Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
339 PM EDT Sun Sep 08 2019
Valid 12Z Thu Sep 12 2019 - 12Z Mon Sep 16 2019
From day 4 Thu into day 5 Fri there is good agreement/clustering
for the deep Bering Sea storm that represents the extratropical
evolution of Lingling and in some fashion incorporation of Faxai
(which is currently near Japan). Over the past day the ensemble
means have trended deeper at the surface and aloft toward prior
operational model consensus. Thus a blend of operational guidance
continues to provide a good starting point for the Bering storm as
well as a leading frontal wave forecast to be near the Kenai
Peninsula by early Fri and track eastward thereafter. The same is
true of a weakening surface low to the south of the Panhandle.
Immediately behind these features the model and ensemble guidance
rapidly diverges (among each other and between consecutive runs)
for specifics of shortwave energy that emerges over the western
Pacific, with significant influence on forecast details eventually
extending as far east as the mainland and Gulf of Alaska. Up to
the arrival of the 12Z GFS, there was a split in most recent
solutions--latest GFS runs and 00Z ECMWF closing off a slow closed
low versus the 00Z CMC/UKMET/NAVGEM with a phased progressive
trough. Ensemble members spanned the full range of operational
possibilities but with a majority progressive enough to yield a
broad trough in the means. Poor continuity in 12-hourly
operational runs up to this point favored trending the days 6-8
Sat-Mon forecast mostly to the ensemble means (with 10 percent of
the 00Z CMC whose pattern was closest to the means) to preserve a
reasonable degree of continuity until some confidence could be
gained for significant adjustments. Note that among the means the
ECMWF mean has been the most stable and thus merited the highest
weight in the blend. This approach would bring the weakening
Bering Sea trough/upper low into the southwestern mainland/Gulf
while surface low pressure settles over the Gulf.
The remainder of 12Z guidance suggests an increasing probability
that the western Pacific energy will separate to some degree but
not to the extent of the 12Z GFS. Maintaining some progression
appears more plausible given the progressive pattern already in
place and progression of upstream flow. However it is still quite
early to have any confidence in a specific scenario such as the
12Z ECMWF/CMC that ultimately show interaction between their
western Pacific to southwestern mainland storm and the upper low
over the northern Bering.
During the latter half of this week expect a broad area of
enhanced winds around the initial Bering Sea storm with leading
moisture bringing significant precipitation to parts of the
southwestern mainland/southern coast with lesser amounts
eventually extending into the Panhandle. The latest model trends
are increasing the potential for another episode of enhanced
wind/precip from the Aleutians into southwestern mainland by the
weekend--but with confidence too low to depict in the
deterministic forecast so far due to poor guidance
agreement/variability.
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