Prognostic Meteorological Discussion
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FXAK02 KWNH 201906

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Tue Mar 20 2018

Valid 12Z Sat Mar 24 2018 - 12Z Wed Mar 28 2018

Already during the first half of the period guidance develops some
fundamental differences in the pattern over the Arctic and
mainland Alaska, with two general clusters evident.  The
GFS/GEFS/NAVGEM form one group which displays more of an omega
block pattern with a narrow ridge.  The GFS/GEFS in particular
move the ridge rapidly northward leading to quicker arrival of
westerly mean flow.  On the other hand the ECMWF/ECMWF
mean/CMC/UKMET show a stronger/southward closed high with
retrograding upper low energy crossing the mainland leading to a
rex block configuration.  Recent ECMWF mean runs have trended
noticeably stronger/southward with the upper ridge.  This trend
along with tendency for upper ridges to be slower/more persistent
than at least the weaker side of the spread, plus yesterday`s
coordinated forecast favoring more persistent Arctic ridging,
provide support for holding onto more Arctic ridging than GFS
runs.  There is still a lot of uncertainty for how much energy
does retrograde across the mainland given guidance spread and
occasional tendency for models to be overly aggressive with
strength/speed of energy retrograding beneath upper highs.
Therefore prefer a blended approach that maintains the integrity
of the Arctic ridge but tones down the detail of any energy that
retrogrades across the mainland.

There is also meaningful uncertainty with northern Pacific/Bering
flow.  As a first consideration, recent GFS runs have been faster
than most other solutions with a shortwave forecast to be emerging
from eastern Asia at the start of the period.  This difference
likely plays a role in the forecast downstream with the majority
of solutions tending to be somewhat more amplified than the GFS
with the upper trough crossing the Bering and showing somewhat
more phasing with short range northwestern Pacific energy.  The
majority evolution aloft leads to potential for a farther north
track of the Aleutians system versus continuity but it is still a
question to what extent.  The 00z ECMWF/ECMWF mean made a fairly
significant northward adjustment from previous runs while CMC runs
have been showing a compromise between the ECMWF and more
suppressed GFS.  For a time the 12z GFS does show a farther north
track than its previous runs.  00z ensembles include the 00z ECMWF
solution but somewhat more members favor a track at least a little
farther southward.  Overall an intermediate solution looks best

Toward the end of the period there is better than average
model/ensemble agreement on the idea of an amplified central
Pacific upper trough with best defined surface low pressure well
south of the Aleutians by the end of day 8 Wed.  Confidence is
much lower in any leading surface lows such as what the past
couple ECMWF runs bring into the vicinity of the southwestern
Alaska Peninsula.  Thus would like to dampen out this feature
before it reaches 50N latitude.

Guidance agrees on the existence of leading system over the Gulf
of Alaska early in the period but with differences in
strength/track.  Ensemble means and other operational models
generally suggest some combination of a better defined and/or
farther north wave than some recent GFS runs.

Combining considerations for each area/system of interest, the
forecast blend started with input from the 00z and 12z/19 ECMWF,
12z GFS, and 00z CMC/UKMET early in the period.  From mid-period
onward the forecast included a steadily increasing weight of the
06z GEFS/00z ECMWF mean (higher percentage for the latter) so that
day 8 Wed used the means exclusively.


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