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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1928Z Jan 11, 2019)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
228 PM EST Fri Jan 11 2019

Valid 12Z Tue Jan 15 2019 - 12Z Sat Jan 19 2019

Today's extended forecast continues to show reasonable agreement
with the broad scale mean pattern but significant uncertainties
for embedded details.  Overall expect mean ridging from western
Canada through Mainland Alaska into the Arctic while an elongated
mean trough extends from the Bering Sea southeastward.  The Bering
Sea/Aleutians part of this trough should become more
narrow/sheared toward the end of the period with the approach of
height falls ahead of a broad trough pushing into the western
Pacific.  Confidence remains below average for specifics of
individual low pressure systems tracking between the mean trough
and ridge aloft but there is still reasonable consensus that this
pattern will promote a multi-day period of focused
moisture/precipitation (with some shorter-term variability) most
likely to extend along the Alaska Peninsula to the Kenai
Peninsula.  Within the mean ridge the question will be
existence/location of a possible closed high and any shortwave
energy around its periphery, while front/low pressure detail
uncertainties persist late in the period over the Aleutians and
North Pacific.

If anything the forecast of the upper ridge extending over the
mainland looks a little more complex today, as a short range upper
high over the Arctic appears to maintain its identity longer into
the extended as it drops down toward the strengthening ridge to
the south.  Guidance presents various ideas on whether the upper
high maintains its identity or in some way merges with the ridge
extending into the mainland from Canada, with another upper high
possibly forming at some point in time.  The most basic guidance
difference comes down to ECMWF-based solutions and the CMC closing
off an upper high farther north/northwest than a number of recent
GFS/GEFS mean runs.  Teleconnections relative to the core of
positive height anomalies near the far northwest mainland in D+8
mean charts suggest that an upper high may end up somewhat
northwest of the 00Z/06Z GFS.  The 12Z GFS has adjusted its upper
high northward to provide some added support for leaning somewhat
more in the ECMWF direction.  However recent ECMWF/CMC depictions
of energy coming around the eastern/southern side of the upper
high have sufficiently low confidence to favor blending of
solutions/inclusion of means to downplay the strength of this
energy.

Details within/ahead of the elongated mean trough are quite
chaotic with individual models/ensemble members simply painting a
band within which a surface low may exist at any point in time. 
This leads to minimal confidence in any specific solution and
favors a blend/mean approach to represent the more agreeable large
scale pattern.  Farther west guidance shows some degree of
consensus/continuity on a front coming into the picture near the
western Aleutians by early day 7 Fri but then diverges regarding
possible triple point/trailing wave development.  Overall prefer
the ensemble means as a base with just enough operational input to
provide a weak depiction of possible wave development near the
Aleutians by day 8 Sat.

Based on above considerations the early part of the forecast
started with primary emphasis on 00Z ECMWF/ECMWF mean/CMC
solutions with a little 12Z GFS/06Z GEFS input.  The blend trended
to a near-even blend of model/ensemble guidance around day 6 Thu
and went 60-70 percent means by days 7-8 Fri-Sat.

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