Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
545 PM EDT Sun Sep 13 2020
Valid 12Z Thu Sep 17 2020 - 12Z Mon Sep 21 2020
...Heavy rain Thursday for coastal southwestern/southern Alaska...
...Overview and Guidance/Predictability Assessment...
An upper low will meander near the Bering Strait during the
period, with shortwaves or upper lows rotating around it generally
over the Aleutians and into the Mainland. Lead system on Thursday
will likely be the most potent, traveling into the AKPEN/Gulf
(parent/triple point low) Thursday and through the Interior
Friday. Models continue to waver on the evolution, but still
prefer the parent low track toward Bristol Bay/SW Alaska with
triple point development just east of Kodiak as the whole system
lifts northeastward on strong southwesterly flow. Blend of the
GFS/ECMWF (which favored the farther west or east low) seemed to
get a good starting point which carried northward. Higher
latitudes will remain in weak troughing, modulated by the lead
system. Trend has been just a bit quicker overall with this first
system. Next system should dive through the Bering, across the
Aleutians, and find its way to the Gulf late Sat into Sun.
Ensembles still lose this system due to the spread, but GFS/ECMWF
and some Canadian blend maintained a feature that has actually
kept good continuity the past few days. Finally, last system out
of the NW Pacific showed good agreement between the 12Z GFS/ECMWF
and 00Z ECMWF ensemble mean (12Z GEFS mean farther south). This
could affect southwestern areas next week as the upper pattern may
retrograde a bit as can be the case.
...Weather/Threats Highlights...
On Thursday, potential exists for much several inches of rain
along the southern coast from the AKPEN/Kodiak through the Kenai
Peninsula into Prince William Sound. This could be enhanced by
triple point development just off the coast. Pattern should turn
more showery after the system moves northeastward/eastward
into/past the Panhandle late Friday. More focused rainfall may
return with the third system next Sun/Mon over the Aleutians per
the forecast of a defined low. Max temperatures will be near to
above normal over the Interior but generally near to below normal
over southern coastal areas into 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
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