Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 PM EDT Fri Sep 28 2018

Valid 12Z Tue Oct 02 2018 - 12Z Sat Oct 06 2018

Today's guidance agrees fairly well upon a blocky and generally
slow to evolve large scale pattern, dominated by a very strong
Arctic ridge/upper high (500mb heights up to 3-4 standard
deviations above normal) and southern Bering Sea closed low. 
Initial establishment of this mean closed low will correspond to
the arrival of the extratropical evolution of current Typhoon
Trami which will rapidly recurve over the extreme western Pacific
in the short range period.

Regarding extratropical Trami over/near the southern Bering Sea
plus the Arctic ridge/closed high aloft... Operational model runs
still appear to be attempting to latch onto specifics for track
and timing of the deep surface low while GEFS/ECMWF means have
been somewhat more stable over the past couple days.  The most
notable trend over the past 24 hours has been for the system to
reach farther eastward early in the period--likely due at least in
part to somewhat of a northward trend for the Arctic upper
ridge/high.  This trend provides some support for including the
faster GFS runs within a general operational model blend during
the first part of the forecast, yielding an intermediate
track/timing with better definition than the means.  Later in the
period operational models diverge for surface low track, with the
06Z GFS and eventually the 00Z ECMWF showing greater retrogression
than the means and other models while the 00Z CMC strays south of
the Aleutians for a time.  Meanwhile an average of operational
models would suggest keeping the Arctic upper high a little
stronger than depicted by the means.  Combining these
considerations leads to holding onto some 00Z-12Z GFS/00Z ECMWF
input through day 8 Sat but with increasing ensemble mean weight
mid-late period due to the increasing detail uncertainty over the
Bering and vicinity.

Consecutive operational model runs have been very inconsistent
regarding details of the col region/weakness between
Arctic/eastern Pacific ridges and Bering Sea upper low/northern
Canada cyclonic flow.  Although ensemble means have been
considerably more stable--favoring their use for the deterministic
forecast--guidance historically has had a notoriously difficult
time resolving important details of such an evolution in the
extended time frame.  Thus confidence is not particularly high yet
for some important specifics aloft over portions of Mainland
Alaska and the Panhandle.

This aforementioned uncertainty leads to significant guidance
spread for how much moisture ultimately reaches locations to the
east of the Kenai Peninsula and the Panhandle.  There is a more
coherent signal for enhanced precipitation early-mid period from
the Alaska Peninsula to the Kenai Peninsula, ahead of the frontal
system extending from the Bering Sea storm.  Overall expect many
locations from the Bering Sea through North Pacific to see a
period of strong winds.  There will also be a tight pressure
gradient between the Bering low and Arctic/northwestern Canada
high pressure to support brisk winds over parts of Mainland Alaska
as well.  

Rausch


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