below average this month, with snow on 7 of 11 days so far, and 21 inches (53 cm) on the ground. Worse is the sea ice still surrounding them and its negative impact on area fisheries. The ice edge is way farther south than usual and, so far, not showing much inclination to retreat north for the summer. St. Paul has been in the pack ice for the longest period since regular record have been kept. Along the AK Peninsula and Aleutians the mostly north winds have been picking up moisture off the ice-free southern Bering Sea and showering it on the volcanic arc in the form of rain and snow showers. At Cold Bay, snow has fallen every day so far this May, and fell 24 of 30 days in April. They have more than doubled their seasonal average, and have been climbing steadily in the Iditasnow standings.

A couple of maps might shed some light on the situation:

This recent sea surface temperature analysis also shows the ice edge in a general way. Look at the very cold water in the southern Bering and quite cold North Pacific: 5C down to around latitude 45N. This heat sink is not going to go away soon.

The 500 mb maps shows a strong low near the Bering Strait streaming cold air out of the polar regions, over the Aleutians and and around the low into Southeast Alaska. This low is forecast to drop south then east and end up near Yakutat. Some relief from the cold is likely after this low dissipates next week.

The interior usually heats up faster in spring than any part of the state due to its distance from moderating ocean influence. With a couple mountain ranges to block cool, moist marine air, the interior responds mostly to radiational effect, which are steadily going more positive with the lengthening days. The warm up is happening now after lingering winter cold in the first few days of May (down to 13 F (-11 C) at McGrath on the 2nd, and 8 (-13) at Bettles on the 3rd. Temperatures are close to average now, with Eagle’s high of 60 F (16 C) yesterday being the first such temperature for the interior since last summer, as far as I can tell.

Spring is springing slower in Southcentral Alaska as well, but nothing too extreme. However, there is still 16 inches (41 cm) of snow in Valdez after the very heavy snow year for the whole region, especially coastal areas.

Southeast Alaska is usually the warmest part of the state in early spring, until overtaken by the interior, and by mid-May has usually had more than its share of sunny, warmish days. Not this year. It’s been wet, cool and windy almost without exception. High temperatures (a better gauge than average temperatures in this case) have been running 5 to 10 degrees below average and precipitation 2 to 3 times average at most stations. Ketchikan has been soaked with around 11 inches (280 mm) of rain already in May, compared to 8.20 inches (208 mm) for the long term average for the whole month. At Skagway, 30-35 mph (50-55 km/hr) south winds paired with mid-40s (~7 C) temperatures are playing havoc with the early tourism business. A handful of cruise ships having already made it to the top of the Inside Passage, but at least one smaller vessel has altered its schedule to avoid bad days on the Lynn Canal blowhole, where 35 kt (65 km/hr) southerlies have been kicking up 7 foot (2 m) seas over the past 24 hours, and close to that on other days.

The Carnival Spirit docked at Skagway Friday, with little action on the dock. Note the webcam's rain splattered lens. Image used courtesy of White Pass & Yukon Route. Current webcam images here.

About now this question tends to surface: Does the cold spring portend a cold summer? It looks like a significant warming is at hand mid to late in the coming week, but that says little about the rest of the summer, so check back for some info on this soon.

How is spring progressing where you are? That info, or any questions or comments are welcome via the comments link below.

Since the leap day checkpoint report, most towns have slowed their snowfall pace. Haines and Yakutat, however, have been running hard, each having added an impressive 50 inches this first half of March.

The Top Ten as of 15 March

Remember, the standings are based on snowfall this season compared to the station’s average yearly snowfall. (No, I did not come up with this rating scheme to just to put Haines in the lead).

The top six positions remain as they were at the end of last month. Yakutat, holding easily to 2nd place, has a surprising amount of snow on the ground for such a mild coastal town. In contrast, snow cover at the Juneau Airport is down to a trace, despite a foot of new snow in the past two weeks. Kind of embarrassing, I would guess, for a front runner. Amazing the difference between Juneau and Haines, just 70 miles apart as the raven flies. I should mention that the Juneau Forecast Office is reporting 9 inches on the ground, a little better, but c’mon…Haines customs is down to 95 inches from 120 on Monday. (Yes, they qualify as a town…around 1,000 people live outside of Haines in the Chilkat Valley, scattered or grouped the 40 miles all the way to the border. The customs station is their official weather station.)

Cold Bay has moved up into the top ten thanks to about 22 inches of snow so far this month, passing up a number of towns, including regional neighbors St. Paul, Bethel and Nome on our chart. The western parts of Alaska have been fairly dry lately, though plenty cold. The Bering Sea ice is well south and surrounding St. Paul and St. George, reducing their biggest snow source (showers forming as cold air off Siberia moves over the Bering Sea water). As a matter of fact the ice is not that far from Cold Bay. (More on Bering Sea ice here.)

The race is not over yet. While it is true that most areas getter drier for the next several months, and precipitation in warmer stations comes as rain more often, member, in the weather business, rules are meant to be broken, as are records. Valdez has had Marches with over 100 inches of snowfall at least thrice and their record for April is 71 inches! Stay tuned.

More snow rollers

I’ve seen wind driven snow rollers first hand four times in my life, and all four times were this winter! Click here for the blog post concerning the first episode on December 15. I saw them in action on Jan 10 (too dark for photography) and on February 28 I saw a bunch more after dark, and took some photos the next day with good light, although the rollers had been snowed in a bit (first photo below). March 6 was yet another day with the right conditions: strong winds and temperatures rising to around freezing after new dryish snow. Those rollers are shown in the next two photos below. The first shows some obvious tracks where the balls rolled (the larger one broke apart after rolling). I’d love to hear your reports or experiences with snow rollers, and see any photos, too.

As usual, click on these photos for larger versions.

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The rankings are based on the snowfall this season as a percent of the climatological average, which I think is a fair handicapping system. Note that the snowfall is only through February, but compared to the whole season average (so I can update the graphs as the winter progresses). So, many places have already passed the yearly average—some around doubled–with six or more weeks left to add to it. Here are graphs for some of the front runners. The solid red line is this year’s cumulative snow, the dashed red is the long-term average. The black lines are for snow depth, same patterns:

Now I may be partial, but any way you look at it Haines is having a big snow year and in fact is breaking the all time record right about as I write this. However, if you take a close look at the table you will see that there are fewer years of data for Haines than for most of the others. This can make it statistically easier to break records, depending on the particular years involved. On the other hand, the other two stations with similar length of record (Haines Customs and Valdez), are back a ways in the pack, aren’t they?

Yakutat does not get the spotlight often…it’s a small, isolated town even by Alaska standards, but is hosts a 1st order NWS weather station complete with upper air balloon soundings. It is also one of the wettest spots in Alaska you might be surprised at how much snow it gets considering its location right on the exposed (and mild) pacific coast. The high snow is mostly due to the huge volume of all precipitation, and in fact the great majority of the precipitation falls as rain. In high snow years, there is still rain, and with more snow on the ground or roofs to absorb and hold it, the snowpack and building snow loads are all the heavier. Cordova was hit with this effect also and had more than a few roofs fail.

Kodiak is not thought of as a heavy snow location, but with the relative ranking system in use we can see why they deserve 2nd place at the moment. But look at the snow depth lines (solid black line for this year, dashed black for average). Storms hit the emerald Isle with such frequency and ferocity, and enough of them are of a warm trajectory, that melting episodes rarely let much of a snow pack build up.

You can se by this poorly scaled graph that Barrow gets way less snow than most areas in Alaska. But the snow season is long with virtually no melting from October through April and usually well into May. This year they are a little better than double on snowfall, but only slightly high on snow depth. I’m not prepared to say why this is, but will say this: It is extremely problematic measuring either snowfall or snow depth in a windy place such as barrow. Studies have indicated significant under measuring of precipitation at arctic coastal locations due to problems of getting a representative amount of snow to drop into the gauge.

