Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Mon Sep 24 2018

Valid 12Z Fri Sep 28 2018 - 12Z Tue Oct 02 2018

...Wet Pattern for the upper Aleutians/Alaskan Peninsula/Southwest
Alaska with locally Heavy Rains midweek into the weekend...

...Pattern Overview and Weather/Hazards Highlights...

Guidance highlights an amplified and blocky larger scale flow
pattern comprised by closed deep layer cyclonic circulations
centering over the southern Bering Sea and over/south of the
Aleutians and the Alaskan Peninsula, with multiple waves moving
through the base of the mean trough in the north Pacific. Eastward
progression of embedded systems remain inhibited by a deep layered
downstream ridge/closed high shifting from the east-central Gulf
of Alaska to the Alaskan interior. Fronts hang up parallel to the
flow on the western periphery of the high as coincident with a
persistent moisture flux to fuel a prolonged pattern favoring
heavy rains for the Aleutians/AK Peninsula and southwest/southern
Alaska midweek into the weekend.

...Guidance/Predictability Assessment...

There is above normal forecast predictability for the
aforementioned larger scale flow pattern and  heavy precipitation
threat. However, uncertainty with the timing and emphasis of
embedded smaller scale systems in the models instead suggest an
ensemble mean forecast approach. Accordingly, the WPC Alaskan
medium range product suite days 4-8 was mainly derived from a
blend of the reasonably compatable latest GEFS and ECMWF ensemble
means. Applied greater weighting in this blend to the more
amplified solution of the ECMWF ensemble mean given pattern
amplitude trends.

Schichtel


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html