Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 PM EDT Sat Sep 22 2018

Valid 12Z Wed Sep 26 2018 - 12Z Sun Sep 30 2018

...Wet Pattern for the upper Aleutians/Alaskan Peninsula/Southwest
Alaska with locally Heavy Rains mid-later next week...

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

The models show a closed deep layer cyclonic circulation drifting
east into the Bering Sea.  As this occurs a deep layer ridge
drifts east from mainland AK and eventually into the Yukon.

An initial surface low emerges from east Asia into the Bering Sea.
Next Thu 27 Sept- Fri 28 Sept the ECMWF and Canadian global
indicate a second area of low pressure moves north from the
northern Pacific and crosses the Aleutians, followed by a merger
of low pressure systems.

The blocking pattern inhibit eastward surface frontal progressions
over the northeast Pacific/Gulf of Alaska. Fronts will hang up
parallel to the flow on the western periphery of the high as
coincident with a persistent moisture flux to fuel a period of
heavy rain that will be locally heavy in the AK Peninsula and
southwest/southern Alaska midweek through later next week.

This is when the agreement breaks down as differences in phasing
lead to differences in location/intensity of the surface low.
There is a large cluster of lows plotted in the spaghetti diagrams
with a lack of clustering/overlap of low positions.  This leads to
a blend of the 00z ECMWF/Canadian models and the ECMWF ensemble
mean/06z GEFS Mean.

The next system moving from the west Pacific mid week may move
south of the lower Aleutians by Fri 28 Sep.  The 00-06z GFS/00z
Canadian/ECMWF all have a low that becomes strong, so this system
is well worth tracking.  A blend of the models and 06z GEFS mean
will help mitigate the location/intensity differences.

Petersen


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