Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
711 PM EDT Wed Aug 18 2021
Valid 12Z Sun Aug 22 2021 - 12Z Thu Aug 26 2021
...Heavy rainfall possible across portions of southern Alaska and
the Panhandle...
...Overview and Guidance Evaluation...
An amplified pattern will take shape over Alaska during the
extended range period. An initial closed low lifting into the
Bering Sea by Sunday should slowly drift eastward into the
Mainland Monday-Tuesday. This should send a frontal system across
the southern Coast towards the Panhandle region Sunday-Tuesday.
Behind this, another shortwave dropping into the Bering/central
Aleutians by mid-week next week will help to maintain broad
extended troughing over Alaska through day 8. To the north, a
compact closed upper low will slide just north of the state,
driving a cold front and additional troughing into the northern
half of the region.
The latest 12z cycle of models and ensembles show good synoptic
agreement on the overall pattern through the period. There is good
enough agreement in the deterministic solutions to warrant a
general model blend between the 12z GFS, ECMWF, and CMC for days
4-5. After that, uncertainty remains high in the details of
individual systems and shortwaves rotating through the base of the
trough into the Aleutians. The 00z ECMWF/ECENS solutions showed
some timing inconsistencies with the rest of the guidance, but the
12z runs came in much closer to the consensus. As such, the WPC
progs for days 6-8 used an increasing weighting of the ensemble
means to help mitigate some of the differences. Did maintain some
degree of deterministic solutions just to help maintain system
definition later in the period.
...Weather/Threats Highlights...
The system expected to track into the southern-southeastern Bering
Sea and the Mainland should bring wet and unsettled weather across
much of southern Alaska and into the Panhandle. Southerly flow
ahead of the attendant cold front in the Gulf will direct
significant moisture into southern Alaska to fuel moderate to
heavy rainfall initially for parts of the Peninsula region on
Sunday, steadily shifting eastward with time across the southern
coast and into parts of the Panhandle Monday-Tuesday. Rainfall
should wane across western portions of this area after the
weekend, but could linger a few extra days across southeast Alaska
and the Panhandle as week troughing hangs out into mid-week. Some
of the highest elevations across southern Alaska may also see
measurable snowfall. Gusty winds are also possible along coastal
locations as the system moves through and eventually weakens as
the cold front moves into western North America. Some
precipitation may also accompany the cold front dropping into the
North Slope region on Sunday, but with lighter totals.
The forecast continues to show below normal high temperatures over
much of the state through the period but with some daily
variability with system progression. Anomalies are likely to be
the most below normal across the North Slope region behind the
arctic boundary and linger into early next week. Overnight lows
should be more moderate with various areas of above/below normal
readings. Best potential for above normal lows will be over
southern and eastern parts of Alaska.
Santorelli
Hazards:
- Heavy rain across portions of mainland Alaska, Sun, Aug 22.
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Mon-Tue, Aug 23-Aug 24.
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