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.

Let’s look at the next level — Why does Southeast Alaska get these warm spells and what’s causing this one? The answer to this question is pretty straightforward, but still interesting. Low pressure systems that track north or northeastward off the coast of the panhandle enhance the warming influence of the Pacific Ocean by pumping warm moist southern air over the region. That pattern has been holding without the reverse setting up (an offshore flow bringing cold air off the continent). Between weather systems there are often breaks with clearing skies and light winds which allow for nighttime cooling, but also solar heating during the day. These breaks usually result in very nice weather at this time of year. Here are some weather charts that show what I’m talking about, both at the surface and aloft, where the real power to move the weather lies.

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.”

The Taku Blows

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