Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
600 PM EST Sun Dec 06 2020
Valid 12Z Thu Dec 10 2020 - 12Z Mon Dec 14 2020
...Guidance and Uncertainty Assessment...
Today's guidance continues to suggest a challenging forecast
regarding specifics within a complicated and blocky evolution
aloft. Models and means are similar to start the period early
Thu--a deep upper low/storm near the Aleutians, an upper low over
the northwest mainland, a southeast-northwest ridge between the
two, and a separate ridge over the Arctic. As time proceeds
solutions go astray for how these features evolve/interact. By
the latter half of the period the low predictability of shortwaves
embedded within progressive Pacific flow also lowers confidence in
forecast specifics over the Aleutians/North Pacific.
The 12Z models as a whole are trending toward some
weakening/shearing of the initial northwest mainland upper low
during the first half of the period but then diverge for how much
upper ridging moves in from the west. Over the past day the ECMWF
and GFS have essentially flipped with the 12Z ECMWF now building a
ridge into the southwest half of the mainland by Fri/Sat--similar
to what yesterday's 12Z GFS showed before the model's later
00/06/12Z runs more closely reflected the 00Z ECMWF. Ensemble
means have maintained a better defined mainland upper low, similar
to yesterday but with a moderately weaker trend. The new 12Z
ECMWF mean accelerates this weakening trend.
At the same time guidance has been split over what proportion of
the Aleutians storm's upper low energy may progress east/northeast
versus retrograde beneath northern Bering/Siberia ridging--or even
just elongate and drift northward per the 12Z CMC if the ridge to
the north retreats enough. Thus far the means have favored the
eastern scenario while the GFS/ECMWF have been more split and
inconsistent. Ahead of this system there is a decent signal for
development of a triple point wave (and/or a frontal wave just to
the east) to the south of the Alaska Peninsula with an eventual
track close to though remaining somewhat south/east of the
Peninsula. By the latter half of the period the best
model/ensemble mean consensus has adjusted this general area of
low pressure a little southwest from yesterday.
One other part of the forecast still up for debate is the surface
pattern from the Gulf of Alaska southeastward as of 12Z Thu.
Operational models show the best defined low pressure to the south
of the Panhandle, albeit still with wide variation. Ensemble
means have been farther north over the Gulf but the 12Z CMC mean
has become more elongated and the new ECMWF mean has done likewise.
The initial forecast blend started with a 12Z operational blend
(with a little 00Z ECMWF included as well) early in the period to
reflect majority trends for the mainland upper low, a non-mean
scenario over/southeast of the Gulf, and detail for the storm near
the Aleutians (which has adjusted north some but not to the degree
of the 12Z ECMWF). Preference to maintain some degree of
continuity in the latter half of the period led to trending the
blend toward the 12Z GEFS/00Z ECMWF means and 12Z GFS/00Z ECMWF by
Sun-Mon. Even with the uncertainty in the upper pattern there
continues to be better agreement at the surface that the mainland
should stay in the gradient between strong high pressure over the
Arctic and low pressure over the North Pacific.
...Weather/Hazards Highlights...
During Thu-Fri ahead of the strong Aleutians storm and its leading
occluded front, expect an area of strong winds along with focused
precipitation over the eastern Aleutians and possibly southwestern
Alaska Peninsula. The winds and precip will also extend
west-northwest across the Bering Sea. Already by Fri some
uncertainty develops over specifics of Pacific wave development,
with precipitation details from the Peninsula through the southern
coast and Panhandle continuing to have lower than desired
confidence for the rest of the period. As a whole today's
model/ensemble guidance has trended a little lighter with five-day
totals but individual solutions vary considerably for
amounts/coverage. Most precipitation farther north over the
mainland should be light and scattered.
Temperatures are most likely to be above normal over the northern
and southeastern mainland plus parts of the Panhandle. There is
better potential for below normal readings over the
central/southwestern mainland and other parts of the Panhandle.
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
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