Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
651 PM EST Tue Dec 28 2021
Valid 12Z Sat Jan 01 2022 - 12Z Wed Jan 05 2022
...Overview...
Guidance continues to show an evolution toward a sharply amplified
and blocky upper pattern, with a ridge extending from the Pacific
through the Bering Sea into the Arctic. The ridge will promote
the south-southeastward progression of initial Mainland Alaska
upper energy over the Gulf of Alaska and Northeast Pacific. Gulf
of Alaska low pressure should bring precipitation (the majority
being snow) to Southcentral and especially the Panhandle during
the weekend. Once this system departs, expect fairly dry
conditions while high pressure tracking across the northern half
of the state should push well below normal temperatures into
southern/eastern areas. By next Tuesday-Wednesday questions arise
over the shape of the upper ridge and surrounding flow.
...Guidance Evaluation and Preferences...
Today's forecast started with a 40/30/30 blend of the 12Z ECMWF,
06Z GFS, and 12Z CMC through the entire period. This approach
provided a good representation of the most common ideas of
guidance or the most reasonable compromise as the case may be, and
maintained definition of features and surface gradients. Note
that the 12Z UKMET was not available due to a local data issue and
the 12Z CMC mean was delayed.
With consensus good for the strong storm tracking south-southeast
from the Gulf of Alaska, there were two primary question marks
both regarding the building upper ridge--its longitude and whether
the northern part would close off a high (with the high's location
being an embedded question). Many recent runs of models/means
have indicated the upper ridge should not reach much farther
eastward than 170W longitude while the ECMWF/CMC means have been
showing a high closing off late in the period. ECMWF runs have
gone along with the longitude of the ridge but not so much for the
closed high until the new 12Z run. At least a couple of CMC runs
have closed a high. Meanwhile GFS runs have been very erratic
with both longitude and shape. Some runs including the 12Z
version pushed the ridge axis east of consensus and did not close
a high, while the 06Z run was much closer to consensus in both
regards (hence its inclusion in the blend). GEFS runs have tended
to be less extreme with eastward extent of the ridge but not
surprisingly have been later to close off a high if at all. 00Z
ECMWF/CMC mean runs were farther north than operational runs with
the eventual upper high, making the preferred model blend a good
compromise for position (over the Bering Strait by early day 8
Wednesday) given the uncertainty over whether a high would close
off. Means tend to be weak with ridges that are agreeably strong
in operational models as well. New 12Z ECMWF/CMC means did trend
somewhat south and a bit stronger for the upper high next
Wednesday, adding support for the manual forecast.
It is worth noting that even with the better than average
agreement from the models used in the blend and some support from
ensemble mean trends, there is usually a fair amount of
uncertainty with the exact details of ridge shape and surrounding
flow in this type of evolution. Thus some meaningful changes for
aspects of the forecast such as the upper weakness that may
develop over or near the Gulf of Alaska and how suppressed surface
features and moisture will be over the Bering Sea and Aleutians
may still be possible in future runs.
...Weather/Hazard Highlights...
The fairly strong storm tracking south-southeastward from the Gulf
of Alaska will likely produce some snow over Southcentral and
heavier precipitation (snow, with some rain possible at low
elevations) over the Panhandle during the weekend. Otherwise a
majority of the state should see fairly dry conditions during the
period. The one exception will be the western half of the
Aleutians where flow around the western side of the upper ridge
may promote organized precipitation and brisk winds. High
pressure crossing the northern half of the state will bring well
below normal temperatures especially into southern and eastern
areas (including the Panhandle) from west to east. Northern areas
will be cold in absolute terms for a time but with less pronounced
anomalies, and then should trend warmer than normal after passage
of the surface high. The strong gradient between the high
pressure (possibly exceeding 1050mb for a time) with Northeast
Pacific low pressure may produce a period of strong winds across
the southern mainland and Panhandle.
Rausch
Hazards:
- Heavy snow across portions of the Alaska Panhandle and mainland
Alaska, Fri-Sat, Dec 31-Jan 1.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sat-Sun, Jan 1-Jan 2.
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Sun-Tue, Jan 2-Jan 4.
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