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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1952Z Jan 14, 2019)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 PM EST Mon Jan 14 2019

Valid 12Z Fri Jan 18 2019 - 12Z Tue Jan 22 2019

Most guidance is showing a pattern transition, from a somewhat
complex blocky regime over Mainland Alaska and surrounding areas
toward one consisting of a broad mean trough settling over the
western/central Pacific and a building ridge over the eastern
Pacific extending into northwestern North America.  Individual
solutions show a variety of detail possibilities over the course
of the period but broadly suggest the potential for one or more
periods of enhanced precipitation/winds from the Aleutians across
the Alaska Peninsula and southern coast to the Panhandle.

Within the initial blocky pattern there has been considerable
spread for an Arctic upper high and energy to the south of this
high.  In recent days the ECMWF/CMC had been most consistent in
forecasting the high to be fairly persistent and positioned along
or north of the northern coast of the mainland.  However today the
CMC has switched toward the GFS/GEFS mean and some other solutions
that displace the high southeast into northwestern Canada while
the ECMWF mean has adjusted its overall upper ridge somewhat
eastward as well.  There is better model agreement regarding a
compact upper low that may retrograde over the northern half of
the mainland but a weaker/northward change in the 12Z GFS tempers
confidence.  A compromise among the 00Z ECMWF and 06Z GFS/00Z CMC
provides the best way to reflect latest ideas for this part of the
forecast.  Meanwhile there is still a decent signal for a low at
the surface and aloft near the southwestern Alaska Peninsula at
the start of the period 12Z Fri but with a lot of north-south
model/ensemble spread (as in previous days) favoring an
intermediate forecast.

Over the past day the ensemble means have improved their
definition of a strong system expected to track northward into the
Gulf of Alaska during the weekend.  The 06Z GFS/00Z ECMWF are
closest to the means for track of this system which should
ultimately weaken by later Sun, with these models and the means
providing decent continuity from yesterday.  The full
model/ensemble envelope has a lot of east-west spread so there is
still a risk of meaningful adjustment in future forecasts.

Well-advertised upper troughing that overspreads the
western/central Pacific will favor evolution of multiple low
pressure centers most likely to track along or just south of the
Aleutians.  The best agreement is for one wave near the western
Aleutians as of early Fri and then development of a stronger low
south of the central Aleutians during the weekend.  Thereafter
models and ensembles show a variety of ideas involving leading
triple point wave(s) to the east/northeast as well as for
additional western Pacific features that may interact with or
displace leading low pressure.  The ensemble means continue to
provide the most stable starting point for the forecast while the
06Z GFS/00Z ECMWF offer some detail enhancement into days 6-7
Sun-Mon.  Confidence is not especially high in the merging
scenario for low pressure depicted by the means by day 8 Tue but
still prefer the means for representing the large scale pattern
since it is probably even more unlikely for an alternative model
solution to have persistence in future cycles.  Note that systems
to the south of the Aleutians on the days 7-8 manual progs
represent two separate features, the first one rapidly weakening
and replaced by the second.

Based on today's considerations the forecast started with a 00Z
ECMWF/06Z GFS/00Z CMC blend on day 4 Fri.  Increasing spread
supported a gradual increase of 06Z GEFS/00Z ECMWF mean weight
thereafter and reaching 75 percent by day 8 Tue.  The CMC's
details departed from preferences earlier than the other models so
it was eliminated after mid-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