Anchorage hosts the start of the Iditarod race. Well, the ceremonial start anyway, but the teams do need to mush across the city to the outskirts, which does require snow. This year snow conditions should not worry the organizers. In some past years they’ve had to shorten the ceremonial start, shift the real start between various location in the Susitna Valley, and one year relocate to Fairbanks! Conditions should also be good for the next day’s Tour of Anchorage ski marathon, the nation’s 2nd largest ski race.

Valdez has always been the crowd pleasing favorite, simply because they are almost impossible to beat in a head-to-head race. I’m still looking for a snowier sea level town on the planet. Let me know if you find one. (By town I mean a place where people live, and at minimum has a post office.) The records held here are only eclipsed by mountain locations such as nearby Thompson Pass. And the weather records we’re talking about are fairly short in years and taken in the new city, reputed to be less snowy than the old city that was abandoned after the 1964 earthquake. The record snow depth seems too low, but I did find a 194 inch snow depth record at another station in Valdez. But let’s stick to apples vs. apples in this evaluation. More sleuthing could done.

Also rans

Before you ask, “What about such and such a place?”, I need to say that I’ve only included inhabited places with decent historical and current snow data. There are surprisingly few stations that meet that criteria. Most of them are on the table above, although I did not bother to add several the are not really in the running this year for heavy snow (Fairbanks slipped in). Some other places deserve mention:

Cordovans where hit hard with a heavy snow load this year (see description under the Yakutat graph) and were in the news for a while. Cordova doesn’t get nearly as much snow as Valdez, but, as mentioned, it’s the water content that counts, and they do get more precipitation overall. Much of Prince William Sound is similar. See my post on snow density.

Mountain stations are in a different class. There are precious few mountain weather stations with snow data, and many of them only operate for a few years. Thompson Pass (2500′) holds most of the snow records for the United States–including 175.4 inches of snowfall in 5 days–based on only a few years of data. The truth is, all the coastal mountains from the northern panhandle to the Kenai Peninsula are similarly snowy. Look at a map and look where the glaciers are. It’s a combination of the coolness of higher altitude and increased precipitation due to upslope lifting that causes the tremendous snow in our coastal mountains. Alyeska and Eaglecrest ski areas of course make their  snow stats easy to find and get some impressive loads. This year Alyeska (near Anchorage) had gotten 629 inches as of 2/29 compared to their seasonal average of around 717 inches. (Alyeska only records snow Nov 1 through Apr 30 thus the actual numbers are no doubt higher). In 2000-2001 they recorded 939 inches…that’s the winter a rash of huge avalanches put most of the Kenai Peninsula under siege. Eaglecrest is closer to average so far this year. There are some links to mountain weather stations given at the end of the post.

Fjords extending inland in SC and SE Alaska tend to get much more snow than more exposed coastal locations, and in fact Haines and Valdez are good examples. A coastal location provides a lot of precipitation, but often too much warmth. Being tucked up in a fjord keeps much of the precipitation but allows more influence from cold interior air that tends to flow out of valleys. There are not too many towns in this situation. Besides the two mentioned there is Hyder, tucked way back in. They are way snowier than their southerly latitude would suggest. Seward is another, with moderately high snowfall, and Whittier is like Yakutat, with extreme amounts of rain and snow. In the same group are two hydro projects near Juneau with very good and current climate records, Snettisham and Annex Creek. They average 214 and 248 inches per year respectively and this year are currently running 120 and 139 percent over that, consistent with Juneau percentage-wise, but way higher in actual amounts, though all three are near sea level and near each other. The two power plants are “up-fjord”…more inland.

Parts of the Alaska Range come close to the coastal ranges in snowfall. This can be seen in some of the valley and foothill locations such as Skwentna, Talkeetna, Chulitna and Cantwell, which get over 100 to close to 200 inches per year, average. That’s just a hint of what is going on higher up. Again look at the glaciers…there are plenty in the Alaska Range and on southwest into the Aleutian Range. The Iditarod trail passes through Skwentna on its way through the mountains at Rainy Pass. It’s much direr on the west side, and some years low snow makes for hard going over the rough farewell burn area. It should not be a problem this year.

Finally, the Bering Sea and Aleutian islands are surprisingly snowy. St. Paul Island averages about 60 inches per year (and has already bettered that this year–see more data in the table above). Adak, the southernmost city in Alaska, averages close to 100 inches of snow per year. That’s 3 times as much as Annette Island even though Annette (at the southern end of the Southeast panhandle and island of the village of Metlakatla) is almost 200 miles farther north, next to a potential source of cold continental air, and receives twice as much yearly precipitation! The seeming discrepancy is collaborated by other Aleutian stations such as Dutch Harbor (95 inches snow per year) and Shemya (85 inches). I find this fascinating and invite readers to submit ideas for why this is so in the comments. I’ll try to address it in replies or a future blog article.

Here are some links to good snow data:

…For most cities and low altitude stations:

http://pajk.arh.noaa.gov/cliMap/akClimate.php  or  http://www.arh.noaa.gov/clim/akcoopclim.php  or http://pafc.arh.noaa.gov/cliMap/akClimate.php

These are the somewhat duplicate sites for searching daily climate stats. Very good, though not official data. I have found some errors, especially in the snow depth reports. On the other hand I’ve found errors in official NCDC data too. Nobody’s perfect.

…For mountain locations:

http://aprfc.arh.noaa.gov/sd_all_sites.html  and  http://www.wcc.nrcs.usda.gov/snotel/Alaska/alaska.html

These sites are good in part because you can easily find SWE (snow water equivalent) info, which, as I’ve said, is more important than the amount of snow. A little harder to use, but you can access mountain sites not found in common reports.

How is the snow in your community? Lots of snow but not on the list? Perhaps you could volunteer to be a cooperative weather observer, or at least a spotter. Check back to see how the snow race turns out and for other weather stories.

It would not be hard to estimate the weight of this thing if we knew its density. And you might want to know the density your snow for a variety of reasons. One of the more common reasons is to figure if the weight of the snow might damage something. Check out the scene from our school a couple weeks ago:

The pieces of mangled metal hanging from the roof on the left were snow stoppers, put on this fairly new covered playground structure to prevent potentially deadly roof avalanches. They were not strong enough, and here is the result. Notice the bulging chain link fence around the small fenced area full of dense old snow. I wonder how much weight is in there.

Of course snow density varies tremendously. Even the density of new fallen snow varies quite a bit, but after the snow is on the ground it is subject to the weight of the newer snow compressing it, and to settling with changing temperatures, and to soaking when rain falls on it, a common occurrence in Southeast Alaska. Through some of my own measurements and more from others, I’ve compiled some approximate snow densities in the table below.

Since snow is frozen water and air, I think the best way to relate the density of snow is by giving the percentage of water in the snow, i.e., comparing the density of snow to that of water. (This is the same as the specific gravity, which is expressed as a decimal, vs the percentage format as I suggest.) For example, if snow has a density of 20% of that of water, it means the snow is 20% water and 80% air. (We can ignore the mass of the air). This method does not depend on what measuring system you use. If you want specifics from that starting point you can choose your units and do the simple arithmetic. For most purposes metric units are super easy, but for such things as snow load on a roof, we’re used to English units, at least here in the US. Elsewhere, building codes might use kilopascals (KPa) to specify snow load criteria. I’ve also included the commonly used ratio of snow to water. In fact the table shows more ways to quantify snow density that you probably wanted, but I hope can be instructional as well as useful. The density of water is shown on the bottom row for comparison. Click for a larger version.

Some surprises for some

First the old 10:1 rule–10 inches of snow equals 1 inch of water—is not even a good average for new snow. Perhaps it was developed in a really wet location, or the round numbers were appealing. Instead, I’d suggest thinking of snow at 10% density as uncommon. 7% is a much better average density for new fallen snow. That’s a 14:1 ratio, considerably less than the old rule of thumb.

And another rule of thumb to put out to pasture is the one that says the colder it is, the less dense the snow. My research shows that if the temperature is even just below freezing and there is no rain involved, new snow does not stray too far from the 7% average I stated above. As the temperature goes down, the density goes down a bit, but then at very cold temperatures it actually goes back up because the smaller crystal structure can pack more densely. This generality is for snow falling without too much wind. With strong winds, the density also goes up some because crystals are broken into smaller pieces and the wind can arrange them yet more densely when they come to rest (wind slab).

