Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
547 PM EDT Sat Sep 12 2020
Valid 12Z Wed Sep 16 2020 - 12Z Sun Sep 20 2020
...Heavy rain likely next week for southwestern/southern Alaska...
...Overview and Guidance/Predictability Assessment...
Deep upper low will slowly progress through the Bering Sea next
week with additional shortwave spokes rotating around the
circulation. Lead surface front into western areas will weaken in
advance of a surface wave out of the North Pacific late Wednesday.
This low will likely deepen and track toward the AKPEN by early
Thursday. The 12Z deterministic models generally agreed on the
synoptic pattern with the UKMET again the apparent outlier. The
ECMWF/GFS took a more NW/SE track (respectively) around the AKPEN
but a blend of these with the Canadian seemed to get a plausible
solution with the main low into SW areas but triple point
development near Kodiak toward the southern Kenai peninsula early
Friday. Ensembles keep this front progressive into Canada with
another surface front driven by a shortwave out of northeaster
Russia drives a cold front across the Aleutians. This may round
the base back to the Gulf next weekend with another system from
the N Pacific perhaps moving through the western Aleutians next
Sunday. Ensembles were rather agreeable throughout with the
GFS/ECMWF near enough to allow for a majority deterministic
weighting to next Sat. This kept good continuity overall.
...Weather/Threats Highlights...
Periods of rain will accompany the lead cold front, with modest
amounts possible over Southcentral (especially south-facing areas
of the Kenai Peninsula). By Thursday, potential exists for much
more rain along the southern coast from the AKPEN/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. This could be enhanced by
triple point development just off the coast per the GFS. Multi-day
rainfall totals may approach 10 inches in favored areas. Pattern
should turn more showery after the system moves
northeastward/eastward into/past the Panhandle late Friday. Max
temperatures near to above normal over the Interior will trend a
bit cooler with time as the upper low and colder temperatures
aloft filter in. Max temperatures over southern coastal areas will
generally be near to below normal through the period. Min
temperatures will be near to above normal through the period.
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
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