Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EST Wed Nov 29 2023
Valid 12Z Sun Dec 03 2023 - 12Z Thu Dec 07 2023
...Overview...
After a northeastern Pacific system on Sunday dissipates or
continues eastward, most guidance has maintained the theme of
Bering Sea into Pacific energy/low pressure consolidating into a
deep Pacific storm that should track toward or into the Gulf of
Alaska by around Tuesday-Wednesday and gradually weaken in place
thereafter. However individual solutions display as much spread
as yesterday if not more, so confidence has not improved regarding
more precise details of precipitation and wind effects across the
Panhandle and westward along the southern coast. Ill-defined
upper troughing will likely set up over the western half of the
mainland as the storm lifts northward. Farther west, latest model
runs have rapidly converged toward the idea of a deep upper low
crossing Siberia during the period and approaching or perhaps even
reaching the far western mainland by next Thursday. Most areas
will see above normal temperatures aside from some below normal
readings primarily over the southwest corner of the mainland.
...Guidance Evaluation and Preferences...
The models and ensembles are still having a difficult time
resolving important upper shortwave and surface low details over
the northeastern Pacific into Sunday. Latest ECMWF runs have
tended to be on the southeastern side of the spread and would have
minimal influence on even the Panhandle, while the 12Z UKMET
surface low is far enough northwest to spread precipitation over a
broad area from Kodiak Island northeastward and eastward across
the southern coast and Panhandle. Recent GFS runs have been
southeast as well, though the new 18Z run has adjusted northward
of the 12Z ECMWF (closer to the 00Z ECMWF). Overall preference
today was an intermediate composite of the 12Z CMC and the
ensemble means which have trended somewhat east versus yesterday.
This provided an element of continuity aside from the slightly
faster adjustment.
As is typically the case, the ensemble means have been the most
consistent solutions for the deep Pacific storm expected to track
toward/into the Gulf of Alaska during the first half of the week,
with the surface low settling to a position just southeast of the
Kenai Peninsula by Wednesday-Thursday as it gradually weakens.
Most operational models suggest the storm will reach a depth into
the 950s mb for a period of time but should weaken somewhat as it
nears the southern coast. Among latest model runs, the 00Z/12Z
CMC and 00Z GFS were closest to the means while the 06Z/12Z GFS
leaned to the west side of the envelope and 00Z/12Z ECMWF runs
were a bit to the east. The 12Z/18Z GFS also kept the storm
farther south than consensus until very late in the period. The
12Z UKMET held onto the strongest depth closest to the coast by
the end of its run. An ensemble mean track with intermediate
depth (by way of a model blend de-emphasizing the GFS) provided
the best continuity along with a slightly deeper trend at some
forecast hours.
Guidance adjustments and consolidation have been pronounced just
in the past 12 hours of runs for the deep upper low now expected
to track across Siberia and approach the western mainland by next
Thursday. Earlier model runs generally had a weaker depiction and
more southward shearing of digging energy, while the ensemble
means had minimal reflection of a defined upper feature (let alone
a deep upper low) before the 00Z cycle in the ECens/CMCens means
or 12Z GEFS mean. For the time being, an operational model blend
with a little tempering by the latest ensemble means provided a
reasonable depiction of this upper system and surface reflection.
This solution leaned away from the somewhat fast 12Z ECMWF late.
Today's starting blend for day 4 Sunday used a combination of
about one-third means (12Z GEFS/00Z ECens) with the rest 12Z
CMC/GFS/ECMWF and least weight of 12Z UKMET to yield the desired
surface evolution over the northeastern Pacific and reflect
consensus for less contentions aspects of the forecast elsewhere.
Then days 5-6 Monday-Tuesday reverted to an all-model blend to
yield a deeper Pacific storm than the means but maintain an
intermediate track. Days 7-8 Wednesday-Thursday reintroduced the
means up to 20-40 percent weight while keeping CMC/GFS/ECMWF input
and eventually trending the ECMWF component more toward its 00Z
run.
...Weather/Hazards Highlights...
Continue to expect the primary precipitation focus during the
period to be along the southern coast and Panhandle, with most of
the interior seeing only isolated/scattered light snow if any
precipitation at all. Far western areas could see some snow move
in from the west around next Thursday as the system crossing
eastern Siberia approaches. Confidence remains lower than desired
for precipitation coverage and amounts along the coast/Panhandle
on Sunday with a leading system. The most likely low track would
bring some moisture to the Panhandle and perhaps just a bit
farther west. Confidence has also not improved regarding
precipitation and wind effects along the southern coast and
Panhandle in association with the deep Pacific storm tracking
northward toward/into the Gulf of Alaska during the first half of
the week. The currently favored intermediate track would provide
the best potential for meaningful precipitation totals over the
Panhandle and westward along the southern coast to the vicinity of
Prince William Sound. Strongest winds would likely remain over
the open waters. A farther west low track would increase
precipitation and wind speeds back toward Kodiak Island and nearby
areas, while a farther east low track could focus stronger winds
over the Panhandle. With continued uncertainty for the low track
and forecast precipitable water values remaining only moderately
anomalous, the 3-7 Day Hazard chart still does not depict any
defined areas associated with this system. Will continue to
monitor forecasts of this storm and its potential effects closely
over the coming days. Farther west, the Aleutians will see
unsettled conditions during much of the period with mostly light
rainfall and winds varying with system progression but tending to
be on the brisk side.
While temperatures may trend somewhat colder as modest upper
troughing sets up over the western mainland, most locations should
tend to stay above normal with the North Slope likely seeing the
warmest anomalies. The far southwestern corner of the mainland
has the best potential to see some below normal readings next
week, especially for highs. A few pockets of below normal highs
may be possible over parts of the interior as well.
Rausch
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