The water in the snow is what really matters

There are so many factors that go into snow density and so many observational issues that can throw it off that one should not assume too much precision or accuracy in reports and calculations of snowfall or snow density. Remember, we are dealing with ice and air. The air part is what makes the density change, but the air part is of no consequence! In reality, the liquid part is all we need to know. There is little difference in shoveling or plowing 1 foot of snow at 10% density or 2 feet at 5%. The liquid part is called the snow water equivalent or SWE. You can get the SWE for new snow from the weather report (the precipitation) or from your own rain gauge. The table above is intended to estimate the SWE of the snow on the ground or on your roof or boat or whatever, among other things.

Another surprise, for me anyway, is that even after umpteen melt-freeze cycles and tons of rain, old snow seldom reaches densities greater than 50%. 30-40% is more common.

How to use the table to figure snow loads

Say you are wondering if your roof can handle the snow load. The most accurate way to figure the pressure would be to take a core sample–a vertical slice of known cross sectional area—and weigh it, then convert the weight to a unit area, say one square foot or 1 square meter. But there is another way that does not necessarily involve going up onto the roof. Measure or estimate the depth of the snow, then multiply by the figure in one of the pressure columns that matches the type of snow you are dealing with. For instance if you’ve got 24 inches snow on your roof that fell in the last week and the temperature has been consistently well below freezing, the density is probably only 10% to at the most 20%, so choosing 20 to be conservative, that’s only 1 lb per inch of depth, or 24 lbs per square foot, worst case. But say you’ve got 24 inches of snow that fell over the last month and the temperature has swung above and below freezing many times and there’s been some rain on it too, the density is probably 25-30% which could make the snow load around 38 lb/ft^2.

So, what was the weight of the big chunk of snow?

I did directly measure the snow load on the shed roof pictured above (carefully from the perimeter) a few days before it slid off. After two different measurements showed 61 and 71 lbs/ft^2 (the latter was in a deeper section) I shoveled the most critical part of the roof for fear of collapse. Since I was expecting warm weather and rain, I shoveled a channel across the rood to encourage separate sections to slide. After it did fall off, I roughly measured the surface area (the two formerly horizontal dimensions) of the block and multiplied by the snow load I had come up with from my core samples, adjusted a bit for the varying thickness. The result…the big guy weighs between 5,000 and 6,000 pounds (2,300 and 2,700 kg). The density of that rain soaked old snow was 30% overall with the densest layer at 42%.

How was your estimate?

The figures show the coldest temperature (degrees F) endured at each city over this past weekend (28-29 Jan). The pattern that jumped out at me is the “warm” Fairbanks surrounded by a bunch of colder towns and villages. Now, I’m not about to make any conclusions based on this one case, but stay with me and I think I can make some sense of this, at least to whet the appetite for further study. About the map–you might point out the Nenana was not much colder than Fairbanks, and after all, some places are just colder than others. To check that idea, I’ve tabulated the long term averages for the stations on the map (some did not have enough data, so I’ve added Hughes to stand in for Huslia, and I also added McGrath for reasons which will become apparent).

The stations are arranged with the coldest average at the top. There is not a lot of correlation between the average temperature in the coldest part of the winter (the table) and the Jan 28-29 cold snap minimums (the map). Fairbanks is only a little warmer than Nenana on this chart, based on WRCC period of record data, but is slightly colder according to the NCDC’s latest (1981-2010) normals. This apparent inconsistency is probably due to the many holes in Nenana’s temperature record, and shows how careful one must be when working with climate data. Anyway, the map is more of a conversation starter than anything. We need more data. Complete, reliable weather records going back far enough to see long term trends are quite hard to come by in Alaska. The best data for this little investigation is a comparison between Fairbanks and McGrath. McGrath is a ways from Fairbanks, about 275 miles (440 km) actually, but for this purpose, having complete, quality data was paramount. There are stations much closer geographically but with less complete and in some cases less reliable data. Those stations also support what I’m trying to say about Fairbanks cold snaps. Click here to see them. What am I trying to say? This graph of the two 1st order stations says it:

click for a larger version

The columns on this graph show the number of days during the given winter that the temperature dropped to –50 F (-45.5 C) or colder, red for Fairbanks, blue for McGrath. The jerky lines above are temperature observations for each station (same colors), showing the average of daily lows for December, January and February for the given winter (December of the year previous to the label). The average lows track as you would expect: each of the two stations follow the ups and downs of the 70 winters in fairly close agreement to each other, or with Fairbanks being a few degrees cooler. The number of –50 or colder days is fairly close as well, through about 1971. In fact up till then Fairbanks has more of the super cold days than McGrath, but that’s nothing unusual since Fairbanks is colder on average–the line graph clearly shows that–and in the winters that Fairbanks had significantly more –50 days (1965 & 1969) their DJF average was significantly lower as well. What begs an explanation is the lack of –50 days after 1971. There is no change in the relative DJF temperatures (almost all the –50 days occur during those three months). Yet from 1971 onward, McGrath has had many more 50 below days than Fairbanks, even while their average temperatures have been very close to or a bit warmer than Fairbanks. In fact, the rate of occurrence of days 50 below or colder does not change much for McGrath or the 4 other interior stations you can see in the supplemental file, but at Fairbanks it drops by 4/5ths!

Alaskan “heat” islands

I think we are dealing with an interesting variation of the Urban Heat Island effect. The UHI effect is the well-known and well-documented rise in temperature of a large urban area as compared to surrounding non-urban areas. In more temperate areas the UHI is only significant for large urban areas, and has mostly to due with radiational effects due to changes in the nature of the land surface. In Alaska we don’t have the kind of urban-ness needed for a significant UHI effect based on the mechanism just mentioned. Even Anchorage, as large as it is, has so many trees, and indeed large blocks of natural forest areas within the city as to mitigate those effects. Apart from Anchorage, no city has the size that would create a significant UHI in the lower 48. But we do things differently in Alaska, as the saying goes. Radiational effects may be small, but emissions of sensible heat, and especially water vapor and particulate matter are what I believe create UHIs in Alaska. Winter low temperatures show a much stronger effect. Even then, the UHI effect for Fairbanks appears to be fairly weak. If it were strong, my graph above might show some divergent trend over the years of population growth of Fairbanks vs McGrath (see graph below), but it sure doesn’t. A study at UAF (Magee, et. al. 1999) looking for a possible UHI for Fairbanks found about a 1°C (1.8°F) warming of winter lows compared to nearby Eielson AFB. It’s just one study of climate records, which as mentioned above, are not always as complete or precise enough to draw too many conclusions. (An actual field experiment in Barrow found a much larger UHI effect, surprising to me given that Barrow is not only much smaller, but also much windier. Winds tend to homogenize the air and wipe out small scale temperature differences. click here to read the study.)

Why would the UHI effect be so evident for extremely low temperatures when it is fairly weakly reflected in average low temperatures ? Several factor present at average low temperatures are sharply increased in the extreme cold.

1. During cold snaps, temperature inversions are sharper and winds tend to be lighter, meaning local effects are not diluted as much by mixing with surrounding air.
2. The colder it get, the more energy must be used to heat buildings, keep cars running, etc., and so the more waste heat enters the atmosphere.
3. The more fossil fuel consumed, the more water vapor and particulates enter the atmosphere, releasing latent heat though condensation, freezing and deposition.

