Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
617 PM EST Sat Dec 28 2019
Valid 12Z Wed Jan 01 2020 - 12Z Sun Jan 05 2020
The best consensus of guidance suggests that elongated short-range
energy aloft will consolidate into a closed low that should track
from near the northern Alaska Peninsula eastward into the Gulf of
Alaska mid-late week, while an east-west ridge extends from
northwest Canada into the northern half of the mainland. This
northern ridge should then weaken and lingering energy will likely
keep upper troughing over the mainland to the end of the forecast
period next Sun. The upper level evolution will support a strong
surface low that should be near Kodiak Island and the Kenai
Peninsula early in the period and then settle into a position a
bit to the east for the rest of the period as the initial closed
low drifts eastward and the overall upper trough weakens
gradually. As was the case yesterday, a blend of the 12Z
GFS/ECMWF best reflects the recent southwestward trends for the
deep surface low early in the period, at least Wed into early Thu.
The UKMET/CMC continue to be farther east as of day 4 Wed.
Interestingly the models and means as a whole cluster better by
day 5 Thu. After early Thu recent GFS runs stray southward of the
means and most other models with the surface low, favoring a
reduction of GFS input at that point. Important specifics aloft
still exhibit some spread and variability so confidence in an
exact solution remains only moderate. Early in the period this
storm will produce strong winds/high waves over some areas and
promote a precipitation focus over the southeastern
coast/Panhandle. These locations will see a drier trend later in
the period.
Farther west there is good agreement in principle that a very
slowly moving trough (anchored by an upper low near Kamchatka)
will prevail over the western Pacific with some degree of upper
ridging over or near eastern Siberia. Progressive flow around the
western Pacific trough should continue across the North
Pacific/Aleutians and far southern Bering Sea, though with a
tendency for a weak mean ridge aligned over the central
Aleutians/Bering. This flow will carry multiple fronts/waves for
which individual model runs have been quite diverse and
inconsistent. From late week through the weekend the latitude of
the axis of lowest pressures has been up for debate as well,
corresponding at least part to strength and shape of the eastern
Siberia ridge aloft. The one wave that has had a fairly
persistent signal for existence reaches the western Aleutians by
12Z Thu. Thus far prefer to lean to the weaker side of guidance,
in the realm of the past couple ECMWF runs (weaker trend than
prior runs) or recent GEFS means, due to the dynamics passing
through a mean ridge. Trailing waves/fronts are likely but with
low confidence for details. By late in the period the average of
ensemble means would suggest that the latitude of lowest surface
pressures over the Aleutians/southern Bering Sea is most likely
south of the 12Z CMC/00Z ECMWF but perhaps somewhat north of the
12Z ECMWF that has the strongest eastern Siberia ridge aloft.
Exactly how North Pacific wave(s) may interact with mainland/Gulf
troughing aloft late in the period is another complexity in the
forecast.
The aforementioned 12Z GFS/ECMWF blend preferred for the deep
storm early in the period transitioned to nearly half ensemble
mean weight and lower GFS weight versus the ECMWF by day 6 Fri.
Ensemble input continued to increase with time, reaching 100
percent weight by day 8, as the means have remained agreeable for
the large scale pattern in the latter half of the extended
forecast time frame.
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
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