Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM EDT Fri Aug 23 2019
Valid 12Z Tue Aug 27 2019 - 12Z Sat Aug 31 2019
Upper level ridging will shift into British Columbia as the medium
range period (Tues - Sat) begins, favoring stretched troughing
from the Gulf of Alaska south-southwestward into the Pacific and a
couple of weak surface low pressure areas moving along the main
baroclinic zone. Initial troughing along the Gulf Coast eventually
shifts inland, being replaced by upper level ridging as some kind
of organized system sinks south of about 50N in the Pacific.
Models today show better agreement than previous days, as previous
runs of the GFS had been advertising a stronger/more coherent low
pressure south of the Aleutians rather than a more stretched
trough as is the consensus today.
Elsewhere across Alaska, a main upper low meanders east/northward
as a possible shortwave rotates around it through the North Slope
region. The ECMWF continues to show a stronger representation of
that idea, and while plausible, a blend with the other guidance
suppressed its depth given continued uncertainty. In the Bering
Sea, high pressure dominates in the beginning of the period before
a system moves eastward along the Aleutians. Models also show
relatively good agreement on this system being reinforced by
rotating shortwaves, but continuing to show a slow buy steady
movement eastward into next weekend.
The blend today features a majority deterministic model blend for
days 4-5 (between the 00z ECMWF/12z GFS/00z CMC), with increasing
weighting of the ensemble means (ECENS/GEFS) from day 6 onward.
This mitigates any normal amplitude/timing differences and also
presents a forecast very close to yesterdays continuity.
In terms of sensible weather, a weakening system in the Gulf will
continue the threat for heavy rains across portions of the
Panhandle on Tuesday, with conditions likely to mostly dry out the
rest of the week. Scattered to locally heavy rain/snow possible
across the North Slope with the passage of a cold front, while the
next system in the Aleutians spreads rain from the western
Aleutians to the Peninsula late next week and into the weekend.
Santorelli
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 the southern portion of the Alaska Panhandle,
Tue, Aug 27.
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