This last item is crucial, but might need some more explanation. When carbon based fuels are burned, the primary byproducts are carbon dioxide and water vapor. There is a lot of energy in water vapor, a potential kind of energy called latent heat. This is an important lesson in Meteorology 101. The latent heat is put there by the sun when it evaporates water, or, in this case, by the heat of combustion. There is very little water vapor in the frigid winter air of uninhabited interior Alaska (almost none) since there is so little heat available to convert or keep the H2O in vapor form. Of course there is very little liquid water to be found lying around either, for the same reason…not enough heat to keep molecules moving enough to stay liquid. But in a city the size of Fairbanks, with water vapor flowing out of every smoke stack, power plant, furnace flue, exhaust pipe and jet engine, there is quite a bit, and it does not want to stay gaseous long. At moderately cold temperatures (+10F down to around 25 or 30 below 0F)  it often makes liquid water fog, which often gets mixed into drier air and turns back to vapor, or become rime or frost on solid objects. Sometimes, if the right kind of particulates are at hand, (remember how burning fossil fuels releases particulates: smoke, etc.) it can form ice crystals in the air which gives us those brilliant halo and sun dog displays. All these instances of water changing to a lower energy state release heat. And when fog or clouds are formed, the fog or clouds can also retard heat loss due to radiational cooling of the ground. But often it keeps getting colder despite these effects.

Alaskan cold

So, the colder it gets, the more moisture is put into the air by humans trying to keep warm or keep their vehicle or aircraft engines running, but the less moisture the air can keep in vapor form. Around –40 (F or C) and colder the liquid and gas phases are basically no longer available to H2O. It’s getting squeezed out of the air as fast as it can be put in, and the result is usually ice fog. And lots of latent heat, since there are two phase changes to get to the solid ice particles that make up ice fog. So let’s look at the recent cold snap and see the difference in moisture issues between Fairbanks and much smaller, but usually warmer, McGrath. Last graph, I promise.

Look especially at the temperature changes. In Fairbanks, when it dropped below about –45 F, the visibility dropped to 1/2 mile, then 1/4 mile or less…ice fog. With the fog thick and constant for the next day and a half, the temperature did not get much colder—nor warmer. In McGrath, the visibility only dropped for short periods and not as drastically as Fairbanks, despite even colder temperatures at the low end. There was less water vapor being put into the air, less fog, and less latent heat release and less to hinder radiational cooling. During the afternoons, the lack of fog allowed some warming due to a little sunlight, though on the 29th it looks like the warming was due to other reasons.

If your fingers and keyboard are not too stiff from the cold, type in your comments or questions below. I’d love to hear your thoughts.

You probably know that where the isobars are drawn with the closest spacing is where the gradient is the strongest and therefore where the wind is supposed to be the strongest. So although most of the state should be somewhat windy, SE AK and on north to Cook Inlet should be especially so. Indeed, here is how the Inside Passage looked like at Haines this afternoon:

The interesting thing is that although this extremely large weather system looks fairly simple on the map—nice smooth isobars defining areas of greater and lesser pressure gradient and, presumably, wind strength—the terrain creates a detailed, small-scale patchwork of wind speeds and directions that even some meteorologists have had trouble believing at first. Let’s look closer at the above photo and situation. On the right end of the dock is a wind sock…stretched out horizontally (click on the photo for a larger view). No surprise, the wind was pretty strong there, perhaps 20-25 kts (knots…1 kt = 1.15 mph or .5 m/s) but less, maybe 10-15 kts where I was standing. But look out in the middle of Lynn Canal, or to be more precise, the far 1/3 of the Canal. See what looks like a thin layer of fog? It’s blowing spray…the winds out there are probably a steady 45-50 kts with gusts to 60 or 65 kts. This is not just an exaggerated guess. At the Skagway airport, about 13 miles north (to the left in the photo) the instruments were (and still are as I write this) reporting sustained 35 kts with gusts to 45-50. About the same distance to the south is Eldred Rock where the lighthouse winds are averaging 55 kts sustained, gusts to 70-81 as you can see from the graph below. The state ferry trying to make its rounds to Skagway and Haines today turned around half way and headed back to Juneau.

–graph from the National Data Buoy Center–

Link directly to the Eldred Pock page at NDBC

What’s at work here is channeling. When mountainous terrain has gaps, valleys or channels, which of course it usually does, the wind can flow through those channels to get to the other side, and that flow is often accelerated compared to what it would otherwise be. The wind “wants” to flow through these gaps when there is a difference in pressure between the two sides, which of course there usually is. Here are some rules about channeled winds:

1. The more stable the atmosphere is, the stronger the channeling effect
2. The narrower the channel, the stronger the channeling effect
3. The wind tends to blow along the channel, not across it
4. The wind blows from the end of the channel with higher pressure toward the end with the lower pressure
5. The greater the pressure difference from one end to the other, the stronger the wind
6. The wind is accelerated through, and for a distance downwind of, the channel, but is often light upwind of the channel

Now look a the map of Lynn Canal:

View Larger Map

You can see the extreme channeling leading to these extreme winds. 80 kts is not unusual for Eldred Rock (A on the map).

What about the direction? Look again at the surface pressure pattern, shown cropped and blown up below, Eldred Rock is at A, approximately. From which direction should the wind blow?

Most weather books for the layman, and even many more advanced textbooks would tell you something like this: “The wind blows counterclockwise around the low (N hemisphere) parallel or angled 20-30 degrees across the isobars toward the low.” With this instruction, the wind at “A” ought to be blowing from the east. In fact it is blowing from the north, about 340 degrees true. Many protest and say “the wind can’t circulate clockwise around the low!” Re-read points 3 and 4 in the channeled rules above and look again at the Google map. The channel is oriented right along 340-160 true…just a tad counterclockwise from north-south. Look at the pressure pattern. The pressure is definitely higher to the north, so the wind blows from north to south, right along the channel.

If it makes you feel better, the wind at “B”, the Fairweather buoy, is blowing from the east southeast, at 40 kts gusting to 50. Over the open ocean the coriolis effect is at play and the textbook answer works. Over highly blocking terrain, the coriolis effect is not at play because the air is initially not (hardly) moving, having been blocked by the mountains (it does not want to go over since the air is so stable…rule 1 above). When the air finds a gap or channel, it races through, more directly from high to low pressure. The coriolis effect does not have the time to “turn the wind to the right,” nor can the wind turn to the right since it is constrained in narrow channel of solid mountain! Don’t throw out the textbook, just add some notes in the margin. Few of them acknowledge this exception to the wind direction rules…maybe because most authors and publisher hail from the flatter east coast.

Skagway is in the same channel as Eldred Rock, but the speeds are a little less since the wind is coming off the land and the land has more friction than the water. In the summer, with predominately south winds, there is less difference in speed between Eldred Rock and Skagway. What about Haines? The wind at the Haines airport has been averaging about 5 knots during this event. Why? Look a the map. The Haines airport is a couple miles west of the dot labeled Haines. It’s in the Chilkat Valley, oriented more Northwest-Southeast than north-south Lynn Canal. In this case the isobars hardly cross it–they are pretty much parallel to the channel, so there is little difference in pressure from one end to the other. See rules 4 and 5 above. [If the low were farther east, say over Ketchikan or BC, The wind would blow faster down the Chilkat Valley, and when this happens it often looks on the large scale map like the wind is blowing exactly clockwise around the low, 180 degrees from the textbook solution!] Certainly other parts of Haines, downtown in particular, have more wind than the airport in the current situation, because of their exposure to the edge of the Lynn Canal wind or a wind coming down a smaller side channel. Also, occasional bursts of wind spill over the ridge along the north side of the valley, violating rule 3 above. No absolutes in the weather business.

Now, if the previous material makes any sense at all, get ready for an even more tricky and interesting case. Look up the coast at location “C” on the surface pressure map. The flatlander textbooks would say the wind should be from the northeast, right? Well, by now you know it can’t be right, but you will probably be surprised by this map:

Do the wind speeds and directions look almost random to you? [don’t know how to read the wind symbols? Think of them as arrows, with the point placed on the weather station, the shaft aligned with the wind direction (with the arrow pointing with the wind), and the fletching (called barbs) showing the wind speed: 10 knots for each full barb and 5 knots for each 1/2 barb. Look at Talkeetna for example, the station furthest NW on this map. The wind there is from the north-northeast at 25 knots.] These wind reports are real and they are from the same time as the pressure map above. If you look closely, you will see that almost all the stations with the strongest winds are in, or just at the exit of, a pass, gap or other sort of terrain channel. Most of those have north or northeast wind since those channels will interact best with the large scale pressure pattern as shown on the pressure map above.

