Why So Cold?

First, before discussing again the extreme interior cold, I would be remiss not to mention the remarkable snowfall in northern Southeast Alaska during the final days of December.  Driven by a persistent influx of moisture aloft, and a very cold air mass locked in at the surface by high pressure to the north, extreme snow accumulations occurred from Juneau northward.  Juneau airport measured a snow depth of 50 inches on the 31st, easily the highest on record (data back to 1949).

Reported multi-day totals in late December were widely near or above 40" at sea-level locations: click to enlarge the following.

Rick Thoman has lots more detail in his post on the event:

https://alaskaclimate.substack.com/p/southeast-alaska-extreme-snowfall

Now back to the headline topic: the generational cold snap that will come to at least a temporary end tomorrow for the central and eastern interior.  Yesterday's high temperature of -44°F in Fairbanks, and the daily mean temperature of -46.5°F, were the lowest values since New Year's Eve 1999; it has been 26 years since this intensity of cold occurred in Fairbanks.  Today's high temperature may be as low as -46°F, the coldest since December 13, 1977.  [Update: it was.]

Here's another remarkable statistic: the 9 straight days (and counting) with a low temperature of -46°F or lower hasn't occurred since December 1975 in Fairbanks.  If it's still -46°F at midnight tonight, we'll have to look back to December 1961 to find an equally long streak of 10 such days.  [Update: the streak reached 10 days.]

Of course, there have been other harsh cold spells since 1975, but none with quite the sustained intensity of this one.  January 1989 is top of the list, with 12 days having lows below -45°F, but the streak was broken by one slightly milder day in the middle (-40°).  The New Year's cold snap of 1999-2000 saw 7 days with -45°F or lower; and early January 2009 produced 8 such days.

What do all these cold snaps have in common (1961, 1975, 1989, 1999, 2009)?  Like this winter, all occurred during La Niña in the tropical Pacific.  La Niña hasn't always produced extreme cold in Alaska, especially in recent years, but extreme cold is much less likely without it.

The more immediate cause of the astonishing cold spell is a sustained confluence of the ingredients that are required for deep cold in interior Alaska, i.e. a cold air mass, clear skies, minimal solar heating, and calm winds.

The first ingredient is the starting point: the circulation pattern has persistently transported cold air from the north and northwest into Alaska, and this pattern has involved a ridge (high pressure) over the Bering Sea and a trough (low pressure) over eastern Alaska and northwestern Canada.

Here are a couple of simple animations to illustrate the flow pattern in the last week: 500mb height (i.e. mid-atmosphere pressure) in the top animation, and 850mb temperature anomaly (about 4000 feet above sea level) in the lower.  Notice the persistent ridge over the Aleutians and Bering Sea, with northerly or northwesterly flow over central Alaska.

The following figure shows the resulting temperatures in and above Fairbanks: the black line shows 850mb temperature, and the blue line shows surface temperature.

The feature to notice here is that while temperatures aloft (850mb) have certainly been cold in the past 10 days, they haven't been that much colder than earlier in December; but surface temperatures have bottomed out, reflecting a strong and sustained temperature inversion.  This occurs when heat energy is persistently lost to space under clear skies (with no real solar heating by day), and when calm winds prevent less-cold air aloft from mixing with the severely cold surface layer.  Yesterday the Fairbanks airport reported zero wind for well over 24 hours, and the sky was clear above the ground-level ice fog, so there was nothing to disturb the cooling process or provide a warming influence.

This exact confluence of circumstances is obviously rare, particularly in terms of the longevity of the event.  And thinking in terms of the ingredients for cold, it seems the key aspect that has been especially unusual and has allowed this event to become so severe is the absence of wind since December 27.  The cold air aloft has not been particularly unusual or extreme; clear skies are not especially rare in Fairbanks; but typically the active jet stream pattern that brings deep cold to Alaska also tends to produce significant pressure gradients across the state, leading to episodes of wind that preclude or break up a strong inversion.  In other words, I suggest that prolonged calm is unusual when the circulation pattern favors cold.  An example (admittedly extreme) of a more typical style of cold was the northerly blast about a month ago:

https://ak-wx.blogspot.com/2025/12/cold-outbreak.html

It will be worth digging into this idea further in a future post: what distinguishes a "normal" cold pattern from one that includes strong inversions and therefore severe valley-level cold?  And how did the pattern manage to persist so long in this case?

These and other questions will be fuel for fruitful comment once all the data is available for a full post-mortem.