Haines High School students throw around a football in the school parking lot on 18 Feb.

After a fairly seasonable winter through the end of 2009 in Southeast Alaska, 2010 soon broke into what many old-timers call a “January thaw” that seems to want to be known as a “January-February thaw.” Ketchikan has been warm and wet, double the precipitation but ZERO snowfall in January. Juneau has had about 50 inches so far this winter (1/3 last year’s running total) but it has melted to bare ground yet again. Here in Haines we’ve had above average snow on the ground but people are complaining because it’s wet, rather than the mostly dry stuff of the last three winters. (OK we’re spoiled). What is going on here? There are answers on several levels…I’ll try to hit three:

Why #1 –compared to what?

Above freezing temperatures have never been too rare, even in mid-winter, in SE AK (sometimes referfed to as the banana belt of Alaska). In 1977 Juneau had 29 consecutive days with highs 40F (5C) or higher starting on Jan 30. In January, Juneau averages about 16 days where the high breaks 32F (0C) and 19 in February. Roughly 2/3 of the panhandle is warmer yet. But since the past several winters have been fairly “wintery” with typical or colder temperatures and plenty of snow (record amounts in some cases), the contrast is hitting us. Summary: yes, it’s a long warm spell, and some records are falling, but it’s not an unprecedented or even all that unusual situation.

So why does SE AK get these warm spells and what’s causing this one? Stay tuned…that’s why #2 and we’ll explore that in the next post.

This chart show what’s happening up at the jet stream level, about 30,000 ft (10 km) up. The shaded areas highlight the strongest winds, and you can see the west-to-east flowing jet stream, with large dips and humps in its flow. There is a deep trough (dip) straight south of the middle of Alaska, with the eastern leg of that trough bringing warm air from close to Hawaii right to the northern end of the inside passage at over 100 kts (160 km/hr) in the jet core. You can see for yourself that the term pineapple express is not an exaggeration. Following the patterns downstream shows a large ridge over SE AK followed by a giant trough over the entire lower 48 sates, setting up some active winter weather for the east coast.

The next map is for the same time but shows the low level flow. Not right at the surface but at about 4,500 ft (1,500 m), the 850 mb level. This chart reflects what’s going on at the surface, as far as highs and lows, but is better for looking at low-level temperature, since it is mostly above localized surface influences that complicate the picture. Notice the dashed temperature lines (isotherms) which shows air near and above freezing circling around a double barreled low pressure system off the coast and into SE AK. (Remember, this is the temperature at 4,500 ft, so at the surface it is usually warmer in this kind of situation). Meanwhile a stronger low over Nova Scotia has no doubt dumped a bunch of snow on the northeast US and southeast Canada. A stronger-yet low is scouring the western Aleutian islands. Note how each of these lows is aligned under the downstream leg of a upper level trough on the first map.

When this kind of pattern—deep troughs in the jet stream throwing warm air up from nearly the tropics—sets in for a long visit, Southeast Alaska gets plenty of melt weather and on-again, off-again rain. The warm air hose can also whip itself west and warm up Southcentral Alaska (Anchorage and neighbors) and even the interior and more northerly locations (Fairbanks etc), though the effects are a little different. Now, some of you are thinking… “Why does this warm pattern set up like this,”or more pointedly, “Why is it staying this way, this long?” That is why #3, to be tackled in a third post on this topic.

The maps in this post are from the University of Wyoming’s excellent weather briefing site. To pull up maps like the ones above click on “Upper Air Observations” then “Upper Air Maps.”

A look at November 25 on the above graph (click on graph for larger version) seems to support the daytime high pattern mentioned in the opening sentence. However, notice how the temperature rise started before sunrise and started to fall well before sunset. On the 26th the temperature rises from midnight to sunrise, falls during the short daylight hours and rises again after sunset. Assumption blasted.

