Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Tue Oct 19 2021
Valid 12Z Sat Oct 23 2021 - 12Z Wed Oct 27 2021
...Overview and Guidance/Predictability Assessment...
Today's models and ensembles agree rather well for the large scale
upper pattern that should feature a mean trough whose axis extends
from eastern Siberia into the northeastern Pacific. To start the
period, a couple low pressure centers south of the mainland will
likely consolidate under an upper low which ultimately weakens.
Then expect North Pacific/Aleutians flow to carry a developing
system just south of the Aleutians around Sunday. Some
combination of this system and/or eastern Pacific low pressure may
eventually lift northward closer to the Gulf of Alaska by next
Wednesday. A weaker wave could cross the Bering Sea around
Tuesday. On the other side of the upper trough axis at least a
couple shortwaves should progress northward from the
mainland/northwestern Canada into the Arctic. The first appears
more likely to produce a defined surface reflection.
As of the start of the forecast 12Z Saturday, the 12Z models have
come into better agreement for resolving two separate surface lows
south of the mainland and their relative position--one to the
south/southeast of Kodiak Island and the other to the west of the
southern Panhandle. These lows should consolidate and weaken in
favor of low pressure near Haida Gwaii by early Monday. Farther
west the models and ensemble means have latched onto the Sunday
system tracking near the Aleutians only in the past day or so and
are still showing some spread and variability for
depth/timing/track. Sensible weather effects over the Aleutians
will be very sensitive to the exact character of the storm. Based
on data through the 12Z cycle an intermediate solution provided a
reasonable starting point, with the 12Z GFS straying to the
slow/deep/north side and the 12Z ECMWF adjusting noticeably
faster/weaker/south versus its 00Z run. The new 18Z GFS has come
in closer to the preferred blend that used a composite of the 12Z
models from early Saturday into early Monday.
The forecast quickly transitioned to a model/ensemble mean mix for
the latter half of the period, reaching 50 percent total ensemble
mean (12Z GEFS/00Z ECens) weight by day 8 Wednesday. This blend
yielded an upper low near the Bering Strait by next Wednesday,
reflecting the general model/mean average. Details become
increasingly uncertain for specifics of energy aloft that reaches
the northeastern Pacific along with corresponding surface low
pressure, with fairly low predictability given their scale.
Meanwhile there is a moderate signal for a wave to cross the
Bering Sea around Tuesday into Wednesday. Detail confidence is
not great here either given progressive supporting flow aloft and
ongoing spread/variability in the models.
...Weather/Hazard Highlights...
Consolidating low pressure to the south of the mainland may
contribute to lingering precipitation along the southern
coast/Panhandle into Saturday but with amounts trending lighter.
The system expected to track just south of the Aleutians around
Sunday would bring a brief period of enhanced rainfall and brisk
to strong winds. Effects on the Aleutians will be very sensitive
to the exact strength/track/timing of the storm so confidence is
currently in the lower half of the spectrum for how much rain and
wind the Aleutians will see. 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 depending on the proximity of
low pressure by then.
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.
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:
- High winds across the western portion of the Aleutians, Fri, Oct
22 and Sun-Mon, Oct 24-Oct 25.
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