Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
653 PM EDT Sat Aug 22 2020
Valid 12Z Wed Aug 26 2020 - 12Z Sun Aug 30 2020
...Overview and Guidance Evaluation/Preferences...
Recent adjustments in the guidance have led to significantly
better clustering for the strong system expected to track into the
Aleutians and Bering Sea by late week/weekend. Overall trends in
the past couple days have been slower and southward, with earlier
solutions that were on the eastern side of the spread (including
the 00Z ECMWF and pre-00Z GFS runs) adjusting westward. Also the
consensus has gathered toward recent days of ECMWF/ECMWF mean
solutions that have generally advertised fairly early and strong
development. Helping to improve confidence is that the 12Z GEFS
mean finally shows a strong evolution after a number of runs were
inconsistent and generally quite weak. Even with the improved
agreement, there may be multiple sources that contribute to the
upper dynamics so the potential remains for some further
adjustment.
Farther east the models and ensemble means have settled into good
agreement for the upper low tracking from the southeastern Bering
Sea/Bristol Bay through the Gulf of Alaska Wed onward. There are
still some small-scale differences for the surface system which
have low predictability several days out in time. After this
system departs expect an upper ridge to build into the Northeast
Pacific and southeastern mainland/extreme western Canada.
Meanwhile the northern mainland and vicinity will be under or a
little south of a col region between features to the
south/southwest and upper ridging north of Russia/an upper low
near the Canadian Archipelago. There is somewhat more of a signal
today that flow aloft could push a cold front into the northeast
corner of the mainland.
In light of the better than average guidance guidance agreement
that extends even to the end of the period, today's forecast was
able to hold onto 75 percent total weight of the 12Z ECMWF/GFS/CMC
out to day 8 Sun after starting with all operational model input
early in the period. The minority ensemble weight came from the
12Z GEFS/00Z ECMWF means.
...Weather Highlights/Threats...
The much-improved agreement in latest guidance provides increasing
confidence in a strong storm system bringing a broad area of
precipitation and strengthening winds across the Aleutians and
Bering Sea after midweek. The enhanced winds and precipitation
with the leading fronts should reach the Alaska Peninsula and the
southwestern third to half of the mainland after Fri. The system
tracking from Bristol Bay and southwest corner of the mainland
through the Gulf of Alaska mid-late week will bring some focused
precipitation to areas along the southern coast and Panhandle,
with broader coverage of less intense/organized activity
elsewhere. Expect a drier trend from west to east before the
approach of moisture ahead of the Aleutians/Bering Sea storm.
The southern-southeastern mainland/Panhandle as well as a majority
of the North Slope will see below normal highs most of the period,
with the former tending to see the coolest anomalies. Near to
above normal highs are likely over some central mainland areas and
over the west. There will be greater coverage of above normal
readings for low temperatures. Highest anomalies for lows should
be over the northern mainland mid-late week and then shift to the
southwestern mainland next weekend with the arrival of
wind/moisture.
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
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