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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1916Z Aug 07, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
316 PM EDT Tue Aug 07 2018

Valid 12Z Sat Aug 11 2018 - 12Z Wed Aug 15 2018

The dominant focus of the forecast--both in terms of weather
effects and detail uncertainty--involves the extratropical
evolution of Typhoon Shanshan.  This system may bring a period of
heavy precipitation and strong winds to locations from the
Aleutians/Bering Sea into Mainland Alaska.

In the short range time frame the Joint Typhoon Warming Center
track has Shanshan brushing Japan and then recurving northeastward
into the northwestern Pacific.  Once it makes its extratropical
transition there is still significant uncertainty regarding
exactly how northern stream flow will interact with the system,
affecting strength and track.  Looking at guidance through the
arrival of the 12Z GFS, it appears that the 00Z ECMWF may be too
weak/suppressed relative to continuity and recent GEFS/ECMWF
ensemble means.  Tendency toward stronger rather than weaker
development when there is stream interaction also favors leaning
away from the 00Z ECMWF scenario.  The individual GEFS/ECMWF
individual members are still very diverse but the mean track,
through the Bering Sea and into the mainland, seems to provide a
good consistent basis for that aspect of the forecast.  This
leaves the 12Z GFS a bit south/southeast for a time.  Overall
prefer the 00Z-06Z GFS and 12Z/06 ECMWF as the best operational
input for the forecast with only modest ensemble mean inclusion
early in the period.  This produces a low pressure system with
depth between the GFS runs and much weaker ensemble means.  Most
guidance has adjusted noticeably faster from 24 hours ago so this
blend captures the trend.  Other notes... The CMC has had
difficulty in properly resolving/tracking Shanshan near Japan so
its forecasts would be considered less reliable.  Meanwhile the
CMC and parallel (FV3) GFS runs show much stronger downstream
ridging aloft versus other guidance.  This is certainly plausible
assuming a strong Bering Sea system but their surface solutions
are sufficiently different from other guidance and continuity to
preclude their use in the forecast.

Behind this system there is good consensus regarding high pressure
building into the Aleutians/Bering Sea.  Then the 12Z GFS becomes
somewhat faster than consensus with low pressure that may emerge
into the northwestern Pacific toward the end of the period.

The favored forecast blend was adjusted to reflect a
better-detailed compromise of an upper low expected to be near the
Bering Strait as of early day 4 Sat.  This area has high
sensitivity in terms of steering flow as the 06Z GFS and to a
greater extent 12Z GFS kick the low eastward across the northern
mainland while the ECMWF/CMC hold it westward and incorporate it
into the evolving northeast-southwest mean trough aloft.  So far
there are no clear trends recommending one side or other of this
broad envelope.  Once extratropical Shanshan moves inland and
weakens there is decent agreement in principle for gradual
eastward progression of the northeast-southwest mean trough aloft.
 The 06Z GEFS/00Z ECMWF means represent this well and agree for
the upstream pattern so the forecast increased total ensemble
weight to 50-70 percent later in the period.

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