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.

More Snow and Cold

Back on December 18, I commented that the extreme cold then affecting northwestern Canada and far eastern Alaska looked likely to shift westward in the coming weeks, and that trend has played out nicely.  Not only had Fairbanks "not seen the worst of it yet", but severe cold has now developed in southwestern and south-central Alaska.

This morning was remarkably cold for many locations across the south and southwest; here's a selection of minimum temperatures since midnight last night (click to enlarge):

The -33°F in Bethel and the -12°F in Homer are both the coldest since 2012, and the astonishing -35°F in Kenai is the coldest since February 1999.  Today's high temperature in Homer has been only +1°F, which is also the lowest since 1999.

Anchorage airport (-17°F this morning) was colder (-21°F) in 2024, but the high temperature so far today is only -6°F.  January 7, 2009 was the only day this century with a colder maximum temperature (-9°F).

As for snowfall, Anchorage received a hefty dump on Monday and Tuesday this week, with 19 inches in the two days.  This is the highest 2-day snowfall since March 2002, and the 5th highest on record (data back to 1953).  It's also a record for highest 2-day snowfall in January, the previous record being just 2 years ago; prior to that the greatest 2-day total in January was only 13.7" (in 1987).

The combination of cold and snow in the past week is extremely anomalous for Anchorage.  For instance, the temperature rose to only 18°F during the snow storm earlier this week, and that makes it the largest 2-day snow with a 2-day high temperature below 20°F; the previous was 16.6" with a high of 14°F, again in January of 2024.

Here's a scatterplot of weekly temperature and snowfall for Anchorage in winter; I've highlighted the latest week and the very analogous week in 2024.

Anchorage has had weeks in the past with either more snow or lower temperatures, but the combination of cold and snow right now is virtually outside the bounds of the historical climate, just as it was 2 years ago.

The interesting thing about the 2024/2026 similarity is that 2023-24 was a strong El Niño winter, but this winter we're in a La Niña.  As I've commented before, this winter resembles a "traditional" La Niña for Alaska, whereas 2023-24 was quite atypical for El Niño.

CPC's latest 6-10 day forecast indicates that the cold is likely to continue shifting westward in the coming days, and Southeast will likely continue to warm up as the flow orientation changes.  There's a lot of uncertainty after the middle of the month, however, and I wouldn't be at all surprised to see a new round of cold across northwestern Canada for the second half of January.

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.

Serious Cold

I'm on the road and away from my usual data resources at present, but a quick post is in order to highlight the remarkable National Weather Service forecast for the next few days in Fairbanks:

This morning's NWS forecast for the international airport calls for daily high temperatures below -40° from today through Sunday, i.e. 5 days in row.  This hasn't happened since the intense New Year's cold snap at the turn of the century (Dec 30, 1999 through Jan 3, 2000).

The anomaly is even more striking in comparison to the last 15 years: there have only been 2 individual days with a high temperature of -40° or below at the airport since January 2009.  Early January 2009 saw 4 straight days with a high of -40° or -41°F.

The Fairbanks record for consecutive days below -40°F is 9 days in December 1961.

And here's the forecast for Fort Yukon: chilling out below -50°F for the next several days.

Looking farther ahead, there's a distinct glimmer of hope for significant relief in the second week of January:

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.

Sea Ice Update

The extreme cold over southern Yukon is making headlines at the moment - and deservedly so, given how persistent it is - but for balanced perspective it's also worth considering the remarkable warmth in the Arctic in recent months.  Rick Thoman notes in his latest post that Arctic sea ice extent is running at a record low for the time of year, as it has been since late November:

https://alaskaclimate.substack.com/p/winter-solstice-2025-arctic-sea-ice

Zack Labe's splendid graphic highlights the anomaly:

The largest deficits are found to the east and north of Svalbard, where ice-free waters extend to nearly 85°N, according to the NSIDC analysis.  This is pretty remarkable in late December:

It might be counter-intuitive given the preponderance of cold in Alaska, but Bering Sea ice is also below normal, mostly because of ice deficits in the central and western Bering Sea.  There is pretty good ice cover to the east of St Lawrence Island:

The warm anomaly in the Arctic has been ongoing since the height of summer.  Of course most recent years have been warmer than earlier decades, but the latter half of 2025 has been particularly unusual:

Extended Cold

My posts this month are taking on a particular theme - cold - as the ongoing cold spell is starting to make this winter feel like an "old fashioned" La Niña winter for eastern and southeastern parts of the state.  (See this post for comments on cold spells in La Niña versus El Niño winters.)

The cold has been most intense for the southeastern interior, where Northway is having its coldest December to-date since 1980, and that's with the first 4 days of the month being very warm.  The contrast with the very persistently and unusually warm autumn is striking:

Remarkably, Northway saw -50°F on 4 consecutive days from the 8th through the 11th, and 1980 was also the last time that happened before the middle of the month.  The only other Decembers this century with -50°F in Northway at any time in the month were 2012 and 2022.

The state's lowest measurement this month came from Chicken, of course: -56°F on the 9th.  With data back to 1997, that's the second earliest date for such cold; the earliest was December 1, 2012.  So far this month, the average temperature in Chicken is below -32°F, and that's on track for the coldest calendar month on record (again, during the short period of record).  There have only been three Januarys with an average temperature below -30°F in Chicken: 2004, 2012, and 2020.

It's really only the far eastern interior that has been extremely cold, as the core of the cold air has been in northwestern Canada, transported southward on the east side of a very strong ridge over the Bering Sea.  The two following figures show the average 850mb temperature departure from normal and the average 500mb height since December 5, when it turned cold:

It's interesting to note that the persistence of this pattern - and it shows no real signs of stopping - was successfully anticipated by long-range forecast models.  My post on December 6 highlighted this, and the latest forecast update shows much the same signal (above-normal 500mb heights near the Date Line) for several more weeks (refer to the bottom of the post for an explanation of this graphic):

If the model (in this case the ECMWF subseasonal model) is correct, the cold will shift westward slightly in the coming weeks, implying that Fairbanks may not have seen the worst of it yet.  But of course the model could well be wrong.

Note on interpreting these figures: the format takes a bit of getting used to.  The idea is to show the progression of anomalies (departures from normal) from top to bottom in time, with longitude varying from left to right; refer to the map cutout at the bottom for a visual reference of longitude.  The values on the chart are averages from 50-80°N.  The top part of the figure (labeled as "ANALYSIS" on the right) is the progression that has already happened, in this case the 30 days prior to December 18.  The lower part of the figure (labeled as "FORECAST" on the right) is the predicted progression from the ECMWF 46-day model, extending out to February 1st in this case.

The last figure shows temperature anomalies and indicates that below-normal temperatures are expected to persist well into January over Alaska and western Canada, and there's a hint of westward progression in the core of the cold.  The previous figure shows 500mb height anomalies (i.e. pressure in the middle atmosphere), and the persistent orange shading near 180°W is the Bering Sea ridge.