Deep Cold: Alaska Weather & Climate
Objective Comments and Analysis - All Science, No Politics
Primary Author Richard James
2010-2013 Author Rick Thoman
Wednesday, March 13, 2024
Sustained Wind Chill
Friday, March 8, 2024
February Climate Data
Climate data for February has arrived, showing that it was a warmer and wetter than normal month overall for Alaska. Both December and January were slightly colder than the 1991-2020 normal, so February was the only month of climatological winter that was on the warm side; but actually all three months were quite close to normal statewide.
As is often the case, of course, "near normal" masks a lot of spatial and - in this case - temporal variability, as Alaska started and ended February with well below normal temperatures. Here's the UAF statewide temperature index for December through February:
For February as a whole, colder than normal conditions were confined to some parts of the eastern interior, whereas western Alaska and the North Slope were significantly - but not dramatically - warmer than the baseline of the last 30 years.
The temperature rank map for Dec-Feb shows a moderately significant cold anomaly in the eastern interior, but more significant warmth occurred across the North Slope and Southeast Alaska.
February precipitation was very high in southwestern Alaska, locally over 300% of normal according to ERA5 data, and this was caused by a persistently strong Bering Sea trough:
Wednesday, March 6, 2024
Yukon at Dawson Still Open
This has been a recurring theme in recent years, and a problem for residents of Dawson, YT: the Yukon River is reluctant to freeze over next to town. As in some other recent winters, it hasn't been possible to build the usual ice bridge across to West Dawson. Here's today's webcam view, suggesting that a complete freeze-up won't occur at all this winter:
And a video confirming the flow of water in the open channel:
November and December were significantly warmer than normal in Dawson, but January and February were both slightly colder than normal, so it seems unlikely that the lack of ice can be blamed on the weather.
Monday, March 4, 2024
Anchorage Wind
A reader inquired about the persistent north wind event of last week in the Anchorage area, so let's take a look at this.
First, the facts on the ground: there was indeed a very lengthy period of stiff northerly winds, lasting from early morning on Wednesday (Feb 28) through Saturday morning (March 2). Here's a graphical look at observations from Anchorage airport, with wind speeds in mph:
Much the same sort of thing was observed up in Palmer, and with higher wind speeds, although there was a break on Thursday morning:
Remarkably, the peak wind gusts were 60mph or higher for 4 consecutive days in Palmer. The maps below (click to enlarge) show the daily peak gusts from Feb 28 (top) to March 2 (bottom). This was a remarkably persistent event:
Taking a quick look at historical hourly data from Anchorage airport, I see only one other event since 2000 with comparable sustained winds: it looks like a very similar event occurred at the same point on the calendar in 2017 (Feb 28 - March 3). Here's the average MSLP for the 3 windiest days in both cases: 2017 (top) and 2024 (bottom):
Clearly the driving factor in both cases was the strong pressure gradient between low pressure in the eastern Gulf of Alaska and high pressure to the west and north.
The long duration of the event reflects the unusually persistent MSLP setup, with low pressure slow to depart or weaken in the Gulf of Alaska, and with building high pressure to the north maintaining the gradient as the low pressure eventually decayed. Here's a sequence of MSLP analyses at 3am AKST for 5 consecutive days: February 27 through March 2.
Farther aloft, the situation was unusual, with a sharp ridge over the Bering Sea progressing slowly eastward behind a broad and deep trough over the Gulf of Alaska. The 500mb map from Thursday afternoon shows powerful northwesterly flow in the very tight pressure gradient over southwestern Alaska, but yet the 500mb wind was out of the south at Anchorage (highlighted with the red circle below):
To me, this highlights the large degree of spin (technically "vorticity") in the atmosphere aloft, which helps explain the longevity of the low pressure in the northeastern Gulf of Alaska. This upper trough and associated jet stream, by the way, is what brought extreme mountain snows to California over the weekend. It was windy down there too: there's a lot of energy in this jet stream, and this is probably attributable to El Niño.
Friday, March 1, 2024
ECMWF AI Forecasts
Wednesday, February 28, 2024
Calendar of Cold
It's back to cold(er than normal) in Alaska again, with a brisk -31°F observed at Fairbanks airport this morning, and -40°F in colder spots to the north, including Bettles. Wind chills are unpleasant on the North Slope too, but as noted by reader Mike, Arctic Alaska is very much still locked in winter. The seasonal minimum in temperature occurs a lot later for the North Slope - and indeed for many of Alaska's coastal locations - than for the interior.
For a visual perspective on the timing of the seasonal minimum, here's a map based on ERA5 1991-2020 data.
