Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
650 PM EDT Thu Aug 20 2020
Valid 12Z Mon Aug 24 2020 - 12Z Fri Aug 28 2020
...Overview and Guidance Evaluation/Preferences...
Most guidance is fairly similar in depicting southeastward
elongation of initial upper trough/low energy over eastern Siberia
and the Bering Sea with the core of the energy ultimately tracking
across the Gulf of Alaska and possibly into western Canada by late
in the week, followed by upstream energy leading to a potentially
stronger Aleutians/Bering Sea feature after midweek.
A delay in 12Z ECMWF output prevented that run from being
examined/used in today's forecast. A blend of the 12Z
GFS/CMC/UKMET and 00Z ECMWF provided a reasonable starting point
for days 4-5 Mon-Tue, reflecting a modest triple point wave that
develops near the southern coast of the mainland. By mid-late
week the models/ensembles diverge for timing of energy that
reaches the Gulf of Alaska region and the system reaching the
Aleutians/Bering Sea, with additional question marks for other
details of this latter evolution. Timing differences essentially
fall into two clusters. The faster consists of GFS/ECMWF/ECMWF
mean runs and the slower includes the CMC/CMC mean/GEFS mean (plus
the UKMET by the end of its run). At least through the 00Z cycle
the ECMWF mean has been the most consistent piece of guidance, and
with deeper low pressure than other means, while the overall
progression of flow appears to provide a little more support to
the faster cluster. However the slower option is well within the
realm of plausibility and typical error range several days out in
time so some account for that scenario is reasonable. Thus the
early-period operational blend trended to a model/mean solution
that included 40/30 weights of the 12Z GEFS/00Z ECens but enough
lingering 12Z GFS/00Z ECMWF input to tilt the end result a bit to
the faster side. The 12Z ECMWF/ECMWF mean generally maintain
support for the progressive cluster, while the ECMWF mean has
adjusted south for the Aleutians system--a nod to some operational
runs.
...Weather Highlights/Threats...
The upper trough/embedded low(s) elongating from Siberia and the
Bering Sea into the Gulf of Alaska, along with possible low
pressure, should support organized areas of precipitation over
portions of the mainland and Peninsula/Panhandle--especially in
the Mon-Wed time frame. Continued progression of the upper
dynamics would promote a drier trend later in the week. Best
potential for highest precipitation totals will be near the
southern coast and Panhandle. By the latter half of the week the
system forecast to reach the Aleutians/Bering Sea will be
accompanied by an area of enhanced precipitation and stronger
winds. Some effects from this system could reach the western
mainland next Fri if the faster half of the spectrum verifies.
The south-southeast and Panhandle should see mostly below normal
highs through next week. Northern areas will likely see above
normal highs on Mon followed by a cooling trend to near or
slightly below normal readings, though some warmer pockets may
persist. Elsewhere expect a mix of above or below normal readings
depending on location/elevation. For min temperatures above
normal anomalies will prevail with only some scattered areas of
near to below normal readings.
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
No significant hazards are expected over Alaska during this
forecast period.
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