Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
759 PM EDT Wed Oct 20 2021
Valid 12Z Sun Oct 24 2021 - 12Z Thu Oct 28 2021
...Overview and Guidance/Predictability Assessment...
Today's main issue in the medium-range period continues to be the
large spread in the model guidance regarding the next cyclone
forecast to track close to the Aleutians next Sunday into Monday.
The latest (i.e. 12Z) ECMWF now takes a stronger cyclone closer
toward the western/central Aleutians on Sunday in better agreement
with the GFS. But then these two models diverge markedly, with
the ECMWF ejecting the low well to the east while the 12Z GFS
keeps the low well to the west, not far away from the Aleutians.
The 06Z GFS did not keep the cyclone as far west as the 12Z run.
On the other hand, the ensemble means are generally right in the
middle of these two model extremes. This calls for adopting
toward an even blend of the ensemble means by next Monday, which
yielded a solution fairly close to continuity from yesterday.
Beyond this time frame, the ensemble means generally agree with
keeping a broad cyclonic circulation to meander in the vicinity of
the Gulf of Alaska.
Therefore, the WPC forecast grids for Alaska were based on a blend
of the 06Z GFS/12Z GEFS, the 12Z ECMWF/12Z EC mean, together with
smaller contributions from the 12Z CMC/CMC mean. The blend
quickly was skewed toward the ensemble means by Day 5 (Monday).
...Weather/Hazard Highlights...
The medium range period will likely begin with subsiding winds
over the eastern Aleutians and the Alaska Peninsula early next
Sunday as a deep cyclone gets absorbed into a broad cyclonic
circulation over the northeastern Pacific. Meanwhile, models came
into better agreement for the next potentially deep cyclone to
track just south of the Aleutians around Sunday, possibly bringing
a brief period of enhanced rainfall together with a risk of high
winds. Effects on the Aleutians will be very sensitive to the
exact strength/track/timing of the storm. The best chance for
high winds should come late Sunday into Monday from the back-side
of the cyclone as it tracks away to the east. Thereafter, flow
around a potential Bering Sea wave may increase precipitation
somewhat over areas between the eastern Aleutians and southwestern
mainland Tuesday-Wednesday. Shortwaves on the east side of the
large scale mean trough aloft may produce occasional episodes of
precipitation over parts of the mainland. The southern
coast/Panhandle could see a modest increase of rain/snow toward
midweek as a broad cyclone is indicated by the models to meander
over or south of the Gulf of Alaska.
Expect above to much above normal temperatures over the northern
two-thirds or so of the mainland through the weekend and into
early next week followed by some moderation as heights aloft
slowly decline. The southern mainland and Alaska Peninsula plus
the Panhandle will likely see a mix of above/below normal readings
(more above for min temperatures) with a trend toward greater
coverage of below normal anomalies by midweek.
Kong/Rausch
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
Hazards:
- No significant hazards are expected over Alaska during this
forecast period.
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