Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
602 PM EDT Fri Sep 11 2020
Valid 12Z Tue Sep 15 2020 - 12Z Sat Sep 19 2020
...Heavy rain likely next week for southwestern/southern Alaska...
...Overview and Guidance/Predictability Assessment...
Changeable upper pattern across the Pacific will carry a robust
upper low through the Bering Sea into western Alaska next week.
Infused with the remains of former Typhoon Haishen, the surface
front is forecast to lift northeastward across the mainland before
weakening in favor of a surface wave out of the North Pacific
around Thursday. The 12Z deterministic models generally agreed on
the synoptic pattern though the 12Z GFS was quite different (slow)
with a lead-in upper low in the Beaufort Sea. Overall the 12Z
ECMWF represented a most reasonable forecast, followed by the
Canadian and GFS. Utilized a blend of mostly these models with
only a small minority ensemble weighting through next Thu to
preserve the incoming surface wave toward Kodiak. The ensembles
were too washed out with a smearing of the old Bering low with the
new Pacific low. By next Fri/Sat, trended toward much more
ensemble weighting as the trailing low lifts into the Interior.
Back to the west, upper trough rotating around the larger upper
low may carry another weak surface front across the Aleutians.
...Weather/Threats Highlights...
Periods of rain and breezy conditions will accompany the lead cold
front, with modest amounts possible over southwestern areas/AKPEN
early in the week along with locally heavier amounts. By next
Thursday, potential exists for much more rain along the southern
coast from Kodiak through the Kenai Peninsula into Prince William
Sound. Ensembles indicate the possibility of several inches of
rain Thu and Fri (each) with a surge of moisture from the south.
Pattern should turn more showery after the front moves eastward
into/past the Panhandle later in the week. Temperatures near to
above normal in advance of the lead front will trend cooler with
time as the upper low and colder temperatures aloft filter in.
Fracasso
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 rain across portions of mainland Alaska and the Aleutians,
Mon-Tue, Sep 14-Sep 15.
- Heavy rain across portions of mainland Alaska, Tue-Wed, Sep
15-Sep 16.
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