Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2
FXAK02 KWNH 211915

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Wed Mar 21 2018

Valid 12Z Sun Mar 25 2018 - 12Z Thu Mar 29 2018

Guidance still displays significant spread for details of flow
aloft over the Arctic into mainland Alaska.  Compared to yesterday
when guidance was more evenly split or slightly in favor of an
ECMWF cluster, now there is less support for the 00z ECMWF/ECMWF
mean scenario of a persistent closed high over the Arctic with a
significant weakness over the northern mainland.  While support
for that exact evolution has waned somewhat, there is still decent
consensus that what ridging exists should be somewhat stronger
than seen in a number of recent GFS/GEFS mean runs.  Farther south
there is improved clustering for the system forecast to track
across the southeastern Bering Sea toward the southwestern
mainland.  An operational model compromise among the 06z GFS and
00z ECMWF/CMC/UKMET compares well to the typically weaker ensemble
means--though for a time the ECMWF mean is not far from a blend of
those model runs.  This blend--including enough ECMWF input to
reflect its consistency thus far in the Arctic and per
coordination with WFO AFG--provides some adjustment from
continuity for details over the Arctic into the mainland but
maintains a non-GFS consensus.

By the latter half of the period the most confident aspect of the
forecast appears to be the development of a deep central Pacific
trough/embedded closed low with a downstream ridge covering the
eastern Pacific.  Recent operational runs suggest the central
Pacific low surface/aloft may end up farther south than the
ensemble means, favoring at least modest inclusion of operational
guidance through the end of the period.

Elsewhere model/ensemble spread is sufficiently great to require
more emphasis on the ensemble means.  Confidence rapidly decreases
in determining shortwave details over the mainland and Bering Sea.
 Also there is significant uncertainty regarding the potential for
a mid-latitude Pacific wave to develop/track into the northeast
Pacific/Gulf of Alaska during the Tue-Thu time frame with possibly
significant wind/precip effects on the southern coast and/or
Panhandle.  ECMWF runs have been showing decent development but
with a 15-20 degrees longitude eastward trend over the past day.
GFS runs had been more muted/suppressed but the 12z run came in
with a strong system west of the 00z and 12z/20 ECMWF runs.
Support aloft comes from progressive mid-latitude Pacific flow
that tends to have lower predictability at the extended time
frame.  Overall there seems to be at least moderate potential for
such a system but so far the spread/variability in models and
ensembles has been too great to depict this system in a
deterministic forecast.  A blend of 70 percent total 06z GEFS/00z
ECMWF mean with the rest lingering components of the 06z GFS-00z
ECMWF/CMC (plus a little manual adjustment) yielded a compact
northern Gulf of Alaska system as of 12z Wed, though with at least
as much support aloft from remaining northern stream energy as
what may travel northeastward from the mid-latitude Pacific.  This
provides a reasonable starting point for depicting system
potential while waiting for better agreement/continuity in the


$$ is the U.S. government's official web portal to all federal, state and local government web resources and services.