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It’s been a cold winter across most of Alaska. It’s not the first and it won’t be the last. During every winter there are relatively colder and warmer periods. This year these cold snaps and warm spells seem to be aligned pretty well to the calendar months: November was cold, December warm, and January…one of the coldest, especially for the rail belt and everywhere west. Check out the write-ups from Weather Service personnel in Anchorage http://pafc.arh.noaa.gov/papers/THE%20COLD%20FACTS.pdf and Fairbanks https://nwschat.weather.gov/p.php?pid=201202012052-PAFG-NOAK49-PNSAFG. February is staring off with a big warm-up, thanks to a strong low barreling into Alaska’s midsection like a gut punch. I’m not making any promise that this trend will hold for February. It does look like a week to 10 days’ break for from the cold, but beyond that, I’d be surprised if we don’t get a fair amount of more cold weather (and snow for the coastal and near coastal areas) before the winter is over. See the Climate Prediction Center for more. For this post, however, I want to zero in on an interesting pattern that seems evident during the worst of the cold snaps. Let’s start with this map, used by permission of the Anchorage Daily News.

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.

Alaska is a big place, and the weather system affecting our state right now is even bigger, but I’m going to show you that is it the small scale that counts when it comes to winds of the south and southeast coasts. As I write this, virtually the whole state is under the influence of the strong pressure gradient between a large, deep low in the Northeast Pacific and strong but gradually weakening high pressure in Siberia and the Alaska interior. (The high was up to a crushing 1060 mb a few days ago, compared to the still hefty 1040 mb on this map) Here is the surface map from 3 pm/00z this afternoon from the GFS model (It’s the 12 hr forecast which is about as good as an analysis):

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 are small, somewhat elusive, and usually quite potent…cool stuff for weather watchers. They are small compared to their more common, mid-latitude big brother lows, up to a few hundred miles across, compared to a thousand or more miles across for the later. They are more like the size of hurricanes. For that reason, and because they sometimes look a lot like hurricanes on satellite imagery, and because they do share some structural similarities, they are sometimes called arctic hurricanes. The Bering Sea is a hot spot (sorry for all the puns) for polar lows but certainly not the only area…the North Atlantic is a good place too. A interesting polar low recently tracked across the southern Bering, strongly affecting the Pribilof Islands: St. Paul and St. George, but each of the two in curiously differing ways, as we’ll see. Here’s the Infrared satellite image from 15 UTC  (6 am AST) on Jan 3rd. The polar low was still about 200 miles west of the Pribilofs. With most typical low pressure systems, they’d be in the thick of it, but this average-sized polar lows is not yet affecting their weather:

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. Read the rest of this entry »

My tips for witnessing unusual, interesting or simply beautiful weather phenomena have always included spending as much time as possible outdoors and keeping your eyes up, as in looking up at the sky often. A couple days ago I found some unusual weather down at my feet. Snow rollers! My other advice is to always have your camera with you, which I did not, but it was close and I was able to fetch it before the mid afternoon dusk turned to complete darkness. Between the duskiness, the falling snow and the flat light, my photos turned out pretty rough, but by the next morning, after a few hours of rain, the snow rollers were history…you would not have suspected a thing. (Please click on the photos to see larger versions.)

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. Read the rest of this entry »

This year’s winter forecast is going to look a lot like last year’s. That’s because last winter was a La Nina winter and this winter almost certainly will be one, (or already is depending on your point of view). And how did my forecast for last winter turn out? Here’s that story. A twist to this winter is the speculation by some that there might be some significance to a 2nd consecutive, or double-dip, La Nina.

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. Read the rest of this entry »

Driving in winter in Alaska can be interesting, to put it mildly. Driving around town is included in this statement but is certainly not like hitting the open highway, crossing vast, unpopulated areas and ascending and descending mountain passes while dealing with any combination of snow, ice, fog, white-out*, wind, extreme cold, long darkness or blinding sunlight. I would not want to miss this kind of adventure, though I approach it with due respect and sometimes apprehension. (*a white-out is a low contrast condition that makes it very difficult to judge distances, to see where the sky ends and ground begins, and to see ups and downs that might be ahead of you. Obviously not good for driving. In the photo below, the visibility is limited, but you can see the road for a ways and the poles on either side, put there for just this reason.)

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? Read the rest of this entry »

Not all of Alaska is in the rainiest time of the year right now, but the southeast arm of the state, also variously known as the panhandle, the Inside Passage, the Banana Belt, or just plain Southeast sure is. In fact, I like the term rain coast. Farther north the rainiest time tends to be earlier in the year. With a few exceptions, the wettest month in Southeast is October, in Southcentral and the Interior it is September or August and along the north coast and most of the Bering Strait and Bering Sea coasts it is consistently August. The southwest coast and Aleutian Islands don’t show as clearly defined wettest month but there is no doubt that September through December or January is the wet time. These graphs illustrate. The green line is the precipitation, the others temperature. Note that the scales are the not the same on all graphs. The precipitation scale (on the right) is the same for all but Ketchikan, where it had to be expanded upward to handle the larger amounts. The temperature scales are less consistent.  (that’s what you get when you borrow your graphs…these borrowed from the Western Regional Climate Center.)

