The Pendulum Swings

In the world of weather, extremes tend to beget extremes, and sometimes in the same place after only a short interval.  Alaska has seen a wholesale reversal from the cold of late January and early February to very unusual warmth in the past few days.

Wednesday was quite extreme, with chinook winds producing widespread 40s and some 50s downstream of the Alaska Range:

Nenana reached 52°F, the highest temperature in February since 1943.  Only February 24, 1943, had a higher temperature there in February: 54°F, and on that same day in 1943 Fairbanks reached 50°F, its record high for the month.

The Fairbanks sounding on Wednesday afternoon reported a remarkable temperature of 9.4°C about 1500 feet above ground.  There are only a few historical events with a temperature that high at any level in a February sounding from Fairbanks, the most recent being in 2010.  It's not surprising then that the Little Chena Ridge SNOTEL site above Fairbanks (2000' elevation) reached 50°F, and Teuchet Creek (1640') reached 49°F.

There'll be no prizes for guessing the mid-atmosphere flow pattern giving rise to the heat: a ridge over southeastern Alaska, with strong southerly flow coming up all the way from the sub-tropics.  Here are two different views of the 500mb height (pressure) pattern on Wednesday.

The deep trough far to the south of western Alaska was a key feature here, because it allowed that southerly flow to tap into very warm air from a low latitude.  That deep trough in turn was connected to a strong west-east jet stream across the central Pacific to the north of Hawaii: winds in excess of 80 m/s, or 180mph.  And this feature - an enhanced jet stream heading east into the southern US states - is very typical of El Niño.  So you can thank (or blame) El Niño for the big warm-up in Alaska.