Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
649 PM EST Tue Feb 02 2021
Valid 12Z Sat Feb 06 2021 - 12Z Wed Feb 10 2021
...Overview and Guidance/Uncertainty Assessment...
Most guidance suggests an evolution toward a fairly blocky regime
during the period. The majority cluster has an upper low dropping
southwestward into the mainland while a varying amount of
elongated ridging persists to the southwest, from near the eastern
Aleutians into east-central Siberia. The approach/arrival of the
upper low will likely promote a significant colder trend, led by a
front that pushes southward over most of the mainland during the
weekend and start of next week. Shortwaves within the cyclonic
mean flow to the southwest of the upper low should hold surface
low pressure over or near the Gulf of Alaska for most of the
period. Meanwhile flow around the western side of the elongated
ridge may bring one or more features northward through the western
Aleutians.
At the start of the period early Sat there has been a significant
degree of spread and/or trending for specifics of low pressure
likely to be over the Gulf. 00Z/12Z CMC runs hold the upper
dynamics/surface low pressure well westward of other 12Z models
and ensemble means, reflective of yesterday's majority cluster.
On the other hand the new 12Z ECMWF has strayed even more
progressive/sheared than remaining 12Z guidance. Latest means
offer the best support for the 12Z GFS/UKMET and 00Z ECMWF in
principle, so the early part of the forecast blend emphasizes
those solutions with only modest 12Z ECMWF input.
Guidance spread and variability thus far keep confidence on the
lower side for specifics of the upper low arriving from the
northeast. There has been a decent signal over the past couple
days for retrogression of height falls anchored by at least an
elongated trough if not a closed low, but by day 8 Wed there is a
pronounced trend toward holding lower heights over the mainland.
The greatest ensemble spread exists to the south and west of the
mean low center, highlighting the ongoing uncertainty. The
00Z/12Z ECMWF runs lie in the southwestern extreme of the spread
while the 12Z CMC mean keeps the upper low north of the mainland
through the period. The operational CMC keeps the center north as
well but does eventually dig a trough into the mainland. Overall
a solution closest to the 12Z GFS and 12Z GEFS/00Z ECMWF mean
cluster seems to provide the best intermediate way to reflect
latest trends. Thus the blend during the latter half of the
period emphasizes those solutions with only minimal ECMWF
inclusion.
...Weather/Hazards Highlights...
Persistent low pressure over the Gulf of Alaska will tend to focus
periods of precipitation over the southeastern coast and Panhandle
through the period. A leading wave arriving by the start of the
weekend should provide the initial push of moisture with moderate
totals expected. After the weekend there is increasing
uncertainty over precipitation intensity over this area, depending
on specifics of the upper low/trough that may move into the
mainland. Current guidance preferences would yield light-moderate
activity most of the time but an upper flow orientation that could
bring greater moisture into the region is a lower-probability but
possible scenario. One or more features lifting up the western
side of upper ridging may support episodes of precipitation/brisk
winds over portions of the Aleutians and southwestern half of the
Bering Sea. The weekend will start with above normal temperatures
over southern/western areas and below normal readings to the
north/east. Then a colder trend from north to south will likely
yield below to well below normal readings over much of the state
during the first half of next week.
Rausch
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
Hazards:
- Much below normal temperatures across portions of mainland
Alaska, Mon-Tue, Feb 8-Feb 9.
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