Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
614 PM EST Fri Dec 27 2019
Valid 12Z Tue Dec 31 2019 - 12Z Sat Jan 04 2020
During the early to middle part of the period (Tue-Thu) the best
clustering of guidance currently shows an axis of lowest heights
aloft from near the Bering Strait across the eastern Bering Sea
and into the Gulf of Alaska. Evolution of dynamics within this
region of troughing will support a northeastward drift of strong
low pressure most likely to be just south of the Alaska Peninsula
as of early Tue and reaching the northern Gulf of Thu. By Fri-Sat
the majority of guidance expects some energy aloft to linger
over/near the mainland, with some debate as to how much eastward
progression may occur. The most likely scenario provides enough
movement to bring the northern Gulf surface low closer to the
Panhandle by days 7-8 Fri-Sat.
For the storm initially near the Alaska Peninsula the ensemble
means and ECWMF runs have exhibited a pronounced southwestward
trend over the past 1-2 days. While some prior GFS runs were a
little too far west and the CMC is still somewhat farther east,
aforementioned trends and the 12Z ECMWF adjustment to the 12Z GFS
in principle for much of the first half of the period led to
starting with an even blend of those GFS/ECMWF runs exclusively as
the starting point for today's forecast into day 5 Wed and with
only very modest ensemble input through early day 6 Thu.
Upstream there is a general signal that a frontal wave will
develop and track along or near the Aleutians during the latter
half of the week. Operational models vary in strength and track
while among the means the GEFS mean is the only solution to depict
an identifiable wave. The operational spread that develops is
reflective of increasingly diverse shortwave differences within
progressive North Pacific/Aleutians flow during the latter half of
the period. By day 8 Sat the ensemble means are remarkably
agreeable for the overall pattern aloft across a large area,
depicting a deep upper low near Kamchatka anchoring a western
Pacific trough, mainland/Gulf troughing, and a ridge in-between.
The 12Z ECMWF becomes particularly questionable late in the period
as it brings upper ridging into the mainland next Sat while it is
also in the vast minority tracking a storm into the Bering.
Overall the 12Z GFS and old 00Z ECMWF are closer to the means.
Preferences after early Thu were to transition ECMWF input a
little more to the 00Z run into Fri while rapidly increasing the
GEFS/ECMWF mean input so that the means provided the exclusive
basis for the day 8 Sat forecast.
The storm system tracking from near the Peninsula into the Gulf
will bring strong winds and high waves to some areas, while
promoting a focus for heaviest precipitation from just east of the
Peninsula to the Panhandle. The early-middle part of the period
will feature below normal temperatures especially over
western/northern areas but with anomalies less extreme than prior
to the start of the forecast period. The southeast corner of the
state should see a brief period of above normal temperatures. By
Fri-Sat the potential eastward drift of the upper trough along
with surface high pressure building in from the west should lead
to a colder trend over much of the mainland.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Wed, Dec 30-Jan 1.
- High winds across portions of mainland Alaska, Mon-Tue, Dec
30-Dec 31.
- High winds across portions of mainland Alaska, Mon-Wed, Dec
30-Jan 1.
- High significant wave heights for coastal portions of the Alaska
Panhandle and mainland Alaska, Tue-Wed, Dec 31-Jan 1.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html