Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Mon Sep 20 2021
Valid 12Z Fri Sep 24 2021 - 12Z Tue Sep 28 2021
...Significant Storms to Bring Heavy Precipitation to the
Southeast Coast and Panhandle...
...Guidance and Predictability Assessment...
The Alaska pattern in the medium range period will be generally
characterized by an upper low drifting from the Arctic Circle
southward to northwest of the state, while spokes of upper
troughing pass across the state. At the surface, a series of
potent low pressure systems are forecast to move across the Gulf
of Alaska/northeast Pacific, causing the potential for strong
winds and precipitation across the Aleutians to Southcentral to
the Panhandle depending on the low tracks.
Model guidance was generally agreeable with this overall pattern,
but with differences in timing and depth of the features. On day
4/Friday, mid/upper heights/vorticity show some differences, with
the 12Z ECMWF/UKMET swinging a vort max toward the Panhandle
through a little faster compared to the 12Z GFS/CMC, with the EC
and GEFS means generally in between. The WPC forecast leaned a
little more to the quicker side but incorporated some GFS to show
the potential for this movement. Then by the weekend, a deep
surface low is expected to move likely south of the Aleutians and
into the Gulf, which is generally agreed upon among the models,
but the CMC is a weaker exception. There are also differences in
the low track and how close it gets to the coast, which will
create differences in the sensible weather for the southern parts
of the state. For days 4-6, used a model blend of the
GFS/ECMWF/UKMET along with the GEFS and EC means, which created a
compromise position for the low. Then by early in the next
workweek, there is some signal for another low strengthening in
the Bering Sea as more upper-level energy drops southeast, but
with variability in position among models and not a clear
consensus. Used a blend of the GEFS and EC means for days 7-8 to
minimize individual model differences, as the ensemble mean
guidance was generally agreeable.
...Sensible Weather...
The storm system first described above is forecast to cause
significantly heavy totals of rain and higher elevation snow to
areas from the far eastern Kenai Peninsula to the Panhandle for
the latter part of the workweek, with 3-5 inches liquid possible
in some areas. Then as the second strong low system tracks south
of the state, another round of heavy precipitation is possible
over the Aleutians depending on the low track, but likely across
the Panhandle once again by early next week. Both storms could
also produce a broad area of brisk to strong winds, but may stay
offshore. Farther north, central/eastern parts of the mainland
could see moisture and increased precipitation chances late this
week, otherwise only light precipitation is forecast. The upper
trough persisting over the mainland will keep temperatures below
normal over much of the state. Coldest anomalies should be over
the southwestern parts of the state and across higher elevations
of the Brooks and Alaska Ranges.
Tate
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Thu-Fri, Sep 23-Sep 24 and Sun-Mon, Sep 26-Sep 27.
- High winds across portions of mainland Alaska and the Aleutians,
Sat-Mon, Sep 25-Sep 27.
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