Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Sat Sep 19 2020
Valid 12Z Wed Sep 23 2020 - 12Z Sun Sep 27 2020
...Overview and Guidance/Predictability Assessment...
A deep upper low in the Gulf of Alaska to begin the period
(Wednesday) should weaken as it shifts eastward into Western
Canada. Meanwhile, another upper low crosses the Bering Strait by
mid-late next week, establishing over the Bering Sea and
eventually the eastern Aleutians by next weekend. Several
shortwaves rotating underneath the Bering Sea low will eventually
send another storm system south of the Aleutian island chain into
the Gulf again to end the period.
There is decent agreement in the models on the overall large scale
pattern, but significant differences in the details, especially in
regards to timing/intensity of shortwaves across/south of the
Aleutians and into the Gulf. For days 4-5, the WPC forecast relied
on a majority deterministic model blend between the agreeable 12z
GFS/ECMWF. After day 5, leaned increasingly heavier on the
ensemble means (00z ECENS/12z GEFS) to mitigate the detail
uncertainties.
...Weather/Threats Highlights...
The initial surface low in the Gulf will continue to bring heavy
precipitation and gusty winds across parts of southern Mainland
and the Panhandle through mid-week. Showers should linger through
the period across the Aleutians into the mainland within an
otherwise unsettled pattern and as the next system enters the Gulf
next weekend. Much of the southern coastal areas and the North
Slope will have maximum daily temperatures near or below average
through the extended period while much of the Interior will
observe above average temperatures underneath of upper ridging.
Santorelli
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