Prognostic Meteorological Discussion
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FXAK02 KWNH 241947

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Sat Mar 24 2018

Valid 12Z Wed Mar 28 2018 - 12Z Sun Apr 01 2018

The large scale pattern from the north Pacific and Alaska is
expected to shift toward a blockier setup during the medium range.
Models/ensembles show good agreement that a decaying/barotropic
low pressure system will be located along the western coast of
mainland Alaska on day 4 (Wed) as it quickly dissipates. Farther
east, model solutions continue to trend toward better agreement on
a stronger ridge initially from the Gulf north to the Arctic
Ocean. The GFS remains the weakest, although it has continued to
trend toward a stronger ridge over the past 24 hours. The ECMWF
and CMC now show similar solutions with respect to the ridge with
support from a significant number of ECENS and CMCE/NAEFS members.
The continued trend toward a stronger ridge/blocker flow has also
resulted in a slower trend for shortwave energy which is now
expected to enter the Gulf on Thu (about 24 hours slower than
previous forecast). The GFS continues to lag this trend and was
thus excluded from the forecast as in previous days. As in recent
days, WPC continued to favor a blend of the ECMWF/CMC along with
the ECENS/NAEFS ensemble means.

Confidence was sufficient to use a majority of deterministic
guidance in the blend on days 4-5. This solution shows a surface
low entering the Gulf of Alaska on Thu, nearing Southeast Alaska
by Fri. Uncertainty continues to increase later in the period as
the pattern becomes more blocky across the North Pacific. The
GFS/GEFS remain very aggressive with building a ridge across the
Bering Sea by mid to late next week, with a deep cutoff low over
the North Pacific, with perhaps formation of a Rex block. The
ECMWF/CMC and their respective ensembles have shown varying
degrees of support for this scenario, but both have trended toward
a stronger ridge in recent runs (especially the ECMWF). All
deterministic solutions by this time continue to show a fairly
significant degree of run-to-run variability with respect to the
exact strength and orientation of the ridge and resultant flow
pattern by late next week. Further, dependent on the amplitude and
position of the Bering ridge, there is some potential for Arctic
shortwave energy to dive southward into the state by next weekend.
Given these considerations, weighting was shifted toward the
ECENS/NAEFS means in the forecast beyond day 5.

In terms of sensible weather, rising heights across Alaska will
mean gradually increasing temperatures through the medium range
period. Initially near average temperatures early next week will
reach several degrees above average by mid-week. Southeast Alaska
will see the greatest potential for precipitation Wed-Fri under
the influence of the Gulf low pressure system. Additional
scattered snow showers will be possible across west central
mainland Alaska as the decaying Bering low lingers through much of
the week. Meanwhile, ridging across east central/northern Alaska
will result in dry and relatively cloud-free conditions at least
through mid-week, with some potential for an increase in snow
shower activity by Friday or Saturday if Arctic shortwave energy
is able to move southward into the region.


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