Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Mon Sep 09 2019
Valid 12Z Fri Sep 13 2019 - 12Z Tue Sep 17 2019
The very start of the forecast period on Fri will feature a strong
but weakening Bering Sea system nearing the western mainland and a
leading frontal wave emerging over the Gulf of Alaska. Behind
this overall system the focus turns to what has been the most
significant forecast problem in recent days, a shortwave forecast
to reach the western Pacific around Thu. Models and ensembles
have depicted a variety of possible evolutions at the surface and
aloft with effects extending downstream through the Bering
Sea/Aleutians and the mainland/Gulf of Alaska.
At the moment an overwhelming majority of guidance (00Z
ECMWF/UKMET/CMC/NAVGEM and the ensemble means) agrees that the
upper trough and likely embedded closed low will be sufficiently
progressive to bring a fairly strong storm system northeastward
through the Aleutians/Bering Sea and likely into the mainland.
The CMC differs in stalling it near the western coast. In
contrast recent GFS runs keep the western Pacific upper low more
separated from the westerlies, ultimately leading to a completely
different pattern downstream. The new 12Z GFS has trended
somewhat faster but is still significantly slower than remaining
solutions. The behavior of guidance leading up to this point
suggests additional changes are possible, but for preparing a
specific deterministic forecast it appears straightforward to
start with an operational blend of the ECMWF cluster early in the
period and then gradually incorporate the 06Z GEFS/00Z ECMWF means
into the forecast. While they offer differing details a minority
component of the ECMWF/CMC still provide a little enhancement to
the means through the end of the period. Remaining 12Z guidance
provides ongoing support for the favored majority cluster.
Over the Arctic the primary forecast consideration is to tone down
the combined depth/southward extent of the 00Z ECMWF upper low
that approaches the northern mainland by next Tue. Previous runs
did not depict this upper low. However the GEFS/ECMWF means show
a steady lowering of heights aloft through the period so there
seems to be potential for a feature to come into the picture over
the Arctic by late in the period. The preferred blend yields a
weak upper low to depict this possibility, with depth less extreme
than the 00Z ECMWF/CMC.
The leading system will produce late-week enhanced precipitation
along the southern coast and Panhandle. The next storm, expected
to track from the western Pacific through the Bering Sea and into
the mainland, would bring a broad field of brisk winds along with
potential for enhanced precipitation most likely to extend from
the Aleutians into the southwestern mainland, southern coast, and
eventually the Panhandle.
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
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Thu-Fri, Sep 12-Sep 13 and Sun-Mon, Sep 15-Sep 16.
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