Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
636 PM EST Tue Feb 18 2020
Valid 12Z Sat Feb 22 2020 - 12Z Wed Feb 26 2020
...Overview...
The best clustering of guidance shows an initial southwestern
mainland-northeastern Pacific upper trough (supporting Gulf of
Alaska low pressure) progressing to the east/southeast during the
weekend, and by early Mon replaced by an upstream ridge that
builds into the area ahead of a North Pacific system. Flow aloft
over the northern mainland should strengthen as the gradient
tightens between this ridge and an upper low that tracks to the
northwest of the mainland. By Tue-Wed the dynamics from the North
Pacific system will likely shear out as a northwestern Pacific
system enters the picture and ultimately tracks into the
Aleutians/southern Bering Sea. At the same time some degree of
upper ridging should extend from western Canada into a portion of
the mainland.
...Guidance Evaluation/Preferences...
The most significant forecast issue involves the
eastward/northeastward progression of height falls aloft and
corresponding surface reflection toward the southwestern mainland,
affecting specifics of the associated wind field and enhanced
precipitation likely to affect areas from the eastern Aleutians to
the southern coast/Panhandle. Guidance appeared more similar
yesterday and has since diverged with ongoing inconsistency. The
00Z ECMWF had adjusted much slower due to a stronger upper ridge
building into the mainland while latest GFS runs have strayed to
the faster side of the envelope. The 00Z ECMWF mean and 12Z
GEFS/CMC means (as well as new 12Z ECMWF mean) are closer together
around an intermediate timing. These means suggest the faster
trend of the 12Z ECMWF is a positive one, though that model run
could still be a little slow.
During the latter half of the period the operational models
diverge significantly regarding details of flow aloft over the
mainland. The last two ECMWF runs alone vary greatly for what
happens to the mainland ridge by next Wed, while the 12Z CMC has
flatter than consensus flow to start the week and then develops a
trough. The 12Z CMC mean has a much weaker version of the
operational run's trough while GEFS/ECMWF means depict broad
ridging. Farther west, given the extended time frame there is
decent agreement among the GFS/ECMWF and their means for upstream
low pressure reaching the Aleutians/Bering Sea by Tue-Wed.
Guidance considerations led to a starting blend that consisted of
the 12Z ECMWF/GFS/UKMET for about the first half of the period.
In addition to aforementioned 12Z CMC question marks, that run
also has a stronger than consensus wave tracking near the southern
Panhandle during the weekend. After Mon the 12Z GEFS/00Z ECMWF
means gained more weight in the blend (50-75 percent) to represent
the agreeable large scale pattern while downplaying operational
run differences. The fact that North Pacific system evolution may
consist of multiple wave developments will keep confidence in
specifics lower than if there were a single strong system.
...Weather Highlights...
Initial low pressure/upper level energy will support light to
moderate precipitation near the central-eastern southern coast and
into the Panhandle during the weekend. From about late Sun or Mon
onward expect strong flow ahead of North Pacific low
pressure/frontal system(s) to bring a period of heavier
precipitation into the eastern Aleutians/Alaska Peninsula and then
to the southern coast/Panhandle. There is still a fair degree of
spread regarding precip totals and how quickly the moisture shield
spreads eastward. Below normal temperatures will progress from
west to east during the weekend into the start of next week,
followed by a trend to above normal readings as the upper ridge
axis reaches western Canada and the central/northern mainland.
Locations to the north of the Brooks Range may stay below normal
for most of the period.
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:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Fri, Feb 21 and Mon, Feb 25.
- Much below normal temperatures across portions of mainland
Alaska, Fri-Sat, Feb 21-Feb 22.
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