Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
602 PM EDT Wed Sep 30 2020
Valid 12Z Sun Oct 04 2020 - 12Z Thu Oct 08 2020
...Heavy rain possible Sunday for portions of coastal Southcentral
to the Coast Ranges...
...Overview and Guidance/Predictability Assessment...
Upper troughing is forecast to move out of southwestern
Alaska/Bristol Bay late this weekend to the Gulf by the middle of
next week. Upstream troughing will move into the Bering Sea around
midweek next week as the flow flattens across the Pacific south of
Alaska. Several embedded disturbances will rotate through/around
the upper low, deforming it as additional shortwaves drop in from
the northwest. The 12Z guidance offered a reasonable starting
point for the forecast with better agreement overall into next
week.
Sunday system will likely pass just south of Kodiak toward/into
Prince William Sound by early Monday (parent low). Front will lift
into NW Canada with the trailing portion soutwest of Hadai Gwaii.
This may allow a wave of low pressure to move northeastward along
the boundary, perhaps nearing the southern Panhandle. Next system
from the northwestern Pacific will enter the far western Bering
around next Tue. By then, the upper pattern will start to flatten,
allowing zonal flow to increase. ECMWF ensembles were still a bit
quicker than the other guidance/ensembles, but the trend has been
for a slightly quicker system. Nudged the forecast quicker but
overall maintained good continuity. Increased ensemble weighting
by next Wed/Thu as the system slips eastward, perhaps over the
AKPEN.
...Weather/Threats Highlights...
Low pressure will spread modest to locally heavy rainfall over
southeastern-facing areas of Kodiak and the Kenai Peninsula toward
Prince William Sound eastward to Yakutat Sunday, gradually
diminishing as showers on Monday. Several inches of rain are
possible along with breezy conditions. Less rainfall is forecast
over the mainland after Monday, but will increase over the western
and then central Aleutians next Tue/Wed with the next system.
Progressive flow should limit heavier rainfall.
Temperatures are forecast to be near to well above normal over
most of the mainland, especially north of the Alaska Range. Near
to below normal temperatures are forecast for the Aleutians,
across portions of Southcentral, and over the Panhandle.
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 precipitation across portions of the Alaska Panhandle and
southern coast of Alaska, Sat-Mon, Oct 3-Oct 5.
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