Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Sat Oct 31 2020
Valid 12Z Wed Nov 04 2020 - 12Z Sun Nov 08 2020
...Guidance/Predictability Assessment...
Models and ensemble means are agreeable and consistent in showing
a Bering Sea upper ridge building into the mainland after midweek,
leading to southeastward displacement of the mean trough initially
over the southern mainland/Northeast Pacific--as well as
associated low pressure. Also there has been good stability in
the mid-late week forecast of a strong Kamchatka/Siberia storm
pushing a broad area of precipitation and strong winds across
parts of the Aleutians/most of the Bering Sea and into portions of
the mainland. Broad ridging aloft will likely persist over the
mainland through the weekend. Guidance expects upstream Bering
Sea and/or Pacific shortwave energy to interact with a mid-lower
latitude Pacific system but with very uncertain details by next
weekend. Thus confidence is low regarding possible weather
effects that may reach the western/southwestern mainland at that
time. With typical spread for specifics, there is a general
model/mean consensus toward another strong Kamchatka storm that
would bring another episode of enhanced winds into the western
Aleutians/Bering Sea by next Sun.
The 12Z operational model blend that is favored from day 4 Wed
into early day 6 Fri uses somewhat less than the usual weight of
the GFS Wed-Thu, as that model is somewhat on the fast/suppressed
side of the model envelope with the system forecast to track
toward the southern Panhandle or Haida Gwaii. Guidance has had
difficulty with individual low centers over this area in recent
days but at least models are maintaining the definition of this
system to follow up pronounced trends in that direction 24 hours
ago. There are also lingering question marks over specifics of
residual low pressure over the Gulf and the system supported by
energy dropping south/southeast from the Alaska Peninsula early
Wed onward. The front extending from the strong Kamchatka/Siberia
storm has trended a little faster over the past couple days but
otherwise there has been reasonable agreement and continuity with
the moisture and strong wind field crossing the Aleutians/Bering
Sea and into the mainland.
By next weekend models/means diverge considerably over the details
of Bering Sea through Pacific shortwave energy and interaction
with a system that briefly cuts off over the mid/lower-latitude
Pacific. ECMWF/ECMWF runs show more stream separation and thus
keep low pressure to the south of the eastern Aleutians/Alaska
Peninsula. Other solutions show greater degrees of low pressure
extending into the eastern Bering, whether incorporating the
Pacific system or in response to Bering Sea dynamics. Over the
past couple days even the GEFS/ECMWF means have shown some
variability with this evolution, keeping confidence in specifics
below average. Forecast uncertainty favors a rapidly increased
ensemble mean component after Fri, leading to 60 percent total
weight by Sun. The blend yields a more phased solution than the
ECMWF but a more conservative evolution than the 12Z GFS/CMC.
...Weather/Threats Highlights...
Wed-Thu system tracking toward the southern Panhandle or Haida
Gwaii may bring meaningful precipitation to the southern
Panhandle. There is still considerable spread over precise track
and timing, keeping confidence in precip coverage and amounts
lower than desired. A drier trend will follow. The broad area of
precipitation and strong winds spreading across the western
Aleutians and Bering Sea on Wed will rapidly extend eastward and
reach the western mainland by Thu-Fri. Highest precip totals will
be over favored terrain in the west/northwest while lesser totals
are possible over other portions of the mainland. The forecast
becomes very uncertain from the eastern Aleutians into the
southwestern mainland next weekend. Potential outcomes range from
most precipitation staying along and south of the Alaska Peninsula
to a significant amount of moisture reaching as far north as the
southwestern mainland. Current deterministic preference would
bring some moisture northward, between the two extremes. By late
next weekend the wind/moisture ahead of an upstream storm may
reach the western Aleutians/Bering Sea.
The approach/arrival and then persistence of upper ridging will
steadily reduce the coverage of below to much below normal
temperatures initially over the southern and eastern mainland
while bringing above normal readings across more of the mainland.
By next weekend only the Panhandle should continue to see well
below normal readings while modest negative anomalies may persist
over parts of the extreme southeast. The North Slope will see
above/much above normal temperatures through the period and a
tendency toward more extreme anomalies with time.
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