Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Mon Nov 08 2021
Valid 12Z Fri Nov 12 2021 - 12Z Tue Nov 16 2021
...Overview and Guidance Evaluation/Preferences...
For a majority of the period the guidance agrees fairly well on
the overall pattern. The axis of a mean trough aloft will extend
from the mainland into the northeastern Pacific as of late this
week and will likely be very slow to progress eastward with time
while an upstream ridge drifts over the Bering Sea and Siberia.
By the latter half of the weekend, energy flowing around a trough
crossing the Arctic may drop into the mainland to reinforce the
mainland part of the mean trough while heights eventually rise
over the northeastern Pacific. The Northeast Pacific troughing
through the weekend should support two prominent surface systems,
one reaching the Gulf Friday-Saturday and another that tracks
toward or just south of the Panhandle around Sunday-Monday.
Meanwhile behind the upper ridge a negatively tilted upper trough
may close off a low near the western Aleutians, with the
associated surface system expected to track near the Aleutians
into the first part of next week. Surface high pressure over the
northern half of the mainland should become stronger with time.
Upper flow details become more ambiguous toward the end of the
period next Tuesday with guidance showing varied ideas for what
becomes of mainland trough energy as well as the upstream ridge
that could either stay open or close off a high over the northern
Bering Sea/Siberia.
Today's forecast started with a composite of 12Z operational
models from early day 4 Friday into early day 6 Sunday, with this
blend representing consensus ideas for meaningful features while
resolving lingering detail differences, and then adjusted to a
model/ensemble mean mix for the rest of the period. The second
northeastern Pacific system has a good signal in the operational
models and latest GEFS/ECMWF means but there is still a moderate
amount of spread and run-to-run variability for timing and track.
12Z and new 18Z GFS runs are a bit on the eastern side of the
spread with this feature. By the latter half of the period the
latest GFS runs have been quicker to break down the Bering Sea
upper ridge than most other guidance--including the GEFS mean--so
the forecast eventually phased out the GFS input. Meanwhile ECMWF
runs develop meaningful detail differences within the mean trough
over the mainland. Thus the late-period blend uses some of the
past two runs for ECMWF input while keeping a little bit of the
12Z CMC, with the 12Z GEFS/00Z ECMWF means providing the ensemble
component.
...Weather/Hazard Highlights...
The Friday-Saturday storm over the Gulf will bring a period of
organized precipitation to the Panhandle and possibly extending
back to the southern coast of the mainland with lesser amounts.
Some locally moderate to heavy activity could be possible over the
Panhandle. The next system around Sunday-Monday should track
farther southeast with the majority of associated precipitation
favoring the southern Panhandle. There is still some track and
timing uncertainty with this latter system though. Expect an area
of precipitation and a period of enhanced winds to accompany low
pressure tracking near the Aleutians Sunday onward.
Below normal temperatures will be most pronounced in terms of
anomalies and persistence over the southwestern mainland, while
the North Slope should see above normal readings (especially for
lows) through the forecast period. The eastern mainland and
Panhandle will initially see highs not too far from normal and
lows above normal, followed by a cooling trend toward more below
normal temperatures.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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