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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?

No, the rain gauges are not malfunctioning, (although that has happened before at some weather stations). It has been very dry in the north end of the Inside Passage. There is a natural dry pocket in this rainforest to the point of really stretching the term rainforest. This year the first part of summer has been drier than usual over the region meaning the driest areas are getting pretty dusty. There have been drier years, but not many.

Here’s the precipitation data for the past three months in map form, then more completely in table form–arranged top-bottom in case you need some help in identifying what towns the numbers on the map belong to. Look at the large variation in a bit over a hundred mile radius, with the dry bulls eye over Skagway:

The “years” column is the number of years of data that went into the average and extremes.

What’s behind the dry spell/dry spot?

Check out this recent surface map, a very recurring pattern this summer. The large high over the North Pacific is a semi-permanent summer feature and is in fact officially called the North Pacific High. Strong highs like this are effective at keeping frontal systems from impacting Southeast with any strength or regularity. That front slicing down through the interior and Cook Inlet brought about a quarter of an inch of rain to Anchorage and other areas, but fell apart (as most have) while trying to rake across the top of the high, and the panhandle got almost no rain.

However, for coastal stations, highs do not mean all is dry and sunny, for they push the ample oceanic low clouds (commonly called the marine layer) onshore, bringing not only clouds, but a mix of fog and drizzle and a smattering of light rain as the moisture gets wrung out of the marine air by our abrupt coastal mountains. But rainfall will stay below average with this kind of pattern. Bet why are the northern inner channels so much drier? They are protected from the both the low marine layer and the orographic (mountain induced) precipitation by the first several ridges of the coastal mountains. In a sense Skagway and Haines are not very coastal, but a little more like the interior. Everything is in shades in the weather business.

What’s next?

Will the dry weather continue? Climatology indicates that things must start getting wetter fairly soon. Even in super-dry 2009 the rains came in August and September, playing a strong catch-up game in the area. Such is the flip-floppy nature of the weather in much of Alaska.

The computer models are not showing a major shift for the next week anyway. There is some weakening of the pattern and there will probably be a little more moisture than we’ve seen, but probably very little in the protected areas discussed above. Our gardens could use a lot more than I think we’re likely to get over the next week. The real weather is the continuing strong lows moving into the Bering Sea, an example of which is pictured in the surface chart above. The west coast is getting way more storms than they probably want. The longer range maps from the Climate Prediction Center are not wanting to deviate from climatology (in other words they’re not sticking their neck out for above or below normal precipitation), except for continued wetter than normal for the next two weeks for the southwest part of the state.

The big issue for Haines is the upcoming Southeast Alaska State Fair coming up in two weeks. Despite Murphy’s laws on rain, its too early to say whether the fair will be warm and sunny, cool and rainy or somewhere in between.

I’d love to hear any comments or questions on this article, or any weather topic. Click on “comments” below, and let me know whether your hoping for rain or sun where you live.

Alaska is a great place to celebrate Independence day, and historically, the weather is decent, more often then not.

This histogram shows the percentage of time each station received the listed amount of precipitation on July 4th over their period of weather records, 68 years minimum. Of these 4 metro (“Alaskan metro”) centers it is surprisingly Anchorage that comes out driest. Another surprise is that Juneau and Fairbanks are right about the same! I’d guess that Fairbanks’ rain is more showery and therefore more brief, allowing more sunshine than Juneau’s marine cloud layers. Barrow has more dry days than Fairbanks as well, but fewer days with more than a tenth of an inch…the kind of rain that can really soak a picnic.

What about this year?

The following computer forecast is for 10 am on the 4th and shows wind barbs at the 700 mb level (~3,000 m/~10,000 ft) and relative humidity through all levels.

There are three lows threatening to rain on our parades. One near Kaktovik in the northeast corner of the state, one over the Seward Peninsula between Kotzebue and Nome, and one offshore of Southeast Alaska. Each is feeding moisture via some strong onshore winds. The Seward Peninsula low will hit a little later, hopefully after the celebrations.  If its not raining it will certainly be cloudy in for the Panhandle, especially the southern parts. Similar for the north slope although leaning more towards low clouds, drizzle and fog. It does not look quite so bad for the the interior and Southcentral…clouds and perhaps some light rain or showers. Click here for a more detailed, up to date NWS forecast. Then let me know how your 4th was, weatherwise and otherwise.