Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 221324
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 AM AKST Wed Jan 22 2020

.ANALYSIS AND UPPER LEVELS...

A mainly zonal flow is in place for much of the region, with
short-wave ridging building across the western Aleutians, ahead of
a western Pacific low. A ~998 mb surface low is also noted in
various observational data, just south of Hinchinbrook Island.
Following last night`s snowfall, as drier air has moved in some
low stratus has developed from the Kenai Peninsula through the
Matanuska Valley, and across portions of the Copper Basin.

The same can be said for portions of southwest Alaska, as stratus
and fog stretched from the Bristol Bay region, inland to Lake
Iliamna.

&&

.MODEL DISCUSSION...

The model continuity has improved over the last 24 hours, with
guidance in pretty decent agreement through early next week. After
that, they only fly in the ointment is the GFS drying to lift an
unusually deep (~482 dm) mid-level low into the Gulf from the
north Pacific. Otherwise, all guidance suggests the main items to
watch is a deep mid-level low developing somewhere across the
Bristol Bay/Alaska Peninsula region, as a strong disturbance dives
into the base of an intensifying trough. This is a slight shift
westward, which makes sense given they now show the ridge axis
stretching from the Pribilof Islands to the eastern Aleutians.

This pattern doesn`t look all that dissimilar from what we seen
around New Years Eve/Day, when a deep mid-level low sat in the
same vicinity with the brunt of the cold air being locater west
and north of the Alaska Range, while also filtering into the
northern Susitna Valley, and most of the Copper Basin. The pattern
also featured several lows developing or moving through the
northern Gulf.

&&

.AVIATION...

PANC...Stratus continues to plague the airfield, with ceilings
varying from around 800 feet to 2500 feet near the terminal. This
should continue through the morning, with conditions improving
around 16z. With very cold temperatures aloft, flurries or very
light snow showers will continue through until these ceilings
increase, due to greater cloud depth/moisture depth.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level trough will amplify over southwest Alaska through
tonight. This will usher in colder air from the west, and
increase the surface gradient. Offshore winds will strengthen
through Thursday as a result. A 500 mb closed low will then
develop along this trough just to the west of the Kenai Peninsula
Thursday morning, dropping southeast into the Gulf Thursday
afternoon. This will help develop a surface low in the Gulf,
tightening the gradient and further enhancing the offshore winds.
Precipitation will end for most areas today. The exception will
be the Copper River basin, where some trapped moisture will
combine with some lifting at the mid-levels to produce some
showers through Thursday morning. Some moisture will wrap around
the Gulf low into the Copper River basin and eastern Prince
William Sound late Thursday through Thursday night bringing a
possibility of some snow to those areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Wednesday
through Friday night)...

Residual snow showers continue to move across the northern Kilbuck
Mountains and are expected to taper off this morning. In its wake,
this front leaves behind ample moisture which has produced
widespread low level stratus and fog across most of Southwest.
Interior areas of Bristol Bay may see prolonged low level stratus
through midday today while the Kuskokwim Delta area should clear
out by late morning.

The next big weather story for Southwest is the arrival of cold
temperatures for the end of the week. As an upper level trough
moves inland and "digs" southward by Thursday morning, a cold
Arctic airmass will be pulled down from the north. As an upper
level ridge over the eastern Bering nears Southwest, gusty
northerly flow can be expected for interior parts of Bristol Bay
and through the usual gaps and passes along the southern Alaska
Peninsula on Friday. This combination of cold air and gusty
northerly flow has the potential to create low wind chill values,
especially for the Lower Kuskokwim Valley.

&&

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

A ridge of high pressure in place over the Bering this morning
will slowly drift eastward as a strengthening North Pacific low
approaches the western Aleutians. Precipitation will start as
snow, however a quick change to rain is expected as warm
southeasterly flow quickly moves in tonight.

The main change to the forecast was an increase in sustained winds
across the western Bering, particularly Thursday night into
Friday morning. Although the low center ultimately never makes it
into the Bering (as it encounters a blocking ridge in place over
the eastern Bering), it is expected to strengthen as it lifts
northward, and thus expect the possibility for some storm force
winds embedded within the widespread gales through early Friday.

Meanwhile, an Arctic airmass settles over the eastern Bering for
the end of the week, bringing widespread areas of heavy freezing
spray to the northern and eastern Bering waters.

&&

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

Gulf of Alaska:

Weak low pressure will persist over the Gulf of Alaska Friday
through Tuesday. Strong high pressure over the mainland coupled
with the low pressure in the Gulf will produce strong outflow
wind, especially out of the Copper River Delta and out of Kamishak
Bay through the Barren Islands that should produce gale force
wind Friday and Saturday.

Bering Sea and Aleutians:

A strong and nearly stationary low centered south of the western
Aleutians will keep widespread gales with occasional embedded
storms ongoing across the central and western Bering Friday. The
Gales will likely be over the western bering Sea Saturday and
Sunday. Seas of 20 to 35 ft are expected over a large area of the
central and western Bering and Aleutians Friday. Strengthening
high pressure over the eastern Bering may cause near gale force
wind out of the Kuskokwim Delta Friday night through Sunday.
Offshore gale force wind is also possible in the Bristol Bay area
Friday through Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

The big picture will be low pressure persisting over the Gulf of
Alaska and south of the Aleutians with strong high pressure over
the eastern Bering Sea and deep cold air over the mainland. For
the mainland this translates to temperatures running around 20
degrees below average through the weekend. This setup favors the
outflow gap winds through the coastal mountains with precipitation
mainly along the coastal regions of the Gulf. Models are in good
agreement with the synoptic features through Sunday then diverge
with respect to the low pressure center location south of the
western Aleutians beginning Sunday night. However, the impact of
these differences is minor, and limited to the western Aleutians.
Where the lows end up will decide whether gales occur over the
western Aleutians or not.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 127 130 131 172 173 174 175 176 177 178 411 412 413
         Heavy Freezing Spray warning 121 129 150 160 180 185
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...SA


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