Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
549 PM EST Tue Nov 24 2020
Valid 12Z Sat Nov 28 2020 - 12Z Wed Dec 02 2020
...Stormy pattern from the weekend into next week favoring
heaviest precipitation over the Panhandle/southern coast...
...Overview and Guidance/Predictability Assessment...
Latest models and ensembles maintain well above average agreement
and continuity regarding the large scale pattern evolution.
Moderately progressive flow during the weekend will bring one deep
storm from near/south of the Alaska Peninsula into the Gulf of
Alaska/southern coast during the weekend, followed by another
strong storm reaching the Aleutians by Sun. As this second system
tracks along/south of the Aleutians the upper flow should evolve
into a more amplified and slower regime featuring an upper trough
axis near or just west of the western coast of the mainland and an
upper ridge over western Canada. This transition should direct
the second storm toward the southwestern mainland during the first
half of next week and potentially lead to a longer-duration
Panhandle/southeastern coast heavy precipitation event compared to
that expected ahead of the first system.
A 12Z operational model blend represented consensus/continuity
well for the Sat-Sun part of the forecast whose primary emphasis
is the storm near the Alaska Peninsula/southern coast. Then
spread and run-to-run variability for specifics required some
adjustments to guidance input Mon-Wed along with modest ensemble
mean inclusion (more 00Z ECMWF mean than 12Z GEFS mean due to the
former being deeper with the second storm). The favored forecast
removed the 12Z UKMET by early Mon and the CMC by early Tue due to
straying enough to the slow side of the envelope for the second
storm that their inclusion could adversely dilute the blend. Thus
far ECMWF runs have provided the best intermediate anchoring,
favoring greater weight versus other solutions (GFS a bit on the
faster side but within reason). ECMWF input incorporated both 12Z
and 00Z runs Mon-Wed to account for the developing detail
differences. By day 8 Wed there is a decent model/ensemble mean
signal for an area of low pressure to reach somewhere south of the
Alaska Peninsula but with a lot of latitude spread. The centroid
of this spread currently appears to be closest to the 00Z
ECMWF--south of the 12Z GFS and north of the 12Z ECMWF.
...Weather/Threats Highlights...
The forecast is agreeable and consistent for the first deep storm
of interest, expected to track along/just south of the Alaska
Peninsula and into the Gulf of Alaska during the weekend. This
system should bring a period of enhanced precipitation to areas
from the Peninsula across the southern coast and into the
Panhandle, along with strong winds. The second storm is still on
track to bring its wind/precip field into the Aleutians on Sun and
continue eastward/northeastward during the first half of next
week. There is some timing uncertainty but decent agreement on a
surface low path crossing the Alaska Peninsula or Bristol Bay.
Gradual amplification of the upper pattern should promote greater
northward extent of moisture with this second system but heaviest
precipitation emphasis will likely remain along the southern coast
and Panhandle. Although the low pressure will weaken as it tracks
inland by Tue-Wed, the upper pattern evolution continues to signal
some persistence of a deep-layer flow of moisture into the
southern coast and Panhandle, leading to heavy precipitation over
portions of this region. Specifics remain uncertain given spread
and variability for the magnitude and duration of best moisture
feed. However the potential exists for a significant event since
some individual solutions depict precipitable water anomalies
reaching up to 2-4 standard deviations above normal for a time.
Continue to expect above to well above normal temperatures over
the state from the weekend through midweek, with a few isolated
pockets of below normal readings over the Panhandle and/or far
western mainland one or more days serving as the possible
exceptions. Min temps should generally be farther above
climatology than max temps. Pattern transition to a slower and
more amplified regime will likely lead to increasing warm
anomalies during next week and more coverage of readings 20F or
more above normal.
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, Sat-Mon, Nov 28-Nov 30.
- Heavy precipitation across portions of the Alaska Panhandle,
Tue, Dec 1.
- High winds across portions of mainland Alaska, Fri-Mon, Nov
27-Nov 30.
- High winds across portions of mainland Alaska and the Aleutians,
Fri-Tue, Nov 27-Dec 1.
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