Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Thu Sep 16 2021
Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021
...Several Major Storms to Threaten Alaska...
...Guidance and Predictability Assessment...
The 12 UTC models and ensembles seem in good agreement into Monday
and a composite blend seems reasonable to smooth the less
predictable smaller scale wrinkles in a period with overall above
normal predictability. While modest model input offers some detail
into days 5-8, opted to switch to mainly the compatible GEFS/ECMWF
ensemble means then in a period of increased forecast spread with
the details of stream interactions and embedded system specifics.
The overall stormy pattern though still offers above normal
predictability. Manual adjustments were applied to WPC progs to
ensure deep offshore low depths and wind strengths consistent with
a pattern with good system upper support and improving
predictability. This plan maintains reasonable WPC product
continuity.
...Sensible Weather...
A powerhouse storm will track across the Gulf of Alaska early next
week. This potent low will present a significant maritime wind and
wave threat as well as a heavy precipitation threat from Kodiak
Island and south-central coastal Alaska to especially the
Southeast Panhandle. Yet another major and large storm will affect
the Aleutians and southern Bering Sea early-mid next week before
working across the Alaskan Peninsula to the Gulf of Alaska. This
deep extratropical low is partially associated with current West
Pacific Tropical Storm Chanthu and additional onset of upper
trough energies, so it presents another heightened threat for high
winds/waves and wrapping heavy rains. There is a threat that this
system may bring a long fetch moisture plume northeastward into
Southeast Alaska by later next week.
Farther north in an also active stream flow around/underneath a
deep closed low working over the Arctic Ocean, expect quite
unsettled conditions with increased rain and snow chances,
especially for the Brooks Range, with expected intrusion of Arctic
air southward across the North Slope and Interior/Mainland next
week. Forecast spread is not stellar, but has improved, bolstering
forecast confidence in the pattern. This would also result in
below normal temperatures for much of the mainland.
Schichtel
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Tue, Sep 20-Sep 21 and Thu, Sep 23.
- Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep
20-Sep 21 and Thu, Sep 23.
- Much below normal temperatures across portions of mainland
Alaska, Mon-Thu, Sep 20-Sep 23.
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