Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
544 PM EST Fri Nov 06 2020
Valid 12Z Tue Nov 10 2020 - 12Z Sat Nov 14 2020
...Guidance/Predictability Assessment...
Today's guidance maintains the theme of an active pattern, with
strong progression especially across the North Pacific. The best
model/ensemble cluster shows another deep Bering Sea system
quickly replacing a leading northern Bering low Tue-Wed. Expect
the second low to linger over the northern Bering for a time as it
slowly weakens, and then drift a bit southeast in concert with
upper dynamics. Meanwhile low pressure should develop along the
southern coast of the mainland by Wed-Thu, with persistence of
surface troughing and/or low pressure over the northern Gulf
through Fri-Sat as the upper trough/low reach the mainland and
Gulf of Alaska. Given recent guidance behavior and progressive
nature of mean flow aloft, there is greater than average
uncertainty for a wave that may track south of the
Aleutians/Alaska Peninsula and into the Northeast Pacific Wed-Fri.
The surface/500mb forecast incorporated the most common ideas from
the 12Z/00Z ECMWF, 12Z UKMET/CMC, and 06Z GFS from day 4 Tue into
early day 6 Thu. Then the blend quickly added 12Z GEFS/00Z ECMWF
mean input, reaching 40-60 percent total by days 7-8 Fri-Sat.
Overall the favored models had the best ensemble support and
represented continuity fairly well for the evolution over the
Bering Sea and vicinity. The 12Z GFS was somewhat west of
consensus with the dominant Bering system while the 06Z GFS was at
least closer even if still a bit westward. The 12Z GFS/ECMWF both
trended significantly weaker/southward with the potential wave
south of the Aleutians/Alaska Peninsula Wed-Thu while the
CMC/UKMET were closer in principle to the 06Z GFS/00Z ECMWF. Thus
far ensemble spread has been sufficient for the means to depict
only a vague hint of a wave and frontal system. The preferred
blend yielded a southward adjustment from yesterday while
maintaining a reasonable degree of definition. This solution will
provide good leeway depending on future guidance trends.
Evolution of low pressure over/near the Gulf Wed onward has higher
confidence Wed-Thu and Sat versus Fri, when details will depend on
what becomes of the uncertain North Pacific wave. The
weak/suppressed North Pacific evolution of the 12Z GFS/ECMWF would
allow for persistence of Gulf low pressure through the entire
Wed-Sat period in contrast to a brief southeastward emphasis on
Fri.
...Weather/Threats Highlights...
Deep Bering Sea low pressure will bring another episode of strong
winds and areas of enhanced precipitation to locations from the
Aleutians/Bering Sea into parts of the western/southern mainland.
With time the precipitation emphasis will transition to the
southern coast/Panhandle as low pressure becomes established over
the Gulf of Alaska and northern Bering low pressure slowly
weakens. The progressive North Pacific pattern will keep light
precip over the Aleutians for a majority of the period. A North
Pacific wave could brush the Aleutians and then extreme southern
Panhandle with some precip around Wed-Fri but recent southward
trends suggest lowering probability of meaningful totals. Best
consensus for highest five-day precip totals extends from the
eastern Kenai Peninsula through the Panhandle.
Expect most of the state to see above to well above normal
temperatures for about the first half of the period, with below
normal readings over the Panhandle likely being the exception.
The mainland should see a significant cooling trend late week into
the weekend as an upper trough and possible embedded upper low
move overhead. By day 8 Sat max temperatures in particular should
be below normal over most areas.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Nov 10-Nov 11.
- High winds across portions of mainland Alaska and the Aleutians,
Mon-Wed, Nov 9-Nov 11.
- High significant wave heights for coastal portions of mainland
Alaska, Mon-Tue, Nov 9-Nov 10.
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