Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
548 PM EST Mon Nov 23 2020
Valid 12Z Fri Nov 27 2020 - 12Z Tue Dec 01 2020
...Stormy pattern from late this week into next week...
...Overview and Guidance/Predictability Assessment...
Today's operational models and ensemble means display remarkably
good agreement in principle through the entire extended period.
Behind a rapidly weakening Gulf of Alaska wave on Fri, two very
strong storms tracking from near the Aleutians into the Gulf
and/or southern mainland will be the dominant features of the
forecast. The first should reach the Gulf or southern coast by
around day 6 Sun. The second may take a slightly farther
northwest track toward the Alaska Peninsula/southwestern mainland
by early day 8 Tue, in response to a western Canada upper ridge
that may be somewhat stronger and more persistent than that ahead
of the first system. Both systems will promote an episode of
focused precipitation and strong winds. The primary evolution of
note at higher latitudes will be an initial weakness aloft over
the North Slope, followed by retrogression of embedded
energy/incorporation of upstream flow to yield a north-south upper
trough near 180 longitude by the end of the period.
Model/mean solutions were close enough to allow for using
exclusively operational models through the entire period to
provide the best detail possible while still accounting for
uncertainty to some degree. The main fine-tuning early in the
period involved focusing on the 12Z ECMWF and CMC/UKMET Fri into
Sat as the GFS was a bit south of consensus with the weakening
Gulf wave and on the northern side of the spread for the first
deep Aleutians system. By Sun the CMC was slightly on the
northern side of the spread for the first storm and then by Tue
the GFS strayed modestly northward with the second. However in
both cases typical guidance spread/error ranges left both
solutions within the realm of possibility. Also the GFS was
somewhat on the faster side of the spread for the second storm but
the 12Z ECMWF/ECMWF mean adjusted modestly faster, narrowing the
gap. Thus the latter half of the forecast used half 12Z ECMWF
with the other half divided evenly between the GFS and CMC.
...Weather/Threats Highlights...
Rapidly weakening Gulf of Alaska wave on Fri may support moderate
precipitation along the southern coast and possibly into the
Panhandle. Expect the first deep storm of interest to track from
the southern Bering Sea/Aleutians into the Gulf of Alaska Fri-Sun,
accompanied by strong winds along with focused precipitation
extending from the Aleutians through the Alaska Peninsula and
southern coast/Panhandle. Specific effects along/near the
Peninsula in particular remain uncertain due to sensitivity to the
exact track of the surface low. The second storm will likely
bring its wind/precip field into the Aleutians on Sun and
continuing eastward/northeastward Mon-Tue. Current forecast of a
track slightly northwest of the first storm would spread more
wind/moisture into the southern mainland by late period versus the
first system. Guidance is suggesting some slowing of the upper
pattern by next Tue, which would result in a more persistent
deep-layer flow of moisture into the Panhandle and vicinity at
that time. Exact amounts differ among the guidance but there is
good consensus on highest five-day totals from near the Kenai
Peninsula through the Panhandle, with lesser but still meaningful
totals extending back through the Alaska Peninsula and Aleutians.
Much of the state will likely see above normal temperatures
through the period. In most cases the anomalies for min temps
should be greater than those for max temps. Min temps could
exceed 20F above normal at some locations on one or more days. A
few pockets of slightly below normal highs could exist near the
western coast of the mainland mid-late period, close to the axis
of the upper trough forecast to evolve near 180 longitude. A few
below normal highs are also possible over parts of the Panhandle
during the period and localized areas over the south/southeast on
Fri.
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 mainland Alaska and the
Aleutians, Thu-Fri, Nov 26-Nov 27 and Sun, Nov 29.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sat-Mon, Nov 28-Nov 30.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Fri-Mon, Nov 27-Nov 30.
- High winds across portions of mainland Alaska and the Aleutians,
Thu-Mon, Nov 26-Nov 30.
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