The physical principles that govern daily temperatures apply anywhere. But in Alaska some of these principles are often more pronounced, and others less. The term diurnal, by the way, is applied in meteorology to things that have some sort of a daily pattern or cycle. The pattern of daily temperature changes mentioned in the lead-in is caused by the sun, and is the true diurnal temperature cycle. Of course at this time of the year in Alaska the sun is pretty much AWOL, which means all the other factors which control the temperature have little trouble overpowering its influence. These include the advection (the moving in on the wind) of warmer or colder air from another area, mixing of warmer (usually) or colder air from layers above the surface, and radiational effects.

Radiation is what the sun is all about, and nothing beats it at this game when it is around. When the sun is down, the earth mostly loses heat through radiation, bringing on the cool half of the classic diurnal cycle. During the Alaska winter we have the radiational heat loss part down well and that is basically why it is so cold in the polar regions. But there is another source of warming radiation in the northern winter…clouds. Yes, clouds. And they don’t simply stop nighttime heat loss as is commonly known (the blanket effect). They do actually radiate heat down to the surface. (Everything radiates infrared heat…and receives radiation from other things. It is when the radiational balance between masses is lopsided that strong warming or cooling takes place.) The clouds simply need to be in a warmer stratum of air than the surface. This is almost always the case in the interior and north and west coast winter, much less so in south coastal areas.

In the Fairbanks example, it is the mixing effect of the wind that is the most pronounced. Even the very slight wind of 3-5 knots (4-6 mph) is enough to stir some warmer air down to the surface in the otherwise calm air, causing a temperature jump.

The cloud cover (column labeled sky conditions in the hourly text reports below) shows a less convincing correlation with the temperature in this case, although the automatic cloud sensing equipment may not be good enough for this kind of scrutiny. In other cases it can be a dominant factor.

Wind can pick up and clouds can move in or out anytime of the day or night, affecting the temperature and causing a fairly large temperature range on the high and low record for the day, but it is not a true diurnal cycle.

Here are the hourly observations from Fairbanks covering roughly the same time as the graph above. The time column is in Alaska Standard Time. Temperature (labeled temp) is shown in Fahrenheit. The wind column gives the direction in the first 3 digits and speed in the last two (in knots).

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak
PAFA  AA 25 1153  FEW080 SCT100 BKN200      10           2  -2 00000     936  83%   2
PAFA  AA 25 1253  FEW080 SCT100 BKN200      10           5   1 20003     936  83%   5
PAFA  AA 25 1353  SCT100 SCT200             10           2  -1 00000     935  87%   2
PAFA  AA 25 1453  SCT100 SCT200             10           0  -4 00000     933  83%   0
PAFA  AA 25 1553  SCT100 SCT200             10           0  -4 19005     932  83% -12
PAFA  AA 25 1653  FEW100 SCT200             10          -2  -7 00000     932  79%  -2
PAFA  AA 25 1753  FEW100 SCT200             10          -8 -12 00000     932  82%  -8
PAFA  AA 25 1853  FEW100 SCT200             10          -9 -13 00000     932  82%  -9
PAFA  AA 25 1953  FEW020 SCT100             10          -8 -13 00000     931  78%  -8
PAFA  AA 25 2053  FEW020 SCT100 SCT200      10          -8 -13 00000     929  78%  -8
PAFA  AA 25 2153  FEW020 SCT100 SCT200      10          -9 -13 00000     928  82%  -9
PAFA  AA 25 2253  FEW020 SCT100 SCT200      10         -10 -15 00000     926  78% -10
PAFA  AA 25 2353  FEW020                    10         -10 -15 00000     924  78% -10
PAFA  AA 26 0053  CLR                       10         -10 -15 04003     922  78% -10
PAFA  AA 26 0153  CLR                       10          -9 -14 04003     920  78%  -9
PAFA  AA 26 0253  FEW090 SCT200             10          -5  -9 04003     918  82%  -5
PAFA  AA 26 0353  SCT100 SCT200             10           1  -4 05005     917  79% -10
PAFA  AA 26 0453  BKN100                    10           5   1 00000     917  83%   5
PAFA  AA 26 0553  FEW070 BKN100             10           5   1 21003     919  83%   5
PAFA  AA 26 0653  SCT090 BKN120             10           4   0 00003     920  83%   4
PAFA  AA 26 0753  SCT085 OVC110             10           3   0 00000     923  87%   3
PAFA  AA 26 0853  SCT080 BKN100             10           4   0 00000     925  83%   4
PAFA  AA 26 0953  FEW080 BKN100 BKN200      10           5   1 00000     928  83%   5
PAFA  AA 26 1053  SCT085 BKN120 BKN200      10           6   1 00000     931  79%   6
PAFA  AA 26 1153  FEW085 SCT120 BKN200      10           5   0 00000     932  79%   5
PAFA  AA 26 1253  FEW085 SCT120 BKN200      10           0  -3 00000     933  87%   0
PAFA  AA 26 1353  FEW085 SCT120 BKN200      10           1  -2 00000     934  87%   1
PAFA  AA 26 1453  FEW080 SCT130 BKN200      10          -2  -5 03003     932  87%  -2
PAFA  AA 26 1553  FEW080 SCT130 BKN200      10          -2  -6 00000     931  82%  -2
PAFA  AA 26 1653  FEW080 BKN130 BKN200      10           5  -6 00000     930  59%   5
PAFA  AA 26 1753  FEW080 BKN100 BKN200      10          10   2 00000     930  69%  10
PAFA  AA 26 1853  FEW080 BKN100 BKN200      10          10   0 00000     929  63%  10
PAFA  AA 26 1953  BKN100 BKN200             10          14   0 00000     930  53%  14
PAFA  AA 26 2053  BKN100 BKN200             10          13   2 00000     930  61%  13
PAFA  AA 26 2153  BKN110 OVC200             10          14   6 00000     930  70%  14
PAFA  AA 26 2253  BKN110 OVC200             10          17   3 02003     930  53%  17
PAFA  AA 26 2353  BKN095 OVC200             10          14   4 00000     929  64%  14
PAFA  AA 27 0053  BKN095 OVC200             10          18  10 00000     928  70%  18
PAFA  AA 27 0153  BKN075 OVC090             10          21  12 00000     926  68%  21