The standout exception is Whittier, which has 35 kts from the southwest. Whittier is at location “C” on the pressure map, and it looks like it is not only violating the textbook wind rules but also my channeled wind rules too! What you need to know is that this particular pressure map is too general and smooth to handle this case (other models are more detailed and do show some of these small scale effects). The pressure patterns are affected by the terrain, which in turn gives rise to more possibilities for crazy looking wind directions. In this case the wind is blocked by the substantial barrier of the Kenai and Chugach Mountains and this blockage builds up the air on the west and north sides, the upwind side. This is the higher pressure side to begin with, but the blocking increases the pressure difference, setting up a situation for accelerated gap winds. This damming effect also allows the high pressure to “bend” around the obstacle. For instance, from the pressure map you might conclude that the pressure at Kenai should be about the same as at Whittier, when, in fact, it is a fair amount higher. This bending allows almost any break or gap to be an outlet to the pressurized air, no matter the direction. So the wind at Whittier blows from the southwest whenever there is a low pressure area to the east, southeast or even a fair ways into the south quadrant. Notice the small circle just to the west of Whittier? That is Portage Glacier Visitor Center, a weather station only 9 miles from Whittier as the crow flies, yet so different weatherwise. The wind there is calm. Remember rule 6. There are several other examples of this and the other rules on this map. Can you spot them?

I’d love to hear your questions, comments, etc on this, or other Alaska Weather topics. Click the comments link.

Polar lows often are most impressive on satellite images (many show a more “closed” look than this one) and less so on surface maps due to their size compared to the scale of the typical maps. Here’s the surface maps for 9 hours later ( as usual click for larger version):

You can see the small polar low in the center of the map, between Dutch Harbor and the Pribilof Islands. Note how much larger the lows to its west and east are. Even though this low is almost a far south as the other two, it is within the polar air mass, north of the polar front (the fronts associated with the mid-latitude lows). That’s where polar lows are found and get their name (for a while they were called arctic lows). The lows that form along the polar front (you would think these might be called polar lows but they’re not) depend on the temperature contrast between the cold polar air to the north and a warmer air mass to the south. Polar lows depend on the temperature contrast between the cold polar air coming off continents (Siberia in the case of Bering Sea lows) and the warmth of the water itself.  Look at this initialization of the NAM model.

You can see the cold polar air mass as the green (and blue) which has been drawn south by the big low in the North Pacific. Note that this low is well drawn by the model. Five or ten years ago the models would have had a hard time even seeing this low and harder yet forecasting its movement and development. That’s why polar lows have been, over the years, first unknown, then mysterious, then elusive: Their size allows them to slip between surface weather stations in the synoptic network…especially the sparse network over the polar oceans. In recent years better satellite coverage, more ocean buoys, and higher resolution computer models have about erased the “elusive” label…but they are still cool. And we still don’t often get the full ground report from a first-order manned weather station, including upper air balloon soundings, as we did in this case. Here are the surface observation from the St. Paul Island NWS office (the times are in AST):

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak

PASN  AP 03 0702  FEW018 BKN042 OVC070      10          19  16 13010     969  88%   7
PASN  AP 03 0731  OVC028                     9 S-       21  12 11011     967  68%   9
PASN  AA 03 0753  SCT025 BKN030 OVC080      10 P        21  14 13017     966  74%   6
PASN  AA 03 0853  FEW018 OVC032              7 S-       21  16 10018     963  81%   5
PASN  AP 03 0905  SCT018 OVC027              3 S-BS-    21  18 11018G25  963  88%   5
PASN  AP 03 0942  OVC017                     2 S-BS-    19  18 11021     962  96%   2  26
PASN  AA 03 0953  OVC019                 2 1/2 S-BS-    20  17 10023G29  962  88%   2  29
PASN  AA 03 1053  FEW015 OVC021              2 S-BS-    20  17 10023     960  88%   2  29
PASN  AP 03 1112  SCT016 OVC020              3 S-BS-    19  18 10020     960  96%   2  28
PASN  AP 03 1134  FEW014 OVC024          1 1/2 S-BS-    19  18 09024G28  958  96%   1  28
PASN  AP 03 1143  FEW014 OVC024          2 1/2 S-BS-    19  18 09022G27  958  96%   1  28
PASN  AA 03 1153  FEW020 OVC026          2 1/2 S-BS-    21  17 09022     957  84%   4  28
PASN  AP 03 1209  FEW015 OVC022          1 1/2 S-BS-    21  18 09022G29  956  88%   4  29
PASN  AA 03 1253  FEW012 OVC019          1 1/2 S-BS-    21  19 09024     954  92%   3  32
PASN  AP 03 1304  VV014                    3/4 S-BS-    21  18 09023G30  953  88%   4  30
PASN  AP 03 1311  VV011                    1/4 S-BS     19  19 09025G31  953 100%   0  31
PASN  AP 03 1320  VV007                    1/4 SBS      19  19 09026G32  952 100%   0  32
PASN  AP 03 1338  VV004                    1/4 SBS      21  21 08029G35  950 100%   2  35
PASN  AA 03 1353  VV004                    1/4 SBS      22  21 07028G34  949  96%   3  35
PASN  AP 03 1419  VV006                    1/4 SBS      21  21 08030G35  947 100%   2  37
PASN  AA 03 1453  VV005                    1/4 SBS      21  21 07032G41  944 100%   1  44
PASN  AP 03 1512  VV004                    1/4 SBS      21  21 07031G41  943 100%   1  41
PASN  AA 03 1553  VV003                    1/4 SBS      20  20 08035G44  942 100%  -1  44
PASN  AA 03 1653  VV004                    1/4 S+BS     20  19 07036G44  941  96%  -1  45
PASN  AP 03 1700                           1/4 S+BS     19  19 07034G45  942 100%  -2  45
PASN  AA 03 1753                           1/4 S+BS     18  18 06034G43  941 100%  -4  47
PASN  AA 03 1853                           1/2 SBS      18  18 05032G42  941 100%  -3  44
PASN  AA 03 1953                           1/4 S+BS     17  17 04032G43  941 100%  -4
PASN  AA 03 2053                           1/4 S+BS     15  14 03035G43  941  96%  -8
PASN  AA 03 2153                           1/2 BS       14  13           941  96%
PASN  AA 03 2253                           1/2 BS       13  12 04031G88  942  96% -10
PASN  AP 03 2311                           3/4 BS-      12  12           942 100%
PASN  AP 03 2341                             1 BS-      12  12           942 100%
PASN  AP 03 2351                           3/4 BS-      12  10 03033G38  942  91% -12
PASN  AA 03 2353                           3/4 BS-      13  11 03033G38  943  91% -10
PASN  AP 04 0033  VV020                    3/4 S-BS-    12  12 03027G38  943 100% -10  47
PASN  AP 04 0040  VV005                    3/4 S-BS-    12  12 03026G36  943 100% -10  47
PASN  AA 04 0053  VV005                      1 S-BS-    13  12 03025G34  943  96%  -8  47
PASN  AP 04 0111  OVC010                   3/4 S-BS-    12  12 03026G34  943 100% -10  36
PASN  AP 04 0130  VV005                    3/4 S-BS-    12  12 03029G33  943 100% -10  36
PASN  AP 04 0142  VV005                  1 1/4 S-BS-    12  12 03027G31  943 100% -10  36
PASN  AA 04 0153  VV005                    3/4 S-BS-    13  12 03025G32  943  96%  -8  36
PASN  AP 04 0204  VV005                      1 S-BS-    14  12 03025G32  943  92%  -7  32
PASN  AP 04 0240  OVC010                 1 1/2 BS-      12  10 03025     943  91%  -9  32
PASN  AA 04 0253  OVC010                 1 1/2 BS-      13  11 03025G31  943  91%  -8  32
PASN  AP 04 0258  OVC005                 1 1/4 BS-      12  10 03026G31  943  91% -10  30
PASN  AP 04 0314  OVC010                     2 S-BS-    12  10 03023     943  91%  -9  30
PASN  AP 04 0328  OVC010                     3 S-BS-    12  10 03023G27  943  91%  -9  30
PASN  AP 04 0337  OVC010                     2 S-BS-    12  10 03020G29  943  91%  -7  30
PASN  AA 04 0353  OVC010                     3 S-BS-    13  10 02023     944  87%  -7
PASN  AP 04 0402  OVC010                 2 1/2 S-BS-    12   9 03020G28  944  87%  -7  28
PASN  AP 04 0448  OVC010                 1 1/2 S-BS-    12   9 01022     945  87%  -8  29
PASN  AA 04 0453  OVC010                 1 1/4 BS-      12   9 01020     945  87%  -7
PASN  AP 04 0504  OVC012                     3 BS-      10   9 01020     945  96% -10