  Read the rest of this entry »

Most Alaskans had reason to complain about the recently departed summer. Not that having a reason has kept anyone from complaining in the past. I invite you to check out the hard data below, compare it with your experience and let’s see if we can come up with an award (sorry no prize money) for the worst 2011 summer in Alaska. I’d like to hear your experience and opinion on this high level competition.

To rate the summer weather we first need to define “summer.” Back on the summer solstice I talked about how that date is only an astronomical event, not one that really marks the start of summer, since most of the best Alaskan summer days are history by June 21 and many of them by June 1st (see the article). And when does summer end in Alaska? There are many markers that could be reasonably argued for the end of summer, and certainly it does not have to end at the same time across this vast land. The fall equinox (which happens to be today) does work pretty well for most of the state, but for the sake of symmetry I’m using May 15-Sep 15.

Here’s a series of graphs for 6 cities across Alaska. You can get the basic picture on this screen…or for more detail click on a graph for a full sized version. I’ll touch on how to read the graphs during the first set of observations, for Barrow:

Read the rest of this entry »

The 500 mb (millibar) weather map has traditionally been the bread and butter of weather forecasters. Today, with so many new, high tech tools at their fingertips, is this map still relevant? You bet! The 500 mb level of the atmosphere is simply the height at which the air pressure is 500 mb (or 50 Kilopascals (KPa) if you want to be a metric purist). Since this height varies from place to place, the 500 mb level is really an undulating surface, perhaps resembling the swells on the sea, if you could see it. Map it out and you have a contour map, very similar to a topo map you might take hiking. Here’s the forecast 500 mb map for Alaska and surrounding oceans valid 4 am Sunday:

At the surface of the earth the pressure is around 1000 mb (or one bar, or atmosphere), so the 500 mb level is about 1/2 way up the atmosphere in terms of pressure or mass. in other words, half the air is below 500 mb and half is above. The actual height is another story. Air pressure does not decrease steadily as you go up in the atmosphere, it decreases logarithmically…the decrease in pressure happen quickly at first then gradually slows. The 500 mb level is  roughly 5,500 meters up (18,000 ft). The contour lines are labeled as to their individual heights.

What does this map tell us?

In general a 500 mb chart, or any upper level chart, shows the pressure pattern, and the wind flow for the valid time, whether that be the time of the actual measurements for a diagnostic chart, or the time of the forecast for a prognostic chart (prog for short). More interesting is what this upper level flow means for the weather down here on terra firma. But first a little more info way up high. The map above does not explicitly show the winds, but they can be deduced from the contours. Upper level winds flow pretty much parallel to the contours, counter-clockwise (northern hemisphere) around lows–an area of low heights on upper level charts being the same thing as an area of low pressure, for all practical purposes. The wind speed is proportional to how close together the contour lines are. In this map there are two lows, one big one in the upper left in the far western Bering Sea, and a smaller one half off the bottom of the frame. If you connected these two with a line, you’d be describing the axis of a larger, more general feature, a trough. To the east, or downstream in the wind flow, the contours bend up (north) then curve back south to describe a ridge. The strongest winds are around the west and south sides of the larger low, over the Kamchatka Peninsula and south of the Aleutian Islands, whereas the Alaska Peninsula is in a slower wind area, for instance.

What does it tell us about our weather?

Unless you are up in an airplane, where you will rely on upper air charts directly, what these maps mean for surface weather is of primary interest. The 500 mb chart is good for showing where surface lows and highs will develop and where and how fast they will move. Pretty useful, I’d say. Here’s the super condensed version of how this works: The area from a trough axis downstream to the following ridge is an area favoring development of low pressure centers and fronts at the surface. The area under and downstream from a ridge is an area favoring high pressure at the surface. Remember, entire books are devoted to this interconnection of the upper and lower layers of swirling air, the dynamics of the atmosphere, so what I’ve just let on could be the mother of all simplifications. Many 500 mb charts, this one included, show an additional parameter in an effort to reveal the dynamical processes at work. It the the vorticity field, and for now lets just say the higher vorticity values (shown in the color scheme as tending toward red), are where the dynamics are more favorable for low pressure development at the surface. This is also a big oversimplification, but I wanted to mention it since it is on the chart. Take a look at the 500 mb chart above and see if you can envision the areas where surface lows and highs might want to be, then look at the computer’s vision on the surface prog for the same time, from the same model:

The serious action at the surface is in the eastern Bering sea under and a little downstream (east) of the 500 mb tough and low. The smaller low at 500 mb (the one near the bottom of the map) also has a surface low counterpart a little downstream. But now look at the 500 mb ridge which has its axis a ways offshore of Southeast Alaska and continuing northwest through roughly Prince William Sound and the northwest arctic coast. On the surface there is high pressure a ways downstream from it, centered over Yakutat at the neck of the panhandle. There’s dynamics at work.

