Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
456 PM EDT Mon Sep 27 2021
Valid 12Z Fri Oct 01 2021 - 12Z Tue Oct 05 2021
...Powerhouse Storms to affect Alaska...
...Pattern and Guidance/Predictability Assessment...
A stormy pattern for Alaska and offshore waters will persist and
pattern forecast predictability remains above average overall.
However, vorticity interactions and phasing issues aloft are still
leading to pronounced main offshore surface low differences. A
composite of the compatible 12 UTC GFS/ECMWF/Canadian/UKMET seems
reasonable day 4/Fri, but I'd still lean more on ensemble means
(especially the ECMWF ensemble mean) days 5-8 given embedded
system development variance, albeit keeping in the blend smaller
input from the 12 UTC GFS/ECMWF for more detail consistent with
forecast spread.ÂThe ECMWF ensemble mean seems a closer match
with the GFS/ECMWF at longer time frames. WPC product continuity
is well maintained with this strategy and again planned manual
edits to ensure 2 main deep storms given upper support and
history.ÂIn particular, phasing aspects inherent in the GFS seems
near worst case for the weekend Gulf of AK storm given forecast
spread in both the other models and all the ensembles. I'd
consider it a less likely but plausible outlier.ÂThat said, as
consolidated in the Gulf it should prove to be a vigorous system
that WPC progs will continue to show. The same can be said for the
day 7/8 low slated to reach the southern Bering and Aleutians in
about a week that is currently Typhoon Mindulle. Overall WPC
product continuity was well maintained with these blends in
conjuncture with manual surface system modifications to ensure
sufficient offshore storm depth/intensity consistent with a highly
supportive and amplified upper flow evolution as a powerful closed
low/trough works from the eastern Bering Sea through the Gulf of
Alaska late week/weekend well in advance of approach from the
west-Pacific by early next week of a strong extratropical low
associated with current west Pacific Typhoon Mindulle.
...Weather/Hazard Highlights...
A vigorous storm system digging through the eastern Bering Sea
this week will slam into a highly unsettled Gulf of Alaska Friday
and the weekend to present a substantial maritime threat for high
waves/winds along with heavy rain/snow for especially coastal
areas of south-central Alaska through the Southeast Panhandle with
moistening onshore flow. Meanwhile, A deep extratropical low
associated with current west Pacific Typhoon Mindulle is slated to
track toward the southern Bering Sea and Aleutians by early next
week to present another significant heavy seas/wind and wrapping
rainfall threat.
Schichtel
Hazards:
- Heavy precipitation across portions of mainland Alaska, Thu-Fri,
Sep 30-Oct 1.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Thu-Sat, Sep 30-Oct 2.
- High winds across portions of the Alaska Panhandle, mainland
Alaska, and the Aleutians, Thu, Sep 30.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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