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Alaska Extended Forecast Discussion
 
(Latest Discussion - Issued 2352Z Oct 24, 2021)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Sun Oct 24 2021

Valid 12Z Thu Oct 28 2021 - 12Z Mon Nov 01 2021

...Overview and Guidance/Predictability Assessment...

The medium range forecast over Alaska shows a persistent pattern
with an upper low initially centered northwest of the state and
drifting west and south with time, while rounds of troughing track
around it. One such shortwave trough is forecast to push across
the Alaska Peninsula Thu and into the Gulf of Alaska Fri, before
deeper troughing establishes itself south of the Aleutians in the
northeast Pacific on Friday and tracks slowly eastward over the
weekend. Today's 12Z model guidance (including the
GFS/ECMWF/UKMET/CMC) is more agreeable with this second trough and
its associated surface low, but with minor differences typical of
the forecast time frame. Despite the differences being relatively
small, small variations in the low track closer to or farther from
the Southcentral Alaskan coast could impact the start time and
amounts of heavy precipitation there. Overall a mainly
deterministic model blend worked for about days 4-6.

Farther upstream, model agreement starts to go somewhat awry as
energy from a possibly closed upper low moves east across eastern
Russia and may combine with energy from what is currently T.D.
Twenty-Five-W before tracking south of the Aleutians in the north
Pacific. This evolution is rather uncertain, which can also be
seen by ensemble mean guidance blending everything out to barely
show upper-level and surface features there. The UKMET seems to be
the first to diverge from consensus and was removed from the blend
after day 5. Confidence is low with these features with plenty of
run-to-run differences in guidance and considerable variability in
ensemble guidance, but luckily the 12Z GFS and ECMWF were not
terribly different, so for days 7-8 ended up with a basically even
blend of the GFS/GEFS mean/EC/EC mean showing additional low
pressure tracking into the north Pacific.

...Weather/Hazard Highlights...

Light to moderate precipitation is forecast for southern parts of
Alaska Thursday, as a relatively weak low pressure system tracks
near Southcentral Alaska and convergent flow continues across the
Panhandle. Precipitation will likely ramp up considerably for the
Alaska Peninsula into Southcentral Alaska possibly by Friday but
especially into the weekend, as a deeper low pressure system spins
over the Northeast Pacific/Gulf of Alaska. Gusty southerly winds
are also possible there, while gusty northerly winds are expected
on the backside of the low for the Aleutians. Meanwhile, generally
quiet conditions are expected for much of mainland Alaska, with
occasional snow showers over the western mainland through late in
the workweek, and snow chances should increase for western and
central parts of the mainland over the weekend with the moist
onshore flow from the low pressure system.

Temperatures across much of interior Alaska will be near to
slightly above normal through the end of the week, while colder
than average temperatures especially in terms of highs are
forecast over southwestern Alaska into the Panhandle. By early
next week, temperatures could gradually warm as heights aloft rise
as the mean upper low shifts away from the state. Low temperatures
are currently expected to be 10-25F above normal for much of the
interior, with highs 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

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