This low brought near blizzard conditions (although the wind met the strict NWS criteria the visibility did not) to St. Paul including sustained winds of 40 mph with gusts as high as 54 mph, snow, blowing snow and wind chills around 10 below F…and quite few hours of the worst of it. The interesting thing is while the winds at St. Paul came up steadily throughout the day on the 3rd, at 50-mile-away-neighbor St. George, the wind initially came up, then from around 2 pm to 11 pm winds were light — even calm much of the time — before returning with similar ferocity as at St Paul. This is because it appears the low center went right over St. George. They were in the eye, if you will…a hurricane sized one. Here are observations for a few hours during that time with  St. Paul’s obs (PASN), copied from above, paired with St. George’s (PAPB). Again, times are in standard time:

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak

PASN  AA 03 1653  VV004                    1/4 S+BS     20  19 07036G44  941  96%  -1  45
PAPB  AA 03 1653  SCT022 SCT120             10          22  19 00000     938  88%  22

PASN  AA 03 1753                           1/4 S+BS     18  18 06034G43  941 100%  -4  47
PAPB  AA 03 1753  BKN055 BKN100 BKN110      10          23  15 00000     937  71%  23     

PASN  AA 03 1853                           1/2 SBS      18  18 05032G42  941 100%  -3  44
PAPB  AA 03 1853  CLR                       10          19  15 08004     935  84%  12    

PASN  AA 03 1953                           1/4 S+BS     17  17 04032G43  941 100%  -4
PAPB  AA 03 1953  FEW013 SCT019 BKN039      10          21  15 00003     933  77%  21

Imagine a St. George resident calling, at around 7 pm, a friend or relative on St. Paul, where the weather is usually pretty much the same, saying, “What a nice evening. My wife and I just went for a walk. It’s calm, mild and what a beautiful moon out! I thought you might want to do the same.” The friend replies, “What! Did you go to Hawaii? I’m not sure I’d be able to force the door open with this blizzard raging.” This is a great example of how tightly wound these polar lows can be. I think they are way cool. What do you think? Click the comments link and let me know.

Here are some good polar low links:

Polar low blog by Erik W. Kolstad  —   http://polarlows.wordpress.com/
National Snow and Ice Data Center page  –  http://nsidc.org/arcticmet/patterns/polar_low.html

Snow rollers form when the snow possesses a certain layered tackiness that allows the top layer to peel off the underlying layer (or sometimes the ground) and stay together as it gets rolled into a ball or tube. The rolling can be bone by either gravity or the wind. No elves or gremlins are needed. If by gravity, a pretty steep slope is usually needed. If wind, then you can guess that a pretty strong wind is needed.

The rollers in this photo were traveling UP a slight incline, so it had to have been the wind, and some strong gusts to boot. Notice the tracks left by the rollers show that the wind was coming from slightly different directions at different times, consistent with periodic gusts strong enough to move the rollers interspersed with weaker wind. At the very top of the photo you can see the fence around our Haines Little League field. The whole area between the foreground of this photo and the ball field, the entire ball field, and a gravel pit to the right was littered with thousands of these roller ranging up to the size of a large rolled sleeping bag. Here’s a couple photo at the ball field. By then using a flash was the only hope of a photo and the small bright spots are snowflakes caught in the flash.

What weather conditions lead to snow rollers?

There are several ways the layered tackiness needed for snow rollers can come about. One way is a rapid warming of a dry snow pack such as can happen when a strong storm moves into an area. Downslope (Chinook type) winds in mountainous areas usually bring both rapid warming and strong winds. This was the case with once incident I published a photo of in a past Alaska Weather Calendar in the eastern Chugach Mountains in which the photographer actually saw the rollers being formed. In the recent Haines case, wet snow fell on top of a hard frozen crust as a front brought a surge of warmth and moisture. Here are the weather observations for the Haines Airport (about 1.5 miles or 2 km west of where I found these snow rollers). The photos were taken between 1600 and 1630 AST on the 15th. (The times shown on these observations have been converted to ADT – Alaska Standard Time). The wind had been gusting to around 30 kts (35 mph or 15 m/s) since morning, and the had been snow coming down pretty heavy and at a temperature right at, or slightly above, the melt/freeze point…the perfect temperature for a nice cohesive kind of snow. (The temperatures shown on these observations have been converted to Fahrenheit). Remember, the snowpack up to that time had been a hard rain crust, hard enough to jump up and down on (and according to one report, hard enough to drive a small truck on top of!) The new snow was not going to bond to that very easily.

Site M/A Day Time Sky Conditions   VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak

PAHN  AA 15 0954  OVC008             1 S-F      32  31 11012     987  96%  22
PAHN  AP 15 1019  OVC008           1/2 SF       32  30 09011G30  986  92%  22  30 
PAHN  AA 15 1054  OVC008           3/4 S-F      32  31 10013G24  984  96%  22  30
PAHN  AP 15 1111  OVC008           1/4 S+F      32  30 11013G22  983  92%  22
PAHN  AA 15 1154  OVC006           1/4 S+F      32  32 11014G27  981 100%  21  28
PAHN  AP 15 1211  OVC006           1/2 SF       32  30 09015G29  981  92%  21  29
PAHN  AP 15 1219  OVC006           1/4 S+F      32  32 09015G28  980 100%  21  29
PAHN  AP 15 1230  OVC008           1/2 SF       34  32 09020G30  980  92%  22  30
PAHN  AA 15 1254  BKN008 OVC013    1/2 SF       33  32 09015G29  979  96%  22  30
PAHN  AP 15 1342  OVC008             1 S-F      34  32 11012G26  977  92%  25  32
PAHN  AA 15 1354  BKN008 OVC012    1 1/2 S-F    33  32 09013G24  975  96%  23  32
PAHN  AA 15 1454  BKN008 OVC014    1 3/4 S-F    33  32 09013G26  974  96%  23  26
PAHN  AP 15 1510  BKN010 OVC014        2 S-F    34  32 10012G28  975  92%  25  28
PAHN  AP 15 1520  BKN008 OVC014    2 1/2 S-F    34  32 10014G29  975  92%  24  29
PAHN  AP 15 1525  OVC010               3 S-F    34  32 10014G29  974  92%  24  29
PAHN  AP 15 1534  OVC009               4 S-F    34  32 11014G27  973  92%  24  29
PAHN  AA 15 1554  BKN009 OVC013        4 S-F    34  32 11016G27  973  92%  23  29
PAHN  AP 15 1621  OVC011               4 S-F    34  34 11016G31  972 100%  23  31
PAHN  AA 15 1654  BKN011 OVC015        5 R-F    35  33 10020G33  973  92%  23  33
PAHN  AA 15 1754  OVC013               7 R-     35  33 11012G25  976  92%  26  31
PAHN  AA 15 1854  OVC015              10 R-     36  34 11011G25  980  92%  28  29
PAHN  AP 15 1911  OVC013              10 R-     36  34 11009G20  981  92%  29
PAHN  AA 15 1954  BKN015 OVC065       10 R-     36  34 10009G22  984  92%  29
PAHN  AA 15 2054  OVC014              10 R-     36  34 11010G21  986  92%  28
PAHN  AP 15 2114  BKN016 OVC022       10        37  34 11011G19  987  89%  29

It is quite likely there were many more snow rollers in our area that day. There are miles of river flats nearby that get this sort of wind or stronger. I would love to hear of anyone who saw snow rollers on December 15th in the Chilkat Valley. Or any time or anywhere for that matter. Snow rollers are pretty uncommon, but not “once-in-a-lifetime” rare as some websites like to spout. Again, it is a matter of being aware, being outside and keeping your eyes open. Of course it does not hurt to live in a snowy area.

