Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EDT Sun Sep 12 2021
Valid 12Z Thu Sep 16 2021 - 12Z Mon Sep 20 2021
...Pattern Overview and Model Guidance...
An active weather pattern is expected to be in place across the
Alaska domain during the medium range period. A strong low
pressure system is forecast to track from the Bering to the
northern Gulf through the middle to end of the week, followed by
another low that will likely take a track farther south by next
Sunday/Monday. Meanwhile, a rather significant pattern change is
expected to materialize across much of mainland Alaska as a deep
Arctic trough builds in and heralds the arrival of much colder
temperatures for the second half of the forecast period.
Given the more amplified upper level pattern in place by mid-late
week, the 12Z guidance suite is in better agreement on the
synoptic scale compared to previous days. Some of the timing
differences in the guidance early on can probably be attributed to
the potential double low structure of the storm system on
Thursday, which evolves into one main parent low over the northern
Gulf as it consolidates. For the second storm system next
weekend, the GFS is suggesting a farther south track compared to
the more northern placement of the ECMWF/CMC, and the ECENS even
farther to the north. For the polar low settling in across the
North Slope region, the GFS is the most impressive with the
southward extent of sub-522 dm heights. The WPC fronts/pressures
forecast was primarily derived from a CMC/GFS/ECMWF blend with
some of the UKMET to begin, followed by slightly more of the ECMWF
compared to the CMC/GFS and increased use of the ensemble means.
...Sensible Weather...
Periods of moderate to heavy rainfall are likely from the Kenai
Peninsula to the southeast Panhandle owing to onshore flow from
the Gulf system for the Thursday-Saturday time period, with
perhaps 1 to 3 inches of rainfall across the coastal ranges. The
heaviest rainfall with the following storm system should stay
mainly offshore for the time being. Some strong winds are also
possible for portions of the Aleutians and coastal southern
Alaska.
Another thing that will likely make weather headlines will be the
colder temperatures in the wake of a strong cold front dropping
south from the Arctic. Readings will likely still be near to
above normal on Thursday for most of the state, followed by well
below average temperatures by next weekend and into next Monday.
Accumulating snowfall is also looking increasingly likely across
portions of the Interior, Brooks Range, and North Slope, along
with widespread subfreezing overnight lows.
Hamrick
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