Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Sun Sep 16 2018

Valid 12Z Thu Sep 20 2018 - 12Z Mon Sep 24 2018

A relatively amplified flow pattern appears likely to persist
across Alaska during the extended period. Interactions between
multiple streams, one crossing the North Pacific, and another
originating in the Arctic, introduce a rather large degree of
complexity and uncertainty into the forecast, with model solutions
continuing to vary from run-to-run. The initial forecast concern
early in the period (Thu) is what happens with small-scale but
energetic shortwave energy across the Interior. Over the past few
days models have waffled between digging the energy south into the
Gulf as it amplifies, or moving it more eastward. Over the past
day, a fairly significant number of solutions/ensemble members
have tipped toward digging the feature southward (with the GFS
being the main outlier keeping it farther north). The ECMWF and
CMC showed similar handling of this feature, albeit with some
differences in timing. By next weekend, a number of models show
upper ridging strengthening south of the Aleutians with a long
wave trough amplifying across the Bering Sea. The CMC becomes the
outlier by day 8 (Mon) with a much flatter and more progressive
solution, whereas more support exists for an amplified and less
progressive setup. Thus, initially based the WPC forecast on a
majority deterministic ECMWF/CMC blend, with more weight shifting
to the ECENS/NAEFS ensemble means from day 6 onward.

In terms of sensible weather, this pattern will become
increasingly wet for Alaska through the extended period.
West-southwesterly mid-level flow, eventually becoming
southwesterly by the weekend as the upper trough amplifies across
the Bering, will spread increasing amounts of moisture across the
Aleutians and western mainland Alaska, which will fall vary
between rain and snow depending on the location.

Ryan


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