Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
700 PM EDT Mon Sep 06 2021
Valid 12Z Fri Sep 10 2021 - 12Z Tue Sep 14 2021
...Multiple rounds of heavy rainfall likely from the Peninsula
region to the Panhandle with accumulating snow possible in the
higher elevations...
...Pattern Overview and Model Guidance...
The weather pattern across Alaska should remain quite active
through the extended range as a mean upper level low meanders in
the Bering Sea through the weekend before eventually pushing east
into the western Mainland early next week. Shortwaves rotating
through the base of the trough will send a couple of surface low
pressure systems eastward through the Gulf and into western North
America. The latest suite of model and ensemble guidance continues
to show very good agreement on the overall large scale synoptic
pattern, especially through about day 6. After that, details of
individual systems becomes a little more uncertain, especially
regarding possible low pressure into the Mainland as well as the
next system coming into the western Aleutians by next Tuesday. The
12z GFS and ECMWF show fairly good agreement through the period
with the ensemble means, with the typical question on timing of
systems as you go later in the period. The WPC forecast relied on
a majority deterministic solution through day 6, trending towards
the ensemble means late period to mitigate some of the
differences. This maintains excellent continuity with yesterdays
forecast as well.
...Sensible Weather...
Moderate to locally heavy rainfall will accompany a deep low
pressure system present in the Gulf of Alaska on Friday as Pacific
moisture streams northeastward ahead of the cold front. Models
suggest the potential for several inches of rainfall along the
southern coast and the Panhandle region on Friday into Saturday
with local enhancement likely along favorable mountain ranges.
Marine hazards and coastal gusty winds are also likely from
roughly the Kenai Peninsula to the Panhandle coast. Behind this
system, the upper low in the Bering Sea moving into the Mainland
this weekend will bring another low pressure system along the
Aleutians and eventually into southeast Alaska by next Monday.
Another round of heavy rainfall is possible Sunday-Tuesday for the
Peninsula-Southern Coast-Panhandle region, with precipitation also
likely farther inland across southwestern and interior Alaska as
well. Lowering snow levels may bring accumulating snowfall to some
of the mountains across southern Alaska. The north slope region of
Alaska should remain dry through much of the period.
Temperatures across northern and eastern Alaska will be above
normal on Friday but should trend back below normal through the
weekend as the upper low moves closer to the region and heights
lower across the state. Elsewhere, clouds and precipitation should
keep temperatures near or below normal the entire period. With the
exception of parts of the southwest, morning lows across all of
Alaska should remain above normal through next weekend and into
early next week.
Santorelli
Hazards:
- Heavy rain across portions of mainland Alaska, Thu, Sep 9 and
Sun-Mon, Sep 12-Sep 13.
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Fri, Sep 10 and Sun-Mon, Sep 12-Sep 13.
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