Chinook Winds

A wave of warmth has inundated Alaska in the last few days, and chinook winds have amplified the warming in much of the interior.  On Wednesday temperatures rose above freezing at locations spanning the state from west to east, including Nome, Kotzebue, Bettles, Fairbanks, and Eagle.  There have only been 10 previous days since 1952 (the common period of record) when all these stations made it above freezing on the same day in December through February; 6 of these 10 days have occurred since 2009.

The only portion of the interior that has stayed well below freezing is the far southeast, where the proximity of the upper ridge prevented winds from mixing down the warmth aloft; Northway has not exceeded 14°F.

Fairbanks airport reached 45°F on Wednesday, as the southerly chinook winds managed to break through the surface-based inversion for just a brief time in the afternoon; the temperature rose from 34°F at 1pm to 45°F at 2pm, and then dropped back to 30°F at 3pm and to 15°F by mid-evening as the stagnant colder air rolled back in.  A more extended period of gusty warm winds yesterday afternoon took the temperature back up to 40°F with a bit of mixed rain and snow.

Wednesday's fluctuations in temperature and wind speed in Fairbanks are evident in the half-hourly observations from UAF's Smith Lake site, see below.  The second temperature spike, with the larger wind speed spike, is the one that produced the high temperature at the airport, but at Smith Lake the temperature jump occurred between 12 and 12:30 (presumably AKST), i.e. at least 30 minutes earlier than at the airport.  The elevation difference between the two sites is only about 100 feet, but this might explain the delay in warming at the airport as the wind burst took time to mix down.

The relative humidity plot from Smith Lake shows the low humidity of the chinook air - see below.  The humidity fluctuations nicely highlight the contrast between the two air masses, one of them cold, humid, dense, and stagnant, and the other warm, dry, less dense, and moving quickly northward.  The density contrast between two such air masses is so great that they might be thought of as immiscible, like oil and water: they simply do not mix.  The warm air tends to slide over the cold air, and in the Fairbanks area the chinook flow doesn't often make it to the valley floor unless the pressure gradient is large or winds become unusually strong.

Similar charts from the Poker Flat Research Range, about 30 miles north of Fairbanks at ~700' elevation on the Chatanika River, show that temperatures were above freezing for two lengthy periods on Wednesday - see below.  The early afternoon spike in wind speed was also observed at this location (note that the time axis appears to be UTC), but the temperature rise was more gradual as there was less stagnant, stable cold air to displace at the higher elevation.