Coldest yet for Fairbanks

The latest round of severe cold has set up a bit farther west, with the lowest temperatures today over central Alaska rather than far eastern Alaska and the Yukon Territory.  Most of the Fairbanks area dropped well into the -40s, and Eielson AFB saw -53°F.  Ice fog formed in the valley, as illustrated by this panoramic webcam view from the university's West Ridge (click to enlarge):

It will be interesting to see if the temperature dips to -50°F at the airport before a modest warming trend ensues.  The negative half-century mark has become quite rare in the official climate record from Fairbanks:

There's little doubt the decline in the most severe cold is attributable both to the local urban warming trend and the background (regional/global) warming trend.  Other, much less developed interior sites also show a decline, but less overwhelming.

Deep Cold

Today's high temperature (so far) of -39°F -38°F at Fairbanks airport is the coldest in 7 years, and it's the coldest February day since 1999.  Since 1930, only two years have seen a daily high temperature this cold in Fairbanks in February: 1947 and 1999.  It also happened in 1910, according to the Ag Farm data.

As for the low temperature, -49°F this morning was also the coldest in Fairbanks since 2017; but there's a chance -50°F could be reached before midnight, with the 5pm temperature already back down to -45°F.  [Update: it was reached]

Unfortunately there's no sounding data to look at the vertical profile of temperature, because the automated balloon launcher is disabled when the temperature reaches -40°.  This is a great pity, in my view, and a loss to science.  If the automated launcher can't operate, then perhaps some hardy soul could be persuaded to launch a balloon manually, as in the old days - like 7 years ago.  Here's the sounding from 3pm on January 18, 2017, the last date Fairbanks saw a high temperature below -40°F (the lowest levels aren't included on the plot, but the data was recorded.)

The 2017 cold snap was much shorter than the current one, but was briefly severe, with more widespread -50s than any day in the current cold spell.  There's a series of posts in the blog archives, e.g.

https://ak-wx.blogspot.com/2017/01/cold-snap.html

Here's a splendid photo published to Twitter today by Cody Moore, showing ice fog filling the valley from a vantage point on UAF's West Ridge:

https://twitter.com/codymoorewx/status/1753549093471998281

Visibility at the airport has been 1/4 - 1/2 mile for most of the day.  Water vapor emissions from the urban environment are to blame for the fog, with a substantial contribution from the power plant plumes; here's another photo, this time from UAF GINA:

https://twitter.com/uafgina/status/1753493360315437259

The FAA webcam on Ester Dome provides another perspective on just how localized the fog is: skies were gloriously clear across most of the interior today.

A simple animation gives a sense of the moisture flowing from the power plants at left towards the Tanana River valley at right.

Cold Snap

Clearing skies allowed temperatures to drop to very low levels in most parts of the interior in the past 48 hours, with Fairbanks airport reaching -51°F yesterday morning.  The high temperature for the midnight-to-midnight period was only -41°F.  These are the coldest conditions in Fairbanks since late January 2012.  Many other locations, distributed very widely across the interior, dropped well into the -50s, with the "winner" being the Kanuti Lake SCAN site at -61°F as of 5am today.

The following map, courtesy of NOAA/NWS, shows reported low temperatures below -50°F in the 24 hours ending around 5am today.

As we noted before, the deep trough over Alaska has imported very cold air aloft, so the low surface temperatures are entirely to be expected under clear skies with very little solar insolation and light winds.  The Fairbanks soundings have measured 850mb temperatures around -20°F in the past couple of days, and it's not unusual to see an inversion of 30°F or more from the surface to 850mb at this time of year.  Under the right circumstances the inversion can reach 40°F or more, but that's more common under high pressure.  The sea-level pressure (MSLP) has actually been considerably below normal in the past several days.

The relationship between MSLP and temperature inversions is illustrated by the pair of charts below, which show the 850mb and surface temperatures on high-MSLP days (top chart) and low-MSLP days (bottom chart) according to the winter-season Fairbanks balloon soundings since 1981.  There is considerably more scatter in the relationship when MSLP is high, because the surface conditions are more decoupled from the free atmosphere, and accordingly strong inversions are more common; notice that there are quite a number of days in the top chart with surface temperature more than 40°F colder than 850mb temperature.  In contrast, this almost never happens when MSLP is low, and it's not coincidental that surface temperatures almost never get below -40°F when MSLP is low.  Interestingly, yesterday had one of the lowest MSLP readings on record for a -40°F surface temperature in the Fairbanks sounding, and this attests to the strength of the upper-level trough.

Here's the 500mb analysis from yesterday morning, courtesy of Environment Canada:

Finally, here are some interesting FAA webcam photos of shallow ice fog layers yesterday at Chalkyitsik, Arctic Village, and Livengood.