Major Thaw

December was very warm in Alaska (relative to normal), but January is shaping up to be even warmer; records have been falling in the past week, and the Climate Prediction Center is showing very high probabilities of unusual warmth in the 6-10 and 8-14 day forecasts.  All the cold air has gone south:

Yesterday was extraordinary: widespread temperatures above 40°F in southern Alaska, and as high as 53°F at Nenana; this is only 1°F shy of the January record (54°F in 2009).  The Fairbanks area reached the mid to upper 40s.  Here are 24-hour maximum temperatures ending at midnight last night (note that the plotted 54°F at Nenana is a rounding error - it was 53°F):

For the second time this month, Anchorage saw a daily minimum temperature of 36°F yesterday, tying the January record both times.  But of more immediate concern were the high winds, which caused considerable disruption and damage across south-central Alaska:

https://www.adn.com/nation-world/2025/01/13/anchorage-schools-closed-due-to-weather-related-problems/

Wind speeds exceeding 50mph were widespread not only in the Anchorage bowl but across much of southern Alaska, with much higher gusts in places.  Here are the 24-hour peak gusts (mph) ending at midnight:

The cause of the wild weather is no big surprise: a very large trough over the Bering Sea and a strong low pressure system moving into the Y-K Delta region yesterday.  There was also a strong ridge over British Columbia, enhancing the east-west pressure gradient across southern Alaska.  Here are the surface and 500mb height charts from 3pm AKST yesterday, courtesy of Environment Canada:

December Climate Data

It always takes a week or so for complete climate monitoring data to become available for the previous month, but now we're in a position to look back at the main climate anomalies from last month and for 2024 as a whole.

December was an unusual month in terms of the circulation pattern over the North Pacific and Alaska: a strong mid-atmosphere trough was dominant across the North Pacific from Japan to the waters south of Alaska, but to the north a very broad ridge prevailed from Siberia to central Canada.

This pattern reflects a strongly positive phase of the Pacific-North American (PNA) pattern (see here for explanation), and this is reliably and strongly associated with unusual warmth in Alaska at this time of year.  Hence temperatures were well above normal nearly everywhere, and it was a top-10 warm December for the state as a whole.

The NCEI data shows the Northwest Gulf division with its warmest December on record, but Kodiak airport actually came in at 6th warmest, with December 2014 still holding the record.

The December pattern was substantially inverse of what is typical for La Niña; here's the average 500mb height anomaly for 10 Decembers with strong La Niña:

But of course, La Niña is not particularly strong this winter; it only recently gained enough amplitude to be officially classified as such by NOAA:

https://www.climate.gov/news-features/blogs/enso/january-2025-update-la-nina-here

December precipitation was a mixed bag.  The major anomaly was the wet pattern for much of the Gulf coast, and especially the lower Alaska Peninsula to Kodiak Island and the southern Kenai Peninsula.  Kodiak reported its 2nd wettest December on record.  Southeast was fairly wet too, a big change from autumn.

As for wind, ERA5 data shows a return to above-normal wind for western Alaska as well as the northern interior.

Here's the situation in terms of sea surface temperatures: still very warm from Japan to the region south of Aleutians, although the persistent trough produced a lot of cooling near and south of 40°N (and warming closer to Alaska):

Looking back at annual-average data for 2024, temperatures were modestly warmer than normal, as a result of 8 months above the 30-year normal and 4 months below (January, May, July, August).  December was the most anomalously warm month, although June was very unusual too.

According to NCEI, the statewide annual mean temperature was 28.9°F, which marks the 5th consecutive year below 29°F after the run of very warm years from 2014-2019.  It seems likely that this reprieve is related to the persistence of La Niña and the negative PDO phase in recent years; this winter is the 4th La Niña winter out of the last 5 years (although last winter was a strong El Niño).

2024 annual precipitation and annual-mean wind speed were more anomalous than the year's average temperature; ERA5 data shows much of western and northern Alaska with much above normal precipitation and wind speed.  NCEI data shows drier conditions in southwestern and south-central Alaska, but I'm not sure how accurate that is; Anchorage certainly had a wet year.  Both data sources show a notably dry year in the southern Alaska Panhandle.

Cold Snap

Clear and calm conditions allowed a few thermometers to register -50°F today, the first time that benchmark has been reached in Alaska this winter.  Umiat saw -52°F, locations on the Yukon Flats were very close to -50°F, and Chicken reached -51°F.

Only one winter in recent decades - 2015/16 - failed to see -50°F somewhere in Alaska at some point in the winter.  The last 3 winters all produced -60°F or lower at Chicken.

This morning's sea-level pressure analysis shows sprawling high pressure across northern Alaska and a strong anticyclone over the Arctic waters to the north.

As is typical under these circumstances, surface air temperatures are far lower than temperatures aloft.  The soundings below show the vertical temperature profile at Utqiaġvik and Fairbanks from this afternoon.

Snowpack

Happy New Year to all.  Let's check in on Alaska's snowpack at the turn of the year.  It's a mixed bag, but generally snowfall has been much less abundant than in the Fairbanks area (relative to normal), according to ERA5 estimates:

Fairbanks has 22" of snow on the ground, according to the official measurement, and that's towards the upper end of the historical range for the time of year - although there was much more (34") on New Year's Eve in 2021.  The SNOTEL sites in the hills above Fairbanks are reporting about 150-200% of normal snow water equivalent, and the Mt Ryan site (2800' elevation) has been at record high snowpack since late October (only recently dropping behind the 2021 trajectory):

In contrast, there is a real dearth of snow across the Seward Peninsula and across nearly all of southwestern and south-central Alaska.  We have to go back to 2016 to find so little snow in late December for southwestern Alaska; but in that case the shortfall was much greater statewide:

Anchorage has 8" of snow on the ground, a lot less than the last two years, and the least since 2019 on New Year's Eve.  A number of SNOTEL sites around South-Central are reporting well under 50% of normal SWE - see below.  Let's hope this situation doesn't worsen in the weeks and months ahead.

