Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
338 PM EDT Sun Sep 23 2018

Valid 12Z Thu Sep 27 2018 - 12Z Mon Oct 01 2018

...Wet Pattern for the upper Aleutians/Alaskan Peninsula/Southwest
Alaska with locally Heavy Rains mid-latter portions of this 
week...

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

The models show a closed deep layer cyclonic circulation drifting
east across the Bering Sea with multiple waves moving through the
vase of the mean trough in the north Pacific.  As this occurs a
deep layer ridge drifts east from mainland AK and eventually into
the Yukon.

An initial surface low in the Bering Sea occludes and weakens Thu
27 Sep.
On Thu 27 Sept- Fri 28 Sept the models 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 on Fri.

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 this week.

This is when the agreement breaks down as differences in phasing
lead to differences in location/intensity of succeeding surface
lows.

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.  The
anticyclone over AK holds, so the current model majority keeps the
closed low south of the ridge over the north Pacific.

The following system from the northwest Pacific has potential to
impact the Aleutians and possibly southern Bering Sea Sun 30 Sep.
The 06-12z GFS have a similar track to the ECMWF ensemble mean,
with the respective 00z today -12z Sat ECMWF runs showing location
differences of 600 nm 12z Sunday from run to run, so confidence in
either run is low. Given the differences in operational ECMWF
runs, more weighting was given to the 06-12z GFS and 00z ECMWF
ensemble mean.

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