Fair weather rollers

Another way the snow can reach the right consistency for snow rollers is the warming of a cold snowpack by the sun. In this case gravity is likely going to be the motive force to form snow rollers since strong winds and warming sun don’t often come together. Sometime snow falling off trees get the ball rolling but sometimes they start on their own. Here’s a beautiful example of some gravity snow rollers taken in April along the Alcan Highway in the Yukon Territory, courtesy of Alan Sorum of Valdez.

There are many more photos of snow roller on the Internet: Here are some good ones with good information. You may notice most of them are from the plains states…wide open fields, snow, quick changing weather and plenty of wind.

http://www.wrh.noaa.gov/otx/photo_gallery/snow_rollers.php

http://www.crh.noaa.gov/ilx/events/roller/roller.php

http://webecoist.momtastic.com/2011/01/25/snow-rollers-tumblin-tumbleweeds-of-icy-white-delight/

So be outdoors, and look up… and down. Let me know what you see.

But first, what is a La Nina, how does it affect our weather, and can it really allow one to make a five month or longer forecast? Very briefly, a La Nina is one phase of a oscillating weather pattern in the equatorial Pacific involving air pressure patterns, winds and sea water temperatures. That a weather pattern roughly 4,000 miles (6,000 km) away can affect Alaska’s weather shows the large, interconnected nature of Pacific weather and climate systems and how important ocean temperatures are to them. The tropical part of this system has been studied for decades and is termed the El Nino/Southern Oscillation, or ENSO for short. La Nina is the cool phase of this tropical system, El Nino the warm phase. It oscillates between the two famous kids on a more or less yearly basis, typically with a lull during the northern hemisphere summer and an intensification in fall and through the winter. Many years the phenomenon is weak or noncommittal…a neutral phase. Here’s a intuitive graph from the NOAA’s Earth Systems Research Lab:

For more information on the the ENSO see the links at the end of the post.

As for what it means for Alaska’s winters, the highly simplified version is that El Nino winters are more often warmer, La Nina more often colder. In the following charts I’ve plotted the average temperature from November through March vs the seasonal snowfall total for four hopefully representative locations. This kind of X-Y scatter plot can tell you how winter temperature and snowfall correlate. Each diamond represents one winter of the 62 plotted since the winter of 1949-50. From the scales you can see that winters that fall much to the upper left were cold and snowy, lower left cold & not so snowy, upper right warm & snowy, lower right warm & not so snowy. Near the center winters were close to average. In addition, and pertinent to this topic, I’ve colored the diamonds according to the ENSO phase for that winter: red for El Nina (14 of the 62), blue for La Nina (16) and gray for neutral (32). The lime green circles indicate the “double dip” La Nina winters. Keep in mind that there are quite a few ways to quantify and define El Nino/La Nina/neutral years and which criteria are used could affect this sort of analysis to some degree. My source seems to leave more winters in the neutral bin than some, possibly understating some relationships.

Starting from the north, here’s Nome and Fairbanks (please click for a better look):

Both Nome and Fairbanks show little correlation between winter snowfall and overall winter temperature. At this latitude I would have expected warmer winters to have more snow and colder ones less. Possibly if we considered the water equivalent of the snow it would show up. A little farther north I’m sure the correlation would be obvious. But that’s not the thrust of this article. It’s how the ENSO plays out for our winters. So look at the colors. Both cities show strong evidence to support the warm El Nino, cold La Nina pattern. In both cities only 2 of 16 La Nina winters were warmer than average by any significant amount (one of Nome’s was last winter) and those only moderately warmer. Some of the La Nina winters were only slightly cold but the majority were roughly 3-6 degrees F (1.7-3.3 C) colder than the 62 year average. That amount of deviation makes a big difference over a winter. Snowfall shows no conclusive pattern in these two cases. Neutral years are scattered fairly uniformly throughout the graph for both temperature and snowfall. Interestingly, most of the very snowy years in Fairbanks were neutral years. Perhaps a Goldilocks situation. By the way, the double-dip La Nina years don’t show anything conclusive. There are only four, and they are spread fairly evenly through the La Nina years. Perhaps if the analysis went went further back more could be gleaned. Also, as mentioned above, the criterion used for categorizing La Ninas is not standardized. The Climate Prediction Center calls the winter of 1950-51 a La Nina (a pretty strong one too, so I don’t know why the FSU ranking I used does not). It was, for three of the four cities (all but Nome), one of the four coldest winters of the 62, and it was a double dipper to boot.

For more southerly climes, let’s look at Anchorage and Juneau:

A major difference from the two northern stations is the inverse relationship between temperature and snowfall that is evident at Anchorage, and strong at Juneau. In other words, warmer winters have less snow, colder ones more. This relationship is expected in temperate areas. With respect to the ENSO phase, La Nina winters tended to be colder at both locations, and to a lesser degree snowier. A strong exception is the double- dip La Nina winter of 1999-2000 when in Juneau it was 3.9 degrees F (2.2 C) warmer than average with 44% of average snowfall. At the other three station that particular winter also did not follow the pattern but was not so extreme: Temperatures near average and snowfall near, to above average. Again the double-dip winters do not tell us anything definitive. One thing skiers can be pretty happy about is that there were no La Nina winters in the last 62 years that were both above average in temperature and below average in snow in Anchorage or Nome and only one in Fairbanks and two in Juneau. Study these graphs more and I’ll bet you can tease out more revealing details about our winters.

Useful for seasonal forecast?

What can be deduced from this data to make a projection for the coming winter? With a moderate to strong La Nina already under way, I think we can expect most of Alaska to have a colder than average winter. Snowfall in Southeast is likely to be higher than average but for the rest of Alaska it is indeterminate. If temperatures are not significantly colder, then near average is the likely result—a significantly warmer than usual winter has little statistical chance of happening in a La Nina year. The idea of a double dip La Nina bears further study before I’d want to say much. Let’s just say it will likely be a cold winter for most of Alaska, and I would not be surprised if if it is not much colder than the long term average in many parts.

Backup

Lest you think I’m something special for making a 5-month forecast, I’m not alone, and I’m not going against the crowd. To see more of the consensus, follow these links to the Climate Prediction Center or Juneau Forecast office’s summary. The CPC’s maps are right there on the home page and are easy to use. For more maps click on 30-day or 90-day outlooks below the home page maps. All the winter 2011-2012 maps you can find will say the same thing: confidently colder than average for the southern 2/3rds of Alaska. These are updated around the 20th of each month but I would not expect any change in the basic idea.

Caveats

It is important to note that the above forecast could prove to be flat out wrong, and likely it will be for some parts of Alaska. Alaska is so big that the chances of every part of it being colder than average are pretty low. In particular, the far north and far west (not really covered in our graphs), i.e.  Barrow, and the western Aleutians, seem to often be exceptions. The strong warming in Barrow recorded since about the mid 70s seems to defy trends and drivers that effect other areas. More study needs to be done, but possible reasons are decreases in extend and thickness of the ice pack, and the urban heat island effect (artificially warming the local populated area only). The latter effect has been well documented in Barrow by Dr. Ken Hinke of Univ. of Cincinnati using in situ measurements.

Even if the winter is colder than average, day-to-day and even week-to-week winter temperatures are volatile and there will be fluctuations above and below the mean. Different factors affect the weather on different time scales. For snowfall there are even more variables than for temperature. That is why we deal with averages and percentages, not absolutes. But the ENSO phase is probably the most powerful seasonal forecast tool yet discovered for a vast part of the globe. Another cycle I’ll briefly mention is the Pacific Decadal Oscillation which, due to its current cool phase (cool for Alaska), bolsters the confidence of this colder-than-average winter forecast. I covered the PDO more in last year’s winter forecast.

