Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Mon Oct 01 2018

Valid 12Z Fri Oct 05 2018 - 12Z Tue Oct 09 2018

Latest models/ensemble means maintain better than average
agreement and continuity for the large scale pattern evolution. 
They indicate that eastern Pacific ridging will steadily
strengthen/amplify over the course of the period--possibly
extending into Mainland Alaska by around next Tue--while a strong
Arctic ridge/closed high sinks southeastward while weakening.  At
the same time a closed low over the Bering Sea, possibly drifting
toward the western Aleutians for a time, should anchor a
persistent mean trough extending through the central Pacific. 
Within this agreeable large scale forecast there are some
pronounced uncertainties with the greatest originating from
tropical systems. 

For the northeast Pacific/Gulf of Alaska region, the focus will be
on the ultimate track and evolution of Hurricane Walaka whose
surface reflection may arrive by late weekend/early next week. 
Confidence remains very low for any specific solution for this
system, given combined uncertainties regarding interaction with a
southern mid-latitude upper low toward the end of the short
range/start of the extended time frames Thu-Fri followed by
northeastward timing/track around the western/northern side of the
eastern Pacific ridge aloft.  Per CPHC forecast the deterministic
charts will follow a compromise between the faster GFS runs and
slower 00Z ECMWF.  Trends/biases offset, with overall trends
appearing to be somewhat slower but guidance can sometimes be on
the slow side with recurving tropical features.  Latitude remains
a big question mark late in the period with strengthening of the
eastern Pacific mean ridge aloft appearing to support a somewhat
farther north track than the 00Z ECMWF.  On the other hand the 12Z
GFS has trended south of its previous runs.  Very low confidence
along with lack of any guidance that can provide a template for
the desired track/timing yield a conservative depiction in the
manual forecast.  Sensible weather effects in terms of wind and
precipitation continue to be very uncertain along the southern
coast of the mainland/Panhandle.

Typhoon Kong-Rey provides the other tropical uncertainty in the
forecast.  The solution envelope begins to broaden to the
southwest of Japan at the start of the period and continues
expanding thereafter, with possible tracks eventually ranging from
the central Pacific to west of Kamchatka.  For now leaning toward
the 06Z GEFS/00Z ECMWF ensemble means late in the period to
represent the most agreeable aspects of the large scale pattern
forecast would downplay any well defined representation of
Kong-Rey, with some hint of its energy getting incorporated into
the overall Bering/Pacific mean trough by day 8 Tue.

Other aspects of the forecast display better agreement/continuity
in principle.  The initially deep low over the Bering Sea
(originally associated with Typhoon Trami and strongest in the
short range time frame) will slowly continue to weaken as it loops
around toward the western Aleutians by the weekend.  During
mid-late period the 06Z-12Z GFS runs have adjusted toward less
southward elongation of upper trough energy which is in favor of
established consensus.  There are indications that one or two
Pacific waves may wrap into the mean low over the Bering but
without enough confidence thus far to depict in well-defined form.
 Early in the period a compromise among the 12Z GFS/00Z ECMWF/00Z
UKMET represents detail well for the Bering low, followed by the
previously mentioned trend toward the ensemble means as specifics
become more ambiguous.  Farther east there is a fairly consistent
signal for a frontal system/associated wave(s) to reach the Alaska
Peninsula/Kodiak Island during the weekend with weak low pressure
then continuing across the Gulf.  For the Arctic ridge/upper high,
guidance appears to have stabilized over the past day after prior
trends toward faster southeastward progression as it weakens.

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