Tuesday, October 11, 2022

Snow Arrives; Winter Outlook

The scene is beginning to look and feel more wintry for interior and northern Alaska, with snow making an appearance in many locations as freeze-up begins.  Yesterday the temperature failed to reach freezing in Bettles, and there's a light covering of snow on the ground.

Fairbanks received its first measurable snow of the season on Sunday, and it amounted to just over an inch according to the NWS.  This is a bit later than usual, but more notable is the fact that the temperature hadn't dropped below 30°F until this morning.  This ties with 2017 for latest arrival of the first freeze in the 20s Fahrenheit.  Interestingly, with the forecast looking chilly, it seems possible there may not be another day with a low temperature above the 20s.  This has happened once before: in 1966.

Lakes and rivers will be undergoing freeze-up in the coming weeks.  North of the Brooks Range, it looks like Teshekpuk Lake is partially frozen over, whereas Toolik Lake is open for now.

What sort of winter might we expect for Alaska?  A key consideration is that we still have La Niña in the tropical Pacific Ocean, and La Niña winters tend to be colder than normal for nearly all of Alaska, drier than normal along the northern Gulf Coast, and snowier than normal for western, interior, and northern Alaska.  This is all because La Niña tends to weaken the semi-permanent Aleutian Low and produce more westerly rather than southwesterly flow across Alaska.  Compare the two maps below: the first shows the 1991-2020 average 500mb height, and the second shows the average during 10 strong La Niña winters.  It's a subtle difference, but a significant one, with fewer chinook-style warm-ups for the interior, less storminess for the Gulf Coast, and a wetter westerly regime to the north and west of the Alaska Range.

Here's the difference between the two: the anomaly produces a northerly perturbation to the mean flow, and hence the cold.


Here are the temperature and precipitation anomaly patterns for a larger set of 20 La Niña winters.  According to this data, 70-80+% of La Niña winters see below-trend temperatures across the interior, southern, and most of southeast Alaska.

La Niña is only one consideration in the forecast, however.  The experts at NOAA's CPC don't foresee a heightened risk of cold, except for the southeast; and there are no strong probability signals anywhere in the state for the December through February mean temperature.

Precipitation, on the other hand, does have a notable tilt in CPC's odds, with wetter being significantly favored for western Alaska:

The absence of cold in the CPC forecast may be a reasonable response to ongoing extreme warmth in the North Pacific to the south of the Aleutians.  The very warm ocean (relative to normal) was probably a factor in last month's damaging Bering Sea storm (ex-Merbok, read more here), and it will certainly boost warmth for Alaska whenever the flow comes from the southwest this winter.  Here's the ERA5 reanalysis SST anomaly map for September:

Above-normal precipitation for western Alaska this winter is supported by the latest NMME forecast, which is a consensus of several long-range computer models:

The NMME temperature forecast also hints at unusual warmth for western Alaska, similar to CPC, but it has a more typical cold La Niña look for southern and southeastern areas.  It will be interesting to see if CPC holds onto "equal chances" for most of the state when the forecast is updated next week.

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