Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Sat Sep 29 2018

Valid 12Z Wed Oct 03 2018 - 12Z Sun Oct 07 2018

Continue to expect a blocky and slow to evolve pattern dominated
by large scale features, though with embedded detail
uncertainties.  A very deep and concentrated closed low over the
Bering Sea at the start of the period will reflect a combination
of extratropical Trami and northern stream dynamics currently west
of Kamchatka.  The upper low should weaken somewhat and broaden
with time, while a couple separate shortwaves emerging into the
northwestern Pacific possibly have some influence on
shape/position of the large scale mean low by late week/next
weekend.  Meanwhile continuity looks good for the anomalously
strong (up to plus 3-4 standard deviations for 500mb heights) and
persistent Arctic ridge/upper high that should meander north of
the mainland through the period.  Farther south a ridge will
become more prominent over the mid-latitude eastern Pacific by
Fri-Sun. 

For the deep Bering Sea storm originating from Typhoon Trami,
early in the period a blend of the 06Z GFS/00Z ECMWF provides a
deeper starting point than the means while keeping the low track
near the middle of the ensemble spread.  Over the past day
guidance as a whole has trended deeper with operational runs
continuing to lead the means toward the deeper solution. 

There is decent continuity in principle on a leading triple point
wave tracking just south of the Alaska Peninsula into the Gulf but
with persistent timing differences, and generally faster trends
over the past day favoring a compromise between the ECMWF and
faster GFS.  Farther west across the North Pacific a number of
operational runs suggest that shortwave energy tracking
along/south of the Aleutians will support some degree of waviness
around late week but with great spread in track of such a wave. 
The composite of ensemble means provides a better match with the
00Z ECMWF versus the faster GFS runs as of early day 6 Fri but
after that time confidence rapidly declines in how the low
pressure/frontal system will evolve.  For now prefer to trend the
forecast toward the 06Z GEFS/00Z ECMWF means which would lead to a
weaker/more progressive system than the past couple ECMWF runs by
day 7 Sat.  This system may ultimately lead to another wave
reaching into the Gulf toward the end of the forecast period.

Also late in the forecast, operational models are a little more
pronounced than the means with the idea that the next northwestern
Pacific shortwave could pull the overall mean trough/upper low
somewhat westward/southwestward.  A model/mean compromise could
offer a reasonable starting point but model divergence for details
farther eastward (including for the eventual path of the initial
Bering Sea low) recommend staying closer to the ensemble means for
now.

A model compromise looks reasonable for resolving modest
position/strength differences for the strong Arctic ridge.  The
ECMWF/UKMET continue to be a bit stronger than the GFS/CMC. 
Differences still exist within the likely col region/weakness over
parts of the mainland and/or northwestern Canada but appear to be
somewhat less pronounced than over the past couple days.

Guidance is consistent in highlighting locations from the Alaska
Peninsula/extreme southwestern mainland to around the Kenai
Peninsula for highest precipitation totals during the 5-day
period.  With somewhat less spread for details within the
aforementioned col region/weakness aloft, today's solutions are
closer regarding how much moisture reaches the southeastern
coast/Panhandle--likely less than for locations to the west but
more than the drier end of yesterday's guidance envelope.  Early
in the period expect very strong winds to the south of the Bering
low with some areas of 40+ kts sustained and higher gusts. 
Elsewhere winds will be less extreme but still on the brisk side,
including portions of the mainland given the gradient between the
Bering low and Arctic/Canadian high pressure.

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