Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
622 PM EDT Fri Sep 17 2021
Valid 12Z Tue Sep 21 2021 - 12Z Sat Sep 25 2021
...Several Major Storms to Threaten Alaska...
...Guidance and Predictability Assessment...
The extended period will be an active, stormy period for Alaska
and the surrounding waters as a couple of robust low pressure
systems move through the Gulf of Alaska. The global guidance and
their respective ensemble means are showing deeper surface lows
within the mean trough over the state. Although this pattern has a
higher than average predictability it continues to have spread
within the timing, location and strength. While modest model input
offers some detail into days 5-8, opted to maintain continuity by
using mainly the compatible GEFS/ECMWF ensemble means a period of
increased forecast spread with the details of stream interactions
and embedded system specifics.
...Sensible Weather...
A powerhouse storm is expected to track across the Gulf of Alaska
early next week which will impact Kodiak Island and the
south-central portion of the coast to the Panhandle with very
strong maritime winds, wave threats and heavy precipitation. There
will be another major and large storm that will affect the
Aleutians and southern Bering Sea early-mid next week before
working across the Alaskan Peninsula to the Gulf of Alaska. This
storm will pack an extra punch since it will consist of additional
upper-level trough energies and energies from the current West
Pacific Tropical Storm Chanthu. Another round of 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. Confidence in this feature has increased with the last
couple runs of the guidance and would also result in below normal
temperatures for much of the mainland.
Campbell
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Tue, Sep 20-Sep 21and Thu-Fri, Sep 23-Sep 24.
- Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep
20-Sep 21and Thu-Fri, Sep 23-Sep 24.
- Much below normal temperatures across portions of mainland
Alaska, Mon-Fri, Sep 20-Sep 24.
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