Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
734 PM EDT Tue Sep 21 2021
Valid 12Z Sat Sep 25 2021 - 12Z Wed Sep 29 2021
...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, after a potent
low causes heavy precipitation across the Panhandle during the
short range period, another surface low is forecast to track
across the northeastern Pacific and cause another round of
precipitation for the Panhandle early next week. Then, another low
that could be reasonably deep for this time of year is currently
expected to move from the Bering Sea southeastward across the
Aleutians into the Gulf of Alaska, which could cause increasing
precipitation chances for southern Alaska and the Panhandle
dependent on the low track.
Model guidance was generally agreeable with this overall pattern,
but with differences in timing and depth of some features.
However, today's 12Z guidance is quite consistent with a track of
this weekend/early next week's surface low remaining fairly far
south of Alaska until it approaches the Panhandle around Monday.
This has led to a southern shift in the low track compared to
yesterday's forecast, which reduces sensible weather impacts
across the Aleutians to Southcentral Alaska, though heavy
precipitation is still likely in the Panhandle. But model guidance
diverges upstream in the Bering Sea, with mid/upper-level
shortwave placement and evolution and thus the surface low
positions. Around Sunday/Monday, the 12Z GFS/UKMET/CMC split from
the 12Z EC/06Z GFS, with the former group having a faster
shortwave and surface low track through the Bering Sea and across
the Aleutians. WPC leaned more toward the slower solutions given
the GEFS and EC means were slower as well, suggesting this was the
safer camp to choose at this point. After that, regardless of
exactly how the vort maxes evolve and combine, even the
deterministic models converge somewhat well on troughing
redeveloping centered over the Gulf by midweek. Overall the WPC
forecast used a model blend of the 12Z EC/GEFS mean/EC mean, along
with a small portion of the 06Z GFS early in the period, and
favored the means more as time progressed.
...Sensible Weather...
After a wet short range period, with some precipitation lingering
into the weekend, the Panhandle is forecast to get another round
of precipitation early next week given the approaching strong low,
while gusty winds are expected offshore. Farther north,
central/eastern parts of the mainland could see moisture and
increased precipitation (some falling as snow) chances this
weekend. Then the next potentially strong low pressure system
could bring gusty winds and precipitation to the Aleutians and
toward Southcentral Alaska and eventually the Panhandle again by
midweek, though uncertainty remains in the low's track and
strength. The upper trough persisting over the mainland will keep
temperatures below normal over much of the state. Coldest
anomalies should be over the southern parts of the state and
across higher elevations of the Brooks and Alaska Ranges. High
temperatures should range from the 40s and 50s in coastal areas,
while highs could remain in the 30s or lower across much of the
mainland.
Tate
Hazards:
- Heavy precipitation across portions of coastal and higher
terrain of southern Alaska into the Alaska Panhandle, Fri, Sep 24.
- Heavy rain across southern portion of the Alaska Panhandle, Fri,
Sep 24 and Mon-Tue, Sep 27-28.
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