Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
728 PM EDT Mon Sep 13 2021
Valid 12Z Fri Sep 17 2021 - 12Z Tue Sep 21 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 across the northern Gulf
through the end of the week, followed by another low that should
track just south of the Aleutians and reach the Gulf region by
next Sunday/Monday. By the end of the forecast period next
Tuesday, a third system could be affecting the central and western
Aleutians. 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 next weekend and into early next week.
The 12Z model guidance suite is in decent overall synoptic scale
agreement for much of the forecast period, although some timing
and latitudinal differences remain. The CMC is slower with
bringing the next storm system to the Gulf region on Sunday and
the GFS on the faster side of things. Most of the guidance keeps
the core of the polar low just north of the Arctic Coast, whereas
the GFS is more amplified and brings the low closer to the Brooks
range and thus a colder overall solution. 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 on Friday, with perhaps 1 to 2 inches of rainfall
across portions of the coastal ranges. The heaviest rainfall with
the following storm system should mainly stay offshore until
Monday when the plume of moisture brings potentially heavy
rainfall to the southeast Panhandle region for early next week.
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 and perhaps Friday 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
Hazards:
-Heavy precipitation across portions of southern mainland Alaska,
Thursday, Sep. 16
-Heavy rain across portions of the Alaska Panhandle, Monday, Sep.
20
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