Howard Pass Again

It's time for another mention of northern Alaska's "pole of cold" for measured wind chill, i.e. Howard Pass.  As noted by reader Gary, conditions have been harsh up there in recent days.  In typical fashion, strong northerly winds funneled cold air through the pass (elevation near 2000 feet) with great ferocity starting last Saturday evening, and it wasn't until yesterday afternoon that the wind chill rose above -60°F for the first time in over 3 days.

The minimum wind chill based on hourly mean temperature and wind speed was -73°F on Sunday evening (-25°F temperature, 80mph wind).  This is respectable but not all that unusual for the location; wind chills this cold or colder are observed at least once in nearly every winter.

The MSLP analysis from Environment Canada shows the very typical setup at 4pm Sunday, with a very tight pressure gradient caused by a ridge to the north and sprawling low pressure across southern Alaska.

In the 2012-present history of the NPS instruments at Howard Pass, -60°F wind chill has been observed as early as November 17 (2021) and as late as April 9 (2013).  As one might expect, it's most common in January and February.  Interestingly, this is the first time the wind chill has stayed below -60°F continuously for more than 80 hours this early in the season.  However, late November 2021 saw a much more prolonged period of substantial cold, with 11 straight days having a wind chill reading below -60°F at some point in the day.

Autumn Climate Data

Autumn seems like a long time ago for much of Alaska, but it's worth making a mention of the autumn climate anomalies, if only for future reference.  I'll use "autumn" here to refer to the standard climatological season of September through November.

Statewide all three months were slightly warmer than the 1991-2020 average, and the North Slope was considerably warmer than the 30-year baseline, but overall it was a slightly cooler autumn than the last two years (but nowhere near as cold as 2021, when November was very cold).  The North Slope was the only region with a large departure from normal temperature, although parts of Southeast were relatively cool according to the consensus of ERA5 and NCEI data:

More significantly perhaps, September through November was the driest such period in Alaska since 2016, according to NCEI, and nearly all western and southern coastal areas encountered this dryness.  All three months were drier than average on a statewide basis, and that's the first string of three consecutive dry months (relative to normal) since spring 2022.  And yet in contrast, the central interior was very wet, with Fairbanks observing its wettest autumn since 2017; only five autumns have been wetter since 1930.  The wet weather occurred mostly in October.

November was easily the driest month of the autumn, as the Aleutian ridge of October shifted north and took up a dominant position over the Bering Sea and western Alaska - see below.

With Pacific storms held at bay, winds were lighter than normal for most of western Alaska in November, and autumn wind overall wasn't dramatically different from normal for most of the state.  That's a big difference from summer, which was exceptionally windy in the western half of the state.

The calm weather in the Bering Sea allowed SSTs to return to near-normal there, eliminating the cool anomaly that developed back in the summer.  However, exceptional and widespread warmth persisted to the south of the Aleutians, and the Gulf of Alaska remained quite cool throughout autumn.

As a reflection of the extreme temperature anomaly differences across the North Pacific, the PDO index became extremely negative throughout the autumn.  Before the past decade or so of exceptional warmth in the North Pacific and in the Arctic, it was commonly observed that (at least southern) Alaska had a strong tendency for unusual cold during pronounced negative PDO episodes, but we don't see the same robust connection these days.  Consider the map below, showing the average temperature departure from trend in 10 past years with a significantly negative PDO index in autumn:

Compare this to the temperature anomaly map for autumn 2024:

The general orientation of the anomaly patterns is approximately the same, but the 2024 map is considerably warmer overall.  I think this illustrates that even with a favorable SST configuration, cold in Alaska these days tends to be very muted by historical standards.

No More Wind Chill Warnings

On Friday the National Weather Service in Fairbanks issued an "Extreme Cold Warning" for two eastern interior forecast zones: the Fortymile country and the White Mountains.  This warning has rarely, if ever, been issued by the NWS in Alaska, so one might think conditions must have been extreme indeed.

But no: in a recent change of procedure, the Extreme Cold Warning simply replaces the old Wind Chill Warning for the NWS nationwide.  The goal is to simplify communication of weather hazards:

"Social science research indicates users of NWS products can become confused by the number of hazard messages NWS issues before and during active weather. Especially while dangerous weather is unfolding, it is critical that NWS hazard messages are simple, short and direct."

So how cold did it get over the weekend?  Well, it was certainly very chilly, with temperatures dropping below -40° in the usual cold spots of the eastern interior, and wind chills of -50°F in some places.  The lowest wind chill measurement I saw was -52°F at Delta Junction early on Sunday morning.  Here's a map of observed wind chill in the region surrounding Fairbanks at about that time (click to enlarge):

The following chart shows the 850mb (approx 5000 feet elevation) temperature and wind at 10pm Saturday.  As is often the case, the cold air mass arrived from the northeast as it traveled clockwise around a high pressure ridge to the north.  (The blank areas on the map are locations where the surface pressure is less than 850mb in the model.)