Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
233 PM EDT Wed May 01 2019
Valid 12Z Sun May 05 2019 - 12Z Thu May 09 2019
The large scale pattern across Alaska will remain relatively
unchanged through the extended forecast period. Mean upper
troughing is expected to persist across the Bering Sea, with
somewhat transient upper ridging centered across the Gulf of
Alaska, and a more persistent ridge of high pressure farther north
in the Arctic Ocean. This flow pattern results in a constant
stream of shortwave energy crossing near/south of the Aleutians
and moving into mainland Alaska, with additional shortwave energy
rotating around the southern periphery of the arctic ridge
affecting the North Slope. The WPC forecast was initially based
heavily on the ECMWF/GFS during days 4-5 (Sun-Mon), with gradually
increasing weight placed on the ECENS/GEFS ensemble means during
days 6-8 (Tue-Thu).
The ECMWF/GFS handle the shortwave/upper low crossing the
Aleutians, and the weakening of the Gulf ridge, similarly during
the Sun-Mon time frame. Farther to the north, differences with
respect to arctic energy rotating westward around the southern
periphery of the arctic ridge are primarily confined to intensity,
so a blend of the two solutions should represent a consensus
solution well. The GFS starts to differ quite significantly from
day 6 onward, bringing a deep low pressure system into the western
Bering Sea, with little additional deterministic or ensemble
support. Thus, the GFS was excluded from the forecast blend
starting on day 6. The ECMWF is in general agreement with ensemble
means that a slowly progressing shortwave and associated
occluded/filling low pressure system should cross the Alaska
Peninsula during the early-middle portion of next week, perhaps
pushing into the interior by Thu as the system quickly weakens.
Overall, the forecast was fairly similar to continuity.
Ryan
Bullet points from the WPC Days 3-7 Hazards chart can be found 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