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.”