Small Craft Advisory 876, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKST Sun Nov 15 2020

.ANALYSIS AND UPPER LEVELS...

An upper level low over persists over the Gulf of
Alaska this afternoon, and this low is bringing a few showers
over the marine areas. Over Southcentral, cold and dry conditions
are spreading across most of the region. Conditions have improved
this afternoon in terms of low stratus and dense fog around the
Mat Valley, finally allowing for mostly clear conditions over most
of the mainland. A tight pressure gradient along the North Gulf
Coast will allow for strong offshore flow out of the typical gap
wind locations to continue. Southwest Alaska remains under the
influence of an upper level ridge building over the area. Cold
temperatures, mostly clear skies, and gusty winds remain out
ahead of this ridge as northerly flow allows cold air advection
to filter across the Southwest. Farther west, a storm force low is
bringing a round of windy conditions and precipitation to the
western Bering and Aleutians this afternoon as it works its way
eastward.

&&

.MODEL DISCUSSION...

Models remain in good agreement for the short term forecast,
giving above average confidence for this forecast package. Gap
winds continue to be the main forecast challenge across
Southcentral, with hi-res models being primarily used for these
locations.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist
through Monday.

&&

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

Cooler temperatures and clearing skies will continue in a
strengthening northerly flow. The N-S pressure gradient will
weaken slightly by Monday afternoon, then restrengthen again
Tuesday into Wednesday. As the 500 mb flow gradually aligns with
the surface flow, gap winds will increase, particularly through
Thompson Pass where gusts up to 60 mph will be possible. Valdez
and Whittier will likewise see gusty winds. The Matanuska Valley
remains challenging, however any winds there will not be as strong
as other locations. Temperatures will fall near the single digits
in areas with less wind. Lows in the Copper River Basin could
approach -20. No precipitation is expected in the coming days.

&&

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

A ridge of high pressure will build into Southwest Alaska from
the Bering tonight with winds becoming light and eventually
transitioning to weakly onshore Monday morning. This will set the
stage for fog to make a return to the Kuskokwim Delta, but it
should mostly mix out during the afternoon. By Monday night into
Tuesday morning, winds will become very light yet again, and a
repeat of foggy conditions is expected. Chances for rain/snow
showers along the coast increase late Tuesday as a weakening front
pushes in from the Bering. Late on Wednesday, another weak front
pushes in, bringing a few rain/snow showers with it.

&&

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

A storm force low will lift out of the North Pacific into the
western Aleutians tonight. Tomorrow this low will move towards
the Pribilofs while gradually weakening to gale force.

On Tuesday, the storm in the central Bering heads towards
Southwest Alaska as it dissipates. The next storm system will be
quick on its heels, with a gale force front approaching the
western Bering/Aleutians early Tuesday. Late Tuesday/Tuesday night
this front moves into the central Bering, while its parent low
moves into the far western Bering/Aleutians. High end gales to low
end storm force winds will accompany this system.

On Wednesday, the western Bering storm lifts north/west out of
the region, with winds decreasing to low end gales by late day in
the western Bering/Aleutians. Its associated front heads towards
the Southwest Alaska coast while weakening/dissipating.

&&

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

Gulf of Alaska:
A deepening low in Southeast Alaska will strengthen gap winds all
along the North Gulf Coast. Northwest small craft winds and waves
with local gale force winds near the Barren Islands will extend
across the Gulf and diminish Thu. A front enters the Western Gulf
Thu into Fri. Widespread southeasterly high-end gale force winds
spread from the Shumagin Islands to across Kodiak Island through
Fri. Upper level troughs ahead of the front will bring local
areas of small craft winds over the Gulf Thu and Fri.

Bering Sea:
A strong Northwest Bering low moves into the Russian Far East Thu.
Its front dissipates in the Eastern Bering Thu. Widespread gale
force winds with the low and widespread small craft winds with the
front diminish Thu. A North Pacific low gains strength as it moves
along the Aleutians into the Southeast Bering by Fri. Widespread
high-end gales with local storm force winds will wrap the low
with gale force winds along the Aleutians through Fri.

&&

.LONG TERM FORECAST (Days 4 through 7...Thursday through Sunday)...
This period looks to feature continued relatively cold and quiet
across much of Southcentral with very active weather across the
Bering/Aleutians. This is mostly due to an impressive polar jet
ripping at 150 kt (300 mb/30,000') across the Western Aleutians
into the North Pacific. The jet then turns northward near the
Alaska Peninsula, serving to support the surface features over the
Bering. Meanwhile, there is very little jet activity over
Southcentral. Thus, the overall theme will be for strong lows
across the Bering to attempt to push decaying fronts into SW AK
before they fall apart against the ridge over Southcentral.

On Thursday, a strong low over the Western Bering will exit the
area as it moves over Siberia. This will leave widespread cold air
over the Bering, providing enough baroclinicity for another
strong system to develop over the Eastern Aleutians late Thursday
into Friday. This low moves quickly up the West Coast while
weakening on Friday. By early Sat, 2