Taku Winds blow in Juneau

I’m in Juneau for the Alaska Math and Science Conference and enjoying the sun and the beautiful blue sky. Blue sky does not necessarily equate to “fair” weather here. The Taku winds were blowing yesterday (10/13) and probably still are, though weakening. The photo above looks east across the Channel at Downtown. It is a wide shot to show the layout of the land but you still can see the numerous vigorous whitecaps coming toward the camera (click on image for a larger version). I’ve posted some closer shots below the weather map and explanation.

Surface map from approximate time of photos.

On the map you can see a high pressure area well inland of Southeast Alaska and a healthy low off the coast of Vancouver Island. The strongest pressure gradient (indicated by where the pressure contours, or isobars, are packed the tightest) is right over Southeast Alaska. Taku winds are named after the Taku Inlet (a little south of Juneau) and famously ferocious there and in Juneau.

The temperature of the source air of these winds is a big factor in their strength. In winter, the air over the continent is usually colder than that over the ocean. Other factors aside for the moment, cold air is more dense than warm air, and gravity is going to take advantage of that fact. As the air is pulled off the continent by the low pressure area, the gravity is aiding its trip down the slopes of the coastal mountains. If the air over the ocean were relatively more dense, the down-moving air would not have as easy a time displacing it. This is more often the case with the Anchorage area Chugach Mountain winds. When the pressure gradient sets up to draw air over those mountains, the source air (usually coming off Prince William Sound) is usually warmer than the air at the base of the mountains (in Anchorage) it is trying to replace. When that happens the mountain winds blow over the top of the cold surface air.  In many cases, however, the up and down wave action set up by the mountains can eventually erode the cold air, allowing the winds to reach the surface.

Some more photos (click on photo to see larger version):

strong blast off mountainside racing across Gastineau Channel

This next shot was taken from the east side of the Channel (near the library) and show how the downdrafts hit the water and spread out in most if not all directions. A windsurfer out there would have to watch his back!

radiating downdraft gust on Gastineau Channel