Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
735 PM EDT Thu Mar 25 2021
Valid 12Z Mon Mar 29 2021 - 12Z Fri Apr 02 2021
...Powerful Bering Sea storm to reach Mainland Alaska early next
week...
...Guidance/Uncertainty Assessment...
The majority of model/ensemble guidance shows good agreement and
continuity regarding the strong Bering Sea system forecast to
track into the western mainland around Monday-Tuesday followed by
a transition of low pressure emphasis to the Gulf of Alaska as the
supporting upper trough (extending south into the Pacific)
ultimately settles over the mainland and northeastern Pacific.
Farther west there is a general theme in the guidance for a trough
to approach/reach the western Pacific while some degree of mean
ridging builds over the North Pacific into the Aleutians/Bering
Sea. However solutions differ in the strength and position of
this ridge depending on how energy within the trough
evolves/ejects onward.
For about the first half of the period the primary consideration
for the operational model blend used as the basis of the manual
forecast was to phase out 12Z CMC input after Tuesday, as that
model developed its Gulf low well southwestward of the dominant
clustering of other models and ensemble means. There was minimal
disagreement for the strong Bering Sea storm. After midweek the
forecast blend tilted about 70 percent total toward the past two
ECMWF runs and 00Z ECMWF mean relative to the 12Z GFS/GEFS mean.
This scenario reflected recent trends over the past couple days
toward stronger North Pacific into Bering Sea mean ridging during
the latter half of the week, with ECMWF/ECMWF mean runs showing
greater lead time in reflecting this ridge. The 12Z ECMWF (and
recently arrived 12Z ECMWF mean) have backed off a bit from the
prior cycle for strength of this ridge, supporting some degree of
compromise from the strongest side of the envelope. This latest
adjustment yielded closer agreement among the ECMWF/GEFS
means--which still suggest the operational GFS may be too flat
with the ridge.
...Weather/Hazards Highlights...
Early in the week the strong Bering Sea storm forecast to track
into the western mainland will support an episode of high
winds/waves along with enhanced snow/precipitation over favored
mainland terrain. As supporting upper dynamics continue eastward
and then settle over the mainland/northeastern Pacific, expect a
wave along the trailing front to develop and reach the Gulf of
Alaska by early Wednesday. Low pressure should linger over or
near the Gulf into late week. The Gulf system will likely bring a
period of heavy precipitation to areas from the southeastern coast
through the Panhandle. There are still timing and magnitude
details to be worked out though. Farther west one or more waves
along a mean frontal boundary around the northern periphery of a
deep-layer ridge centered over the central Pacific may produce
precipitation over parts of the Bering Sea and vicinity from about
midweek onward, but confidence in any specifics is currently in
the lower half of the spectrum.
On Monday expect below normal temperatures over the eastern
mainland/Panhandle while above normal readings (especially for
morning lows) prevail over western areas with the approach of the
Bering Sea low and trailing front. The above normal min readings
will continue overspreading the state over the next day or so but
clouds and precipitation will tend to keep daytime highs near to
below normal. The upper trough settling over the area by mid-late
week will yield below normal temperatures over many locations in
the mainland and Panhandle by the latter half of the week, with
anomalies tending to be more below normal for highs.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Mar 30-Mar 31.
- Heavy snow across portions of mainland Alaska, Sun-Mon, Mar
28-Mar 29.
- High winds across portions of mainland Alaska and the Aleutians,
Sun-Mon, Mar 28-Mar 29.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards 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