According to this analysis, the seasonal temperature cycle bottoms out before the turn of the year for a few areas in western and interior Alaska, but most of the interior sees its coldest "normals" in the first half of January. Cold peaks later for the North Slope - the first half of February for the eastern North Slope - and the highly maritime climate of the North Pacific sees a seasonal minimum even later, in late February or even early March. The pronounced lag over the ice-free Pacific is related to the very large heat capacity of the ocean's upper layers, as well as the ample cloud cover that prevents a strengthening sun from bringing warmth to the ocean surface.
I'm a little surprised by the early seasonal minimum for coastal western Alaska, where I would have expected sea ice to keep temperatures suppressed to a later date. The ERA5 results are confirmed by the NCEI 1991-2020 daily normals: Nome has its lowest normal temperature around January 12, and Bethel around January 9. For comparison, Fairbanks also bottoms out around January 12. I don't know why the west coast doesn't have more of a maritime influence in this aspect of the climate.
But a couple of caveats are worth noting in this analysis. First, I'm using harmonic functions to smooth the seasonal temperature cycle through the year, and the details of the method (e.g. the number of functions) can affect the timing of the estimated seasonal minimum. Second, the sample size of 30 years also implies some uncertainty in the date of the minimum, because the random timing of major historical cold and warm episodes will affect the details of the calculated cycle. It would be interesting to quantify the uncertainty with some statistical experiments.
On a broader scale, the map below shows that the European side of the Arctic Ocean has a later seasonal minimum than the Pacific side: it even extends past March 15 for a small area near Iceland.
And expanding the domain into the mid-latitudes, we see that the eastern North Pacific has the latest seasonal minimum of anywhere in the hemisphere: as late as March 29 according to this analysis.
Another question I can't answer is why the eastern North Pacific has so much more lag than the western North Pacific. Perhaps it's as simple as the western ocean being influenced by continental air from the vast land mass of Asia, which warms up quickly in late winter.
Wednesday, February 21, 2024
More Context for Cold
Another round of downslope/chinook warming brought another widespread thaw to the Tanana River valley and Fairbanks-land yesterday. Temperatures above 40°F were widespread, including 45°F at Fairbanks airport, the highest February temperature in 20 years. The year-to-date average temperature in Fairbanks is now only 2.2°F below normal.
Looking back again at the late January and early February cold snap, I was curious to see how the lower atmosphere (not surface) temperatures compared to past cold spells on various time scales. However, with NWS weather balloons no longer going up during severe cold, there's some missing data for the recent event; so I extracted data from the ERA5 reanalysis. I looked at Fairbanks 850mb temperatures and 1000-500mb thickness, with the latter being an excellent measure of the average temperature of the lower half of the atmosphere.
Here's the annual (winter) minimum for 3-day average 1000-500mb thickness:
On this time scale, the recent event was one of the coldest since 2000, but it was nowhere near as cold as the more extreme events of earlier decades. The complete absence of seriously cold events after 2000 is quite remarkable, and suggests that the cold tail of the distribution has been dramatically curtailed since then.
Below is another perspective, showing annual minimum values of 3-day 850mb temperature (y-axis) versus 1000-500mb thickness (x-axis). The values are quite highly correlated, of course, but there's some variability in the extent to which cold is concentrated at lower atmosphere levels like 850mb. On a 3-day basis, the recent event was relatively less unusual in terms of 850mb temperature, with several other events being colder in recent years.
The 7-day and 14-day charts below show quite similar findings on these time scales, with the recent event being a bit more anomalous overall on a 14-day basis, as I noted before; but it was still not in the least unusual compared to earlier decades.
The two "granddaddy" cold spells of 1989 and 1999 stand out clearly here; those two events were much more extreme than anything in recent years, and also worse than anything that happened in the otherwise colder 1950s, 60s, and 70s.
Finally, it's interesting to observe that two quite extreme cold spells occurred in the last 20 years when we look at a 30-day average: late February and early/mid March of 2007, and January of 2012.
I hadn't previously realized how anomalous that 2007 event was, but in fact ERA5 shows the event having the lowest 30-day average temperature since 1950, by both metrics (thickness and 850mb temp above Fairbanks). Given that it was so late in the winter, the surface temperatures were nowhere near as low as they would have been a few weeks earlier; but nevertheless March 2007 was the second coldest on record in Fairbanks, and 27 of 31 days had a low of -10°F or lower (the highest such number for the month).
Here's a look at the 500mb height pattern for the 30-day period: what a classic!
The 500mb height anomaly:
And the 850mb temperature anomaly, according to the older (but still basically reliable) NCEP reanalysis: