Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
614 PM EDT Mon Sep 21 2020
Valid 12Z Fri Sep 25 2020 - 12Z Tue Sep 29 2020
...Overview and Guidance/Predictability Assessment...
One upper low rotating south of the region will drive a surface
low into the Gulf and northward towards the southern coast next
weekend into early next week. Another upper low drops through the
Bering Strait next Friday, eventually establishing mean troughing
over much of the state by early next week. Finally, the next storm
enters the picture by day 8/Tuesday with a deepening surface
low/frontal system just south of the Western Aleutians.
There is decent agreement in the models on the overall large scale
pattern, especially days 4-5. After that, there remains question
on the details of the surface low in the Gulf as it drifts
northward/weakens and also with a shortwave rotating underneath
the broader Alaska trough, well south of the region. With this,
the WPC blend was based on a majority 12z GFS/ECMWF compromise
days 4-5, with increasing usage of the ensemble means (00z
ECENS/12z GEFS) thereafter to sort out detail uncertainties. The
next storm system entering the western Aleutians late period
actually has good agreement between the 12z GFS/ECMWF, so opted to
maintain some of the deterministic run as late as day 8 to help
maintain a meaningful surface low.
...Weather/Threats Highlights...
Heavy rainfall is possible along the Southern Coast into parts of
the Panhandle region associated with the surface low entering the
Gulf and a frontal boundary lingering for a few days. Some snow
may fall in the higher elevations. Rainfall should wane as the
surface low weakens, but another low lifting towards the region
may bring renewed moderate to locally heavy precip late period,
especially for the Panhandle region. Rainfall/unsettled conditions
should be on the increase by early next week across parts of the
Western Aleutians as the next storm system moves eastward into the
region. Mean troughing over much of the state, should keep
temperatures near or below normal for much of Alaska.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Fri-Sat, Sep 25-Sep 26.
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