Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
715 PM EDT Wed Sep 15 2021
Valid 12Z Sun Sep 19 2021 - 12Z Thu Sep 23 2021
...Several Major Storms to Threaten Alaska...
...Guidance and Predictability Assessment...
The 12 UTC GFS/ECMWF and GEFS/ECMWF ensembles seem in good
agreement Sunday into Monday and a composite blend seems
reasonable to smooth the less predictable smaller scale wrinkles
in a period with overall above normal predictability. A composite
blend also seems reasonable through longer time frames overall,
but the 12 UTC ECMWF may be over zealous with stream phasing
compared to the more stream separated GFS, GEFS/ECMWF ensemble
means and 00 UTC ECMWF run to run continuity. 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.
...Sensible Weather...
A powerhouse storm will track from the Aleutians this weekend to
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 moist system is the deep
extratropical low associated with current West Pacific Tropical
Storm Chanthu, so it presents another heightened threat for high
winds/waves and wrapping heavy rains.
Farther north in an also active stream flow around/underneath a
deep closed low working gradually 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.
- Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep
20-Sep 21.
- High winds across portions of mainland Alaska and the Aleutians,
Sat, Sep 18.
- Much below normal temperatures across portions of mainland
Alaska, Mon-Tue, Sep 20-Sep 21.
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