Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Sun Jan 16 2022
Valid 12Z Thu Jan 20 2022 - 12Z Mon Jan 24 2022
...Heavy precipitation threat along the southern coast and
Panhandle late week into the weekend...
...Overview...
The overall forecast is consistent in showing a much wetter and
warmer regime for a majority of Alaska from late this week onward,
as eastern Pacific upper ridging extends its influence farther
northward into the mainland and western Canada while troughing
(with at least one embedded upper low) prevails to the west.
Heaviest precipitation should be along the southern coast and
Panhandle Friday into Saturday but some locations may see
significant totals thereafter. Specifics over this area, and
magnitude/coverage of precipitation farther north, will depend on
yet to be resolved details for two main North Pacific systems that
are most likely to track into the southeastern Bering Sea.
...Guidance/Uncertainty Assessment...
To start the period, latest model runs have trended somewhat
weaker and faster with low pressure expected to reach the Gulf of
Alaska or Panhandle by the start of the period early Thursday.
Most ensemble means are actually a tad stronger than the 12Z model
average so a slight account for their solutions could be feasible
given continuity. Specifics have been dependent on
low-predictability smaller scale details.
There is still some debate over the ultimate evolution of
elongated mainland trough energy as of early Thursday. The
southeastern side of the spread for the trough/upper low (now
represented by the 12Z UKMET) continues to see a gradual loss of
support from consensus given persistent western solutions in the
ensemble means (but with a trend toward more troughing as models
have been showing in recent days) and a general westward trend in
the ECMWF over the past couple days. The exact depth and position
of the upper low retrograding into the Bering Sea will be crucial
in determining the evolution of the first North Pacific system as
it is drawn into the southeastern Bering Sea and beyond. For now
prefer to lean away from the deep and westward solution seen in
the 12Z ECMWF around Friday-Saturday, in favor of surface low
pressure/troughing holding closer to the western coast of the
mainland. Some GFS runs have been showing a less extreme
variation of the idea though, so it is not completely out of the
question.
Guidance also diverges for specifics of the next Pacific system
that is most likely to reach the eastern Aleutians/southeastern
Bering Sea by Sunday-Monday. The 00Z ECMWF mean was on the
east/southeast side of the spread by day 8 Monday while the new
12Z run adjusted somewhat northwest in the direction of earlier
runs. There is also increasing timing spread at that time with
the 00Z GFS and 12Z CMC straying quite fast while more recent GFS
runs as well as ECMWF plus 12Z CMC/GEFS means are on the slower
side--forming the greater majority cluster.
Guidance comparisons led to starting with 80 percent weight of the
12Z operational models early, followed by increasing total 12Z
GEFS/00Z ECMWF mean weight to half by days 7-8 Sunday-Monday.
Preference to lean away from 12Z ECMWF specifics over the Bering
Sea around Friday-Saturday led to reduced weight of that run in
that time frame, while the ensemble component tilted a little more
toward the GEFS late when the 00Z ECMWF mean became somewhat
questionable.
...Weather/Hazard Highlights...
A wave forecast to reach the Gulf of Alaska or Panhandle by the
start of the period early Thursday may produce an initial episode
of precipitation over the Panhandle and perhaps a little farther
northwestward. Then guidance continues to show a strong signal
for an atmospheric river event aimed at the southern coast and
Panhandle from Friday into the weekend. GFS/ECMWF runs still
depict a strong southerly low level flow of moisture ahead of a
North Pacific into southeastern Bering system bringing
precipitable water values as high as 3-4 standard deviations above
normal for a time while the corresponding means are now plus 2-3
standard deviations. Also consistent is the idea that peak values
should be around Friday followed by some decrease but still
remaining well above normal. Heavy rainfall along the southern
coast and Panhandle on Friday may focus more locally over the
Panhandle during the weekend. Less extreme but still meaningful
precipitation may extend farther west into the Alaska Peninsula.
There is a less concentrated depiction of moisture ahead of the
next Pacific/southeastern Bering system from the weekend into
early next week, with best focus over the Alaska Peninsula and
parts of the southern coast while amounts should decrease somewhat
over the Panhandle as the upper ridge strengthens. The extent of
this trend is uncertain at this time though. The flow of moisture
ahead of both systems may also spread precipitation well north
across the mainland. Significant totals are possible over some
terrain-favored areas over the southern and western mainland but
with a lack of confidence in specifics at this time. These
systems will also bring episodes of precipitation and brisk to
strong winds to portions of the Aleutians.
Expect the North Slope and parts of the western mainland to see
below normal temperatures extend through Friday or Saturday while
above normal readings prevail over most other parts of the state.
Regardless of above/below normal anomalies, there should be a
gradual warming trend. More pronounced warming during the weekend
and early next week will likely expand the coverage of much above
normal readings across a majority of the state.
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