Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
525 PM EDT Sat Sep 26 2020
Valid 12Z Wed Sep 30 2020 - 12Z Sun Oct 04 2020
...Overview and Guidance/Predictability Assessment...
Model/ensemble guidance is agreeable and consistent in showing a
very amplified upper pattern during the medium range, featuring a
strengthening ridge over western Canada and a mean trough (with a
couple of embedded lows) over and extending well south from the
western mainland. Upstream, a high amplitude ridge over northeast
Russia means there is confidence the closed low centered south of
AK over the north Pacific will be slow to move through next
weekend.
Guidance suggests above average confidence in the first cyclone
moving north across the upper Aleutians/AK peninsula on Wed into
the southeast Bering Sea/coastal southwest AK Thu morning. This
means a forecasts consisting of a blend of the ECMWF/GFS/UKMET
should work to mitigate minor timing/intensity differences.
There is more spread for the following Fri-Sat cyclone, as the
GFS/ECMWF/Canadian models continue to have run to run changes plus
differences with each other. Consequently, the forecast gave low
weight to the 06-12z GFS and 00-12z ECMWF, as well as the
respective GEFS Mean and ECMWF Ensemble Mean. The
multi-model/multi-ensemble blend was needed, given significant run
to run changes in the GFS and ECMWF, which means low confidence in
any one model run yet until models start to converge and have
improved continuity.
...Weather/Threats Highlights...
The pattern evolution will favor increasing potential for heavy
precipitation from the AK Peninsula to the northern Panhandle.
The first surface low tracking northward towards Kodiak Island,
should advect moisture into the Gulf of Alaska and eventually the
southern Coast/northern Panhandle early next week, with gale force
winds likely to accompany the cyclone.
The next system will probably bring another round of moderate to
heavy rainfall late next week into next weekend, but the timing
and intensity details remain in question. The strength of the
lows in the ECMWF, GFS, Canadian, and German ICON favor another
surge of high winds and heavy rain for Kodiak Island to the
southern coastal areas, including areas from the Kenai peninsula
east to the northern panhandle.
If this storm materializes at the end of next week, a flooding
threat will develop as well, so this system is well worth
monitoring.
Expect widespread coverage of above normal lows prevailing across
much of the Alaska mainland. The one area of below normal
temperatures is near the upper trough in the lower Aleutians.
Petersen
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