Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EST Mon Feb 26 2024
Valid 12Z Fri Mar 01 2024 - 12Z Tue Mar 05 2024
...Very cold temperatures over a majority of the state late this
week into the weekend...
...Overview...
There is good agreement and consistency for the amplified upper
pattern forecast to prevail late this week into early Saturday,
with a trough axis over the eastern mainland and upstream ridge
aligned from the Pacific through eastern Bering Sea and Siberia.
Very cold temperatures over most of the state will accompany this
regime. Meanwhile trough energy to the west of the ridge should
support a leading Bering Sea frontal system and then a trailing
Pacific wave that lifts into the southern Bering Sea by early
Saturday. Then through the rest of the weekend into early next
week, recent guidance runs have varied wildly among each other and
consecutive runs for how the overall pattern may evolve--with the
most pronounced sensible weather questions being how persistent
below normal temperatures may be later in the period and
precipitation/wind details over and near the Gulf of Alaska.
...Guidance Evaluation and Preferences...
Overall a 12Z operational model composite represents consensus
well for the pattern from day 4 Friday into early day 5 Saturday,
with only minor refinements from continuity. Over the past day
the GFS has joined remaining guidance in signaling that compact
low pressure reaching the southern Bering Sea by early Saturday
will originate from a northward-moving Pacific frontal wave, while
consensus position has trended to the southeastern part of
yesterday's spread (represented at the time by the UKMET). In
order to improve the transition to the rest of the forecast, the
favored blend utilized more 12Z ECMWF relative to the GFS and
CMC/UKMET.
Thereafter, model/ensemble runs from the last several cycles have
varied remarkably for how the pattern will ultimately evolve. The
last two CMC runs offer the most dramatic contrast, going from
eastward progression of the amplified upper features in the 00Z
run to reinforcing a deep trough/upper low over the mainland in
the 12Z version. In general the 12Z cycle has trended toward
greater potential for the approaching ridge and trailing trough
energy to split and allow for more mainland troughing into next
week. The 12Z GFS is extreme in having its closed low retrograde
to the southwestern coast of the mainland, while the GEFS mean
keeps the trough centered over the mainland. The 12Z CMCens mean
is much weaker/elongated with its mainland upper trough versus the
operational run. The 12Z ECMWF (and ECens mean, arriving after
forecast preparation) have trended toward a little more mainland
troughing but also in modest fashion. The new 18Z GFS has come in
with an upper trough much closer to the 12Z GEFS mean.
Reinforcing the lack of confidence in how assertively to follow
the apparent 12Z guidance trends, only one of five 00Z
ECMWF-initialized machine learning models showed significant
mainland troughing by the latter half of the period.
Ultimately, the erratic behavior of guidance thus far suggested
only a measured adjustment toward the mainland trough scenario by
way of the 12Z ECMWF and 12Z CMCens/GEFS and 00Z ECens means.
This preference led to slower and weaker low pressure translating
from the southern Bering Sea into or near the Gulf of Alaska after
Saturday, and somewhat less precipitation reaching the
southeastern coast/Panhandle, versus the deeper mainland upper
trough scenario. Either way, there is a noticeable trend for high
pressure to hold on for a longer time over the northern mainland,
tempering moderation of initially cold temperatures.
Farther west, there is actually some degree of continuity with an
elongated frontal system reaching into the Bering Sea by
Sunday-Monday (though with the parent low staying farther west),
with the fast-moving front most likely merging with leading
Northeast Pacific/Gulf low pressure early next week. There is
some signal for a potential Pacific system to begin having an
influence on the Aleutians by next Tuesday.
...Weather/Hazards Highlights...
As highlighted on the Days 3-7 Hazards Chart, the pattern late
this week will support a broad area of hazardous cold temperatures
across a majority of the mainland to the south of the Brooks
Range. This will be from a combination of actual air temperatures
(possibly approaching -40 degrees over the typically coldest
eastern interior locations) or wind chills. Temperatures should
gradually moderate above hazardous criteria from west to east but
could still remain well below normal after Saturday in light of
latest model trends. In addition on day 3 Thursday the
combination of eastern Pacific through northern mainland high
pressure and Northeast Pacific low pressure will support a tight
surface gradient and thus strong gap winds that should be most
pronounced from the southern Alaska Peninsula through Kodiak
Island and to around Prince William Sound.
A Bering Sea frontal system and trailing wave Friday-Saturday will
spread organized precipitation across the Aleutians/Bering Sea
into/near the Alaska Peninsula. Latest guidance trends suggest
this moisture shield will not extend as far into the southwestern
mainland as forecast yesterday. Corresponding to uncertainty with
specifics of the mainland flow pattern aloft, confidence is low
for details of how quickly and strongly the Saturday southern
Bering system moves into the Northeast Pacific/Gulf. At least
there is reasonable agreement on the general wetter/snowier trend
for the southeastern coast/Panhandle from late weekend into next
week. Current preferences lean on the later/weaker side for the
surface low, with somewhat less precipitation than the other
extreme of the envelope, but it is hard to rule out any portion of
the solution spread at this point. Another frontal system quickly
tracking into and through the Aleutians and Bering Sea by early
next week may produce a second episode of precipitation but likely
with fairly light totals.
Rausch
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html