Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Tue Apr 30 2019
Valid 12Z Sat May 04 2019 - 12Z Wed May 08 2019
The large scale pattern across Alaska will remain relatively
unchanged through the extended forecast period. A mean trough will
slowly progress eastward across the Aleutians through the period,
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 during days 4-5 (Sat-Sun), with gradually
increasing weight placed on the ECENS/GEFS ensemble means during
days 6-8 (Mon-Wed).
The ECMWF idea of at least periodically breaking down the northern
extent of the Gulf ridge in response to passing shortwave energy
remains the preferred solution at this point during the early
portion of the forecast period. Teleconnections associated with
some of the stronger hemispheric 500 hPa height anomalies do not
show significant support for a strong Gulf of Alaska ridge,
leading to the conclusion that at least a temporary/periodic
weakening of the ridge northern extent seems feasible. An occluded
low pressure system is forecast to slowly move east across the
Aleutians Sat-Mon, moving into the southeastern Bering Sea by Tue.
Models/ensembles show general agreement on this scenario.
Differences begin to increase Tue-Wed once again with respect to
the Gulf ridge. The ECMWF/CMC and ensemble means support
rebuilding the ridge somewhat in response the further upstream
trough amplification across the Bering Sea/Aleutians. The GFS
differs, weakening the ridge and depicting flatter westerly flow,
which would direct more shortwave energy eastward into Southeast
Alaska and British Columbia. Also prefer the ECMWF/CMC/ensemble
idea here, at least partially restrengthening the ridge and
keeping the stream of shortwave energy directed more toward
mainland Alaska. The interior of Alaska will remain in a rather
chaotic regime in between the north Pacific upper-level easterlies
and arctic easterlies. Potential interactions between small-scale
features in the two streams introduce additional complications and
reduce predictability when it comes to the specific details. The
preferred blend should, however, at least provide a broad
representation of the forecast for that area.
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