Small Craft Advisory 1015, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKST Mon Dec 28 2020

.ANALYSIS AND UPPER LEVELS...

A broad, vertically stacked ~950mb low is currently weakening in
the central Bering. As such, gusty winds and rain/snow showers
will continue across the entire Bering/Aleutians. The heaviest
precipitation has ended over Southwest Alaska, however widespread
rain/snow showers are continuing as a number of shortwaves
propagate around the main upper low. Farther east, a potent vort
max is tracking south to north across the Gulf coast. This
combined with strong southerly flow aloft between the
aforementioned upper level Bering low and a ridge centered over
the Panhandle is leading to widespread precipitation. Heaviest
snow and strongest winds are confined to higher elevations over
the eastern Kenai Peninsula, while much of the western Kenai
Peninsula and Anchorage bowl will remain downsloped in cross-
barrier flow, with only brief light rain/snow showers likely under
the aforementioned shortwave/vort max.

&&

.MODEL DISCUSSION...

Synoptic model agreement remains good. Winds in Anchorage remain
somewhat in question. The latest NAMNest shows gusty
southeasterly winds making it into town. This would significantly
warm temperatures, possibly into the lower 40s. In any case, even
if winds remain light/variable, minimal precipitation is expected
given the cross-barrier flow.

&&

.AVIATION...

PANC...VFR ceilings will generally persist. Ceilings may briefly
drop to MVFR as brief showers are possible over the terminal,
particularly through the morning. Southeasterly surface winds are
likewise expected to develop this morning, becoming light and
variable by early afternoon. Moderate wind shear will continue
early this morning until surface winds switch to southeasterly and
become unidirectional with winds aloft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A strong frontal boundary extends from Prince William Sound and
southward through the eastern Gulf of Alaska. This front is nearly
stationary today and will slowly move eastward tonight. This will
keep the precipitation heaviest in eastern Prince William Sound
with some rain and snow showers persisting in the eastern Kenai
Peninsula. The western Kenai Peninsula and Anchorage and the
Mat-Su Valley will mostly downslope out though some light
precipitation may sneak across the mountains.

The biggest change in the forecast today is in the Anchorage and
Matanuska Valley where the southeast winds are expected to move
into the Anchorage bowl early this morning and persist most of the
day. This will also increase temperatures and highs around 40 are
now expected around the Anchorage Bowl. These winds are already
mixing into the Matanuska Valley and pushing temperatures toward
40 degrees there as well. Temperatures should be lower Tuesday as
the winds will have subsided which are the main driver of the
temperatures today, but it will remain rather mild for this time
of year around much of Southcentral into Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Widespread light snow is expected to develop from interior
Bristol Bay north to the Kuskokwim Valley this morning as a
trough lifts across the Alaska Range. As scattered snow showers
wind down this morning over the Kuskokwim Delta, a second
shortwave lifting north will allow snow to redevelop over the
beginning this afternoon. This system will lift to Norton Sound
by early Tuesday morning, signaling an end to the light snow from
Kipnuk to Hooper Bay.

An area of low pressure will then lift north across the AKPEN on
Tuesday with gusty easterly winds increasing and a mix of rain
and snow developing along the coast, first from Dillingham and
Togiak Tuesday morning to the Kuskokwim Delta coast by early
afternoon. The combination of snow and gusty winds along the
Kuskokwim Delta coast could produce periods of blowing snow
through late Tuesday before the precipitation comes to an end.
Areas of rain and snow are then expected to develop over interior
Bristol Bay Wednesday as low pressure spins up north of Kodiak
Island. By late Wednesday, this round of precipitation is expected
to wind down as the low moves farther away and a ridge builds
over Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

Widespread gales will continue from the Pribilofs south and west
to the Central and Western Aleutians through Monday as a 954 mb
low slowly tracks westward over the Western Bering. Scattered rain
and snow showers will also persist over the Bering and much of
the Chain today, enhanced by multiple upper-level shortwaves
pinwheeling around the main upper-level low. One shortwave moving
over the the Western Aleutians this afternoon will usher in colder
air in its wake, changing any mixed rain/snow showers to all snow
by Monday evening.

This shortwave will phase with a second shortwave lifting north to
the AKPEN for Tuesday, helping to draw a low south of the AKPEN
northwestward. This low will parallel the Southwest coast with
southeasterly gales and mixed precipitation through Tuesday
night.

By late Wednesday, a rapidly strengthening North Pacific low will
track north toward Attu. This system will lift a storm-force front
over the Western and Central Aleutians by Thursday morning as the
expected sub 940 mb low approaches Shemya.

&&

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

Gulf of Alaska:
Good forecast confidence on Wednesday/Thursday for no significant
winds or waves. Model guidance shows the potential for a small
area of gale force easterly winds in the southern portions of the
Gulf on Friday.

Aleutians/Bering:
Model guidance are in good agreement on a sub 940 mb low lifting
north from the Pacific and moving over the central Bering and
western Aleutians Wednesday night to late Thursday. There is a
risk for more widespread coverage storm force winds than currently
forecasted Wednesday night through Thursday night for the western
and central Bering/Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

An active pattern will hold over the central Bering and western
Aleutians Thursday through Friday with a rapidly intensifying (sub
940 mb) low lifting south of the Aleutians and moving over the
Bering. Gusty winds and the potential of heavy precip, including
wintry mix, will cross the Aleutians Wednesday night through
Friday. After Friday, this low weakens and moves over the western
Bering. Model guidance diverge with the next potential storm
force low paralleling the Aleutians, but unsettled weather will
the continue with the risk for winds and precip over the weekend.

For Southcentral on Thursday, a trough moves from the western
Gulf to northeastern Pacific with a ridge over mainland AK. A
couple of weak shortwaves passes through the southern portions of
the gulf and AKPEN Thursday through Sunday. Southcentral will be
dry Thursday through Fri, but model guidance diverge with timing
of the next precip late Saturday through Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 119 125 150 160 165 170 172-180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...MF