Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
750 PM EDT Mon Oct 25 2021
Valid 12Z Fri Oct 29 2021 - 12Z Tue Nov 02 2021
...Heavy precipitation is forecast for the Alaska Peninsula and
Southcentral Alaska with gusty winds from the Aleutians to
Southcentral late week into the weekend...
...Overview...
The medium range period will begin with an upper low initially
centered northwest of the state that should drift west with time,
causing ridging to build into portions of Interior Alaska from the
east. Additionally, persistent troughing is likely in the
Northeast Pacific with an upper low centered just south of the
Aleutians, helping maintain surface lows with strong moist onshore
flow that will cause precipitation and gusty winds for southern
parts of the mainland this weekend.
...Guidance/Predictability Assessment...
Model guidance begins in fairly good agreement with the deep
surface and upper-level low pressure system in the Northeast
Pacific, with minor differences typical of the forecast time
frame. Despite the differences being relatively small, small
variations in the surface low track closer to or farther from the
Southcentral Alaskan coast could impact the start time and amounts
of heavy precipitation and winds there. A multi-model blend heavy
on the 12Z GFS/ECMWF and initially including the 12Z UKMET and CMC
was able to be used for this feature early in the period.
Farther upstream, model agreement starts to go somewhat awry as
energy from a possibly closed upper low moves east to the south of
the Kamchatka Peninsula and may combine with energy from what is
currently Tropical Storm Malou. Malou has its own forecast
differences, with the 12Z UKMET/CMC slower than recent GFS and
ECMWF runs, and it appears the Joint Typhoon Warning Center takes
a track in between these clusters. The evolution of the energy
possibly combining is uncertain and thus confidence is low. The
UKMET seems to be an outlier as it tracks a shortwave quickly
east, while the CMC becomes out of phase by Sun over the Aleutians
as it appears to combine the western energy with the initial upper
low more quickly than other guidance, so those models were quickly
removed from the forecast blend. Another apparent issue is with
the 12Z GFS, which somewhat like the CMC produces one upper low,
which is centered over the Aleutians around Monday. These
scenarios remain possible but not favored for now. Also, around
Sunday/Monday the EC mean was a bit out of phase with the CMC/GEFS
mean, but came back around to cluster well with other guidance by
the end of the period. Thus the most prudent compromise at this
point for the middle to latter part of the forecast period was a
blend of the 12Z ECMWF, 06Z GFS, and 12Z GEFS mean, and increasing
weighting of the 00Z EC mean by the end.
...Weather/Hazard Highlights...
After some leading precipitation possible across the southern part
of Alaska on Friday, precipitation is likely to increase
substantially over the weekend for the Alaska Peninsula into
Southcentral Alaska with the deep low pressure system spinning
over the Northeast Pacific providing moist onshore flow to the
region. Some strong southerly winds are also possible in those
areas, while gusty northerly winds are expected on the backside of
the low for the Aleutians Friday into Saturday. Precipitation
should continue into early next week. Farther north, some snow is
likely to spread across much of the western and central parts of
the state with the moist inflow from the low pressure system.
Temperatures across much of interior Alaska will be near to
slightly above normal through the end of the workweek, while
colder than average temperatures especially in terms of highs are
forecast over southwestern Alaska into the Panhandle. By the
weekend into early next week, temperatures should gradually warm
as heights aloft rise as the mean upper low shifts away from the
state and upper ridging builds in. Low temperatures are currently
expected to be 10-25F above normal for much of the interior, with
highs generally 5-15F above average.
Tate
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of mainland Alaska, Sat-Mon,
Oct 30-Nov 1.
- High winds across portions of mainland Alaska and the Aleutians,
Fri-Sat, Oct 29-Oct 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