What does it all have to do with the Northern Lights?

Actually, the atmospheric patterns aloft or at the surface have nothing to do with the aurora (or northern lights). But they have everything to do with whether you will be able to see the potentially strong aurora likely to be happening over the next couple of days at the very upper edge of the atmosphere, roughly 30 times higher than the 500 mb level. The 500 mb ridge is nicely situated to give Southeast Alaska the best look at any auroral activity. The southcentral region may get some breaks, but it is not as well situated as the panhandle, were the offshore flow should make for talk-of-the-town good weather. The eastern interior, which has had its share of nice weather recently and is drier by nature won’t even do as well as the panhandle, as the high will probably not be strong enough to clear things up completely. For the panhandle, Sunday should be the best day, for the other areas, not so much difference between Saturday and Sunday. The western half of the state. on the other hand, things will be gong downhill fast as the dynamics of an upper trough come into play. Lots of moisture and even more wind are headed for the west coast and the Aleutian chain. The NWS has high surf advisories out already for many west coast areas.

You can check for yourself, not only the 500 mb chart, but all the levels, plus the latest info on solar activity that might spark a good aurora. Here are some links:

Upper air:
http://pafc.arh.noaa.gov/gfemodel/index.php
http://weather.uwyo.edu/upperair/uamap.html

Aurora:
http://www.swpc.noaa.gov/forecast.html
http://www.spaceweather.com/
http://www.gi.alaska.edu/AuroraForecast

Please let me know if you see the aurora, a coastal storm, or something in between. Use the comments link below for your report or any questions or comments you might have.

Fairbankskans and others in the Alaskan Interior have been enjoying plenty of sun and warm afternoons for the past week. The NWS office there even put out a statement about it, as if to rub it in to the rest of us who have been layering on the sweaters, if not rain coats, lately.

PUBLIC INFORMATION STATEMENT…CORRECTED
NATIONAL WEATHER SERVICE FAIRBANKS AK
316 AM AKDT TUE AUG 30 2011

CORRECTED THE HIGH TEMPERATURE AT FAIRBANKS

…THE UNSEASONABLY WARM WEATHER CONTINUES IN THE INTERIOR…

A RIDGE OF HIGH PRESSURE ALOFT HAS BEEN RESPONSIBLE FOR THE
WARM WEATHER DURING THE LAST SEVERAL DAYS ACROSS MUCH OF THE
INTERIOR. MONDAY WAS THE WARMEST DAY OF THE BUNCH IN MANY AREAS.
HERE ARE SOME HIGH TEMPERATURES THAT WERE OBSERVED MONDAY AFTERNOON:

DENALI NP HEADQUARTERS….74
FAIRBANKS INTERNATIONAL…72
NORTHWAY………………71
FORT WAINWRIGHT………..70
NENANA………………..70
TANANA………………..69
EIELSON AFB……………69
EAGLE…………………69
DELTA JUNCTION…………68
FORT YUKON…………….68

Only the southern end of the SE Panhandle could get near the interior’s highs today. Nothing record breaking, but very nice nonetheless. What is not mentioned in the statement is the low temperatures, which are getting downright chilly. This is no surprise, as the same weather pattern which allows the solar heating to warm the afternoons also allows the radiational cooling to cool the nights and early mornings. I covered this topic more thoroughly in this post. So it is also no surprise that the state’s high for today and low for last night are only a stone’s throw apart (Alaskan speaking) at Northway (68 F/20 C) and Eagle (31 F/-1 C). Eagle was 70 F/21 C yesterday.

Now take a look at the Fairbanks Airport temperature trace for August (shy a few hours):
(click for larger version)

The small magenta spikes near the bottom of the chart show precipitation (rain) which is a good indicator of the kind of heavy cloud cover which moderates the temperatures, keeping the diurnal range low. Most of the middle of the month was in this mode, with a few days of relief interspersed. The first five days and the last week were dry and more clear, allowing the large swings in temperature. This is nothing new to many, and as I mentioned, nothing new to this blog (here’s the other article).

What you might not have thought about, however, is how the daily average temperature records can mask the differences between these two regimes. I’m talking about the human experience…how nice it is or was for outdoor activities. To get this story from the weather records, the  daily average temperature downplays what you and I experience as the day’s weather. The recent interior warmth is a good example. During the past week it was so nice that the NWS folks made official acknowledgement. Sunny, warm, pleasant weather with the highs about 6-9 degrees F warmer than the climatological average. However, since the lows dropped lower with the clear weather (they were fairly close to the climatological average or “normal” in NWS lingo), the daily average [(high+low)/2)] was only 3-6 degrees warmer than usual. So when considering how the weather affects daily life, looking at high temperatures makes more sense to me than the day’s average temperature so commonly given in climatological reports. However, in the midst of the winter, I think low temperatures are the more important factor for the human experience. I’ll save that explanation for another post. What do you think…of which measure to use and whether those Fairbankskans deserve this good weather?