Please let me know what you think via the comments link below. And remember, don’t kill the messenger just because you don’t like the message.

Links:

NOAA NCEP ENSO page http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

NOAA ESRL ENSO page http://www.esrl.noaa.gov/psd/enso/

NOAA PMEL ENSO data links http://www.pmel.noaa.gov/tao/elnino/measurements.html

Haines Highway, near the summit, with drifting snow, low visibility. Difficult, but drivable. The poles on either side of the road really help.

Since the winter weather has the potential to put a complete halt to highway travel, having an accurate forecast well in advance of a trip could save much time, money, missed opportunities, or worse. I consider this one of the most important uses of weather forecasting. Is the state of the art up to the task?

The state of the art

The raw truth is that there is precious little information available to meteorologists and the travelling public about the current weather (what’s going on right now) along Alaska’s highways. When you don’t have a good idea of the current conditions, its that much harder to make a forecast. The main problem is the enormous distances between traditional weather reporting stations. What about web cams? Yes, they do help fill some of the gaps, but they are restricted to areas with electrical power and telecommunications, so we are still left with vast stretches of mystery highway. Weather radar does not reach most of the areas of interest due to distance, and blocking of the radar beam by mountains (remember, the mountainous areas are the most critical for driving and also for affecting the weather). Satellite data is of limited help since it is not so good for inferring things like how much snow is piling up and whether there is reduced visibility due to falling or blowing snow. Much of the diagnostic work is done just as the prognostic work: by looking at the various charts and inferring the conditions on the ground, using local knowledge as a guide. All these challenges mean that weather info for the more remote sections of highways is not good enough to make a go/no go decision on a trip more than a few hours in advance. The good news is that if your are properly equipped with a good vehicle/tires, winter driving skills and provisioned for any contingency, you can most always make it through despite the weather, though the time needed may vary.

The targeted highway forecast

A few years back the NWS put out “motoring” forecasts for all the main year-round highways in the state. With two exceptions these have been absorbed into the “zone” forecasts. The thought was that the zone forecasts now cover the entire state, so the highways are covered in the various zones they fall in. So there is no need for a separate forecast. Well, I believe the change resulted in a decrease in service to the highway traveler. The reason is related to the reasons why it is so hard to forecast or even determine the weather along the highways. The lack of data encourages a forecast more tuned into the population centers and weather reports in the zone. The mountainous areas in a zone often have different weather and usually few, if any, weather stations. But the highways go through the mountains, nonetheless (yes, through passes, the low spots, so things aren’t as bad as they could be). The zone forecast can only stretch so far, and the highways (passes in particular) don’t get the treatment they used to. There are a few passes explicitly mentioned as locations included in the zone forecast, and that may help some, but there are only a few. Another issue is convenience: The route from Anchorage to Fairbanks passes through six zones. Other routes aren’t going to make you look up and piece together quite that many forecasts, but at least two or three.

The exceptional highways

There are two routes which avoided the axe: The Klondike Highway from Skagway to Carcross and the Haines Highway from Haines to Haines Junction. Maybe the need was seen for these two since they are two of the most weather-affected drives around, or maybe it had something to do with the international nature of the routes: both cross the coastal mountains while traveling between US and Canada. Perhaps the Canadian forecasters, who write the sections on their soil, wanted to keep it going. Either way, it is good that it is still done. In keeping with the difficulty of the task, the forecast is only valid for the current and next day (the evening version only for that night and next day, i.e.., 24 hours), and it doesn’t try to cover whether the road will be slick, icy, wet, dry etc. The forecast is released from October through April, which covers most of the possible poor driving conditions. Most, but not all. In June 2008 a large section of the Haines Highway was blasted with as much as a foot of snow!

These two remaining highway forecasts can be found at http://pajk.arh.noaa.gov/TextFcsts/textProds.php#prods=public.

Zone forecasts can be found at http://pafc.arh.noaa.gov/pubfcst.php.

Reading between the lines

As sketchy as they may be, checking the current conditions before heading out is crucial. Here’s a portion of the road reports for the two Southeast Alaska access routes mentioned above. This product comes out only once a day, in the morning. Notice that info from Frasier is missing. The report from the summit is valuable…those conditions are marginal but drivable. Another important bit here is the temperatures at customs (38F or +3C) and the summit (30F or –1C). The drop from above freezing to below freezing is most likely going to be the slickest part of the drive, and in this case it comes in the steepest section of the highway. Better have really good tires.

900 AM Saturday October 29, 2011

South Klondike Highway Weather Observations Between 7-9 AM Saturday

Mile	Location	SkyWx	Temp	Vsby	Wind	24Hr Pcpn	24Hr Snow	24Hr Max	24Hr Min
0.0	Skagway Airport	Light Rain	45	10+	SW14G21	0.02	M	47	42
6.8	US Customs	Rain and Fog	38	7	S10	M	0.0	M	M
15	Summit/Border	Snow and Fog	30	3/4	S15	M	3.0	M	M
22.5	Fraser	M	M	M	M	M	M	M	M
66	Carcross	Cloudy	31	10+	NNE4	T	1.2	31	26

For the Haines Highway it might at first appear there is more complete reporting, but look at the mileage figures. The first 40 miles from Haines are along the river flats…very little elevation gain. After passing the border the road climbs aggressively and crosses several high points, followed by many ups and downs. [more details including a satellite photo and nifty elevation profile can be found at the website of the bike race that takes place on this route each June, the Kluane Chilkat International Bike Relay.] By the time you get to Blanchard, the next point with any weather information, most of the weather issues are behind you (assuming you are northbound). At least on the Klondike Highway there is some information about the actual pass, here, nothing. That the visibility is zero at the border and only 4 miles at Blanchard could indicate a slow drive.

Haines Highway Weather Observations Between 7-9 AM Saturday

Mile	Location	SkyWx	Temp	Vsby	Wind	24Hr Pcpn	24Hr Snow	24Hr Max	24Hr Min
0.0	Haines	Light Rain	40	5	CALM	0.09	0	43	37
3.5	Haines Airport	Light Rain	44	10+	ENE13G22	0.02	M	45	39
23.8	Chilkat Rvr Bridge	M	35	M	CALM	0.03	M	42	32
36.6	Klehini	M	32	M	CALM	0.15	M	40	31
40.4	US Customs/Border	Cloudy	32	0	M	0.00	0.0	41	30
88	Blanchard	Cloudy	28	4	CALM	T	0.8	34	21
151	Haines Junction	M	M	M	M	M	M	M	M

The reports from 23.8 and 36.6 are from DOT automated wx stations with web cams. You can get them through the DOT website at http://www.dot.state.ak.us/iways/roadweather/forms/AreaSelectForm.html, or the state 511 website (so names because you can dial 511 on your phone to access the information) at http://511.alaska.gov/. The latter I find slow and cumbersome but at times contains valuable road condition reports that the DOT does not, since the latter does not try to give road conditions, just weather. More web cams can be found at http://akweathercams.faa.gov/. Although indented for flying weather these cams can help with highway weather as well.

More info

Don’t forget to check the basic weather stations. They can be found many places on the web. Two good ones for this are http://pafc.arh.noaa.gov/obs.php and http://climate.gi.alaska.edu/Wx/current.html. The latter is map based,which should lend itself to highway planning (but the highways are not shone). Be careful with this one…for some reason the times of the observations are in east coast time, and, I’ve found several stations that are not mapped in the right place. It would be great to have a really good map-based web page that showed all possible observations, cams and reported road conditions (and forecasts for that matter). Another thing I’d like to see is a way for drivers to report conditions they encounter and have those reports available to other drivers. We get reports from airplane pilots and ships and boats, why not cars and trucks? What do you think? I’d love to hear your opinions, comments and questions via the comments link below.

Winter forecast

Yes, I’m working on a post covering what might happen weatherwise during the coming winter. It might not be out by Halloween (it could be scary enough), but soon after.