Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 231218
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 AM AKDT Fri Oct 23 2020

.ANALYSIS AND UPPER LEVELS...
The latest satellite image has a classic mature low impacting
much of the southern tier of the state. The occlusion spans from
Southwest AK and is bisecting the Bering. The cold front begins
south of Kodiak and extends into the north Pacific. While the jet
is near the AKPEN and Kodiak. A new wave is forming on the frontal
boundary. The latest ASCAT pass has pinged into gale force winds
over the Barren Islands and portions of the Bering Sea. The upper
level ridge spans from the Brooks Range to the Copper River
Basin. As of 3 AM AKDT, Talkeetna was 11 degrees cooler than the
previous 24 hr observation and Gulkana was 3 degrees cooler. On
VIIRS imagery and webcams auroras were detected in Eagle, Fort
Yukon, Bettles and a few other interior locations. However, the
front is going to put the kibosh on potential aurora viewing the
next couple of nights/early mornings for Southcentral.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement into early next week.
Looking at precipitation type (thickness values) there are some
eye catching differences with the GFS, NAM, ECMWF and Canadian.
This will be the trickiest aspect of the forecast the next 72
hours.

&&

.AVIATION...
PANC...Cirrus signaling an approaching front was observed at PANC
overnight. As the front continues its track expect the cigs to
decrease. This morning VFR conditions and light winds will
persist. Cigs will drop to 5,000 feet this TAF package.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Sunday night)...
A frontal system in the vicinity of Kodiak Island this morning will
lift northward across the western Gulf of Alaska through tonight,
making very slow progress due to the presence of a strong ridge
of high pressure sitting to the north and east. While the overall
storm system is now in a weakening phase, funneling of winds
through Stevenson and Kennedy Entrances will lead to a small area
of storm force winds from the Barren Islands to Kamishak Bay
beginning this afternoon. The front will gain some momentum
Saturday through Saturday night as the upper ridge flattens and
shifts eastward. Winds will rapidly diminish across the northwest
Gulf while gale force winds overspread the northern Gulf.

Precipitation with the frontal system will also make slow northward
progress, as it moves into a very dry airmass. The latest surface
observations show dewpoint depressions on the order of 10 to 15
degrees. Thus, it will take some time to saturate the airmass and
for precipitation to reach the ground. The northern extent of
precipitation for today will hover near the southern tip of the
Kenai Peninsula, with light rain becoming likely around Homer.
Precipitation will attempt to overspread Southcentral tonight
through Saturday with limited success. South to southeast low
level flow will lead to upslope enhanced precipitation across much
of Prince William Sound as well as the northern to western Susitna
Valley. For the remainder of the area, if precipitation reaches
the ground it will be brief and light. Warn air advection will
lead to a transition to mostly rain along the coast while inland
areas can expect all snow.

A new storm system will make its way up from the Pacific Saturday
night through Sunday night and deepen out ahead of the Bering
trough. All models currently indicate a track to Prince William
Sound. This looks to be a significant precipitation producer.
However, the track and depth of the low will be critical to
determining precipitation distribution as well as precipitation
type. With the warming which occurs prior to the arrival of this
system, it appears this may be a mostly rain event, with snow
confined to the mountains and interior Southcentral. Stay tuned.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Friday
morning through Sunday morning)...

A frontal system is producing strong winds over the Bristol Bay
region with areas of rain and snow spreading north across the
Kuskokwim Delta and Bristol Bay area. This front will weaken
today with winds slowly weakening by this evening through Saturday
morning. The precipitation will gradually decrease in coverage
this afternoon as the front moves north and weakens. Some snow is
possible this morning from near Bethel and areas north and west
in the Kuskokwim Delta, one to two inches of snow are possible
this morning before changing to rain by this afternoon. Tonight
through Saturday another shortwave trough will bring another round
of rain to the area with winds continuing to decrease. Periods of
rain are possible Saturday night through Sunday as a weakening
low pressure system moves towards Kuskokwim Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Friday
morning through Sunday morning)...

A frontal system across the northern Bering Sea will bring rain
and a broad swath of Gale force winds today gradually weakening to
small crafts by Saturday morning. A low pressure system near the
central Aleutians will bring another area of Gales across the
eastern Aleutians and the eastern Bering today these winds will
weaken to small crafts by Friday night. The low pressure system
will move north towards the Pribilof Islands on Saturday. This low
will weaken with winds decreasing below small crafts by Saturday
night. Otherwise periods of showers are expected across much of
the region Saturday through Sunday morning.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Gulf of Alaska:

There is medium confidence that a slow moving front will lead to a
barrier jet across the northeastern Gulf coast on Sunday. Differences
in models exist over the strength of the winds associated with
this feature and its extent. For now, a NAM/EC solution is
preferred with gale force winds from Prince William Sound east.
After this, forecast confidence decreases with an area of low
pressure from the North Pacific late Sunday afternoon. At this
time, the majority of model guidance indicates broad southwesterly
flow from the southern Gulf into southeast Alaska on Monday.
However, small craft to gale force winds are a possibility,
including continuation of the barrier jet from the day before.
These intricacies will be better determined with future model
runs. This low looks to move inland across southern Alaska and
dissipate Monday into Tuesday.

Bering Sea:

There is high confidence that the low currently over the eastern
Bering will continue to sit there and weaken through Sunday. As it
weakens from a low pressure center to a trough, it will be
absorbed by an area of low pressure south of the eastern
Aleutians Sunday night into Monday morning. Winds should remain
at small craft criteria or below during this time. Additionally, a
complex low pressure system off the Kamchatka Peninsula will bring
a wave of fronts over the western Aleutian waters, bringing small
craft winds to the area. Expect the first front Sunday,
dissipating Monday, and the second front Monday moving into the
central Aleutians through Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Models have started coming into better agreement in regards to the
ridging and longwave patterns on Sunday/Monday. A high amplitude
trough and associated weakening low slowly push east towards the
Southwest shore on Sunday, while a ridge builds over the North
Pacific. Warmer air flows in behind the ridge, increasing
temperatures slightly for Southern Alaska. The GFS was the
previously the biggest outlier with how far inland this warmer air
goes, however, the ECMWF has started showing more agreement with
the GFS, bringing warmer air into the region. The trough pushes
east, flattening the ridge, before being forced south by an
incoming strong wave out of the north. Model solutions are
agreeing well that this arctic wave pushes southeast across the
state Tuesday into Wednesday, which will bring colder air towards
Southcentral along with it. Any precipitation associated with this
wave will likely be snow.

A ridge sets up over Southwest/eastern Bering behind the longwave
trough. This pattern may be short-lived as a low pushes out of
the south towards the Central Aleutians late Wednesday night. Path
is relatively consistent across the models, but strength and
center location vary greatly.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 120 132 136 137 138 411 412 150 160 170 172 174 177
178 181 185 351 352. Storm 130 131.


&&

$$

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RLC
MARINE/LONG TERM...CK/KM


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