Big Changes Underway

Dramatically warmer conditions are on their way north and will bring major relief from western and interior Alaska's deep freeze in the next couple of days.  Huge temperature rises are expected as temperatures recover all the way up to or even above the freezing mark in places.  Strong winds and blizzard conditions will occur in the mountains, heavy snows will be widespread outside of the chinook zones, and unfortunately it will be so warm that some places see freezing rain and ice accretion.  If it's not one thing, it's another when it comes to Alaska's winter climate.

As if to put an exclamation point on the recent cold spell, Tanana reached -58°F yesterday.  Meanwhile, the cold has been unrelenting in the past week in southwestern Alaska; today is the 9th straight day with a daily high temperature below -10°F in Bethel, and that hasn't happened since January 2012 (Bethel's coldest month on record).

In Fairbanks, today is the 32nd consecutive day with a daily high temperature below 0°F, and that's a record for the NWS/Weather Bureau era.  However, a longer stretch of 41 such days was measured at the University's Experiment Farm in 1917-18 (Nov 24 - Jan 3).  December 1917 had an overall average temperature of -33°F at the farm, more than 10°F colder than last month.

Looking back at last month as a whole, it was the coldest December statewide since 1980, according to NOAA/NCEI, and it was the first calendar month to be more than 10°F below the 1991-2020 normal since January 2020.  However, the cold was only really anomalous from the eastern interior to northern Southeast Alaska.  Here's one perspective based on ERA5 data, showing some areas within the coldest 5% of Decembers since 1950.

NCEI data shows the interior divisions in the coldest 10% since 1925, and the central and northern Panhandle divisions were the third coldest on record, trailing only 1964 and 1933 (the latter being by far the coldest).

Here's the culprit for the cold: a very strong and persistent ridge over the Bering Sea, as illustrated by the mid-atmosphere pressure (500mb height) anomaly.

Compare the monthly mean 500mb height (below, top) to the long-term normal (below, bottom).  Under normal circumstances there is a trough over the Bering Sea and the mid-atmosphere flow is directed from the southwest, preventing excessive cold in Alaska.  Last month the flow was mainly from the north and northwest, and Arctic and Siberian air was imported across eastern Alaska.  Moreover, the pattern produced relatively clear skies, allowing for ample cooling during the mid-winter dark.

It's a far-off memory now, but much of southern Alaska from Yakutat westward was drier than normal in December.  Southeast was much wetter, with Juneau northward seeing extreme snows at the end of the month.

Nobody in South-Central will be surprised to learn that the month was also much windier than normal for most of southern Alaska, owing to powerful episodes of cold flow from the north.  It's an interesting fact that windy Decembers across southern Alaska are more typically very warm across the state, with strong westerly or southerly flow; but last month was more akin to December 2012 or December 1964, with strong northerly winds instead.

Looking farther afield at sea surface temperatures across the North Pacific, it's interesting to see that there is still a lot of unusual warmth to the south of Alaska as well as in the southern and western Bering Sea.

However, the North Pacific basin as a whole has certainly cooled off quite a bit (relative to normal) since summer.  The following post from mid-November noted the cooling trend emerging during autumn, and at that time I speculated:

"Given that La Niña will prevail during at least the first part of this winter, the disappearance of the positive NPM [i.e. cooler North Pacific] probably means a greater chance of cold in Alaska..."  

https://ak-wx.blogspot.com/2025/11/autumn-climate-data.html

It's going to be interesting to see whether the second half of winter takes on a very different character, or if further cold episodes develop even as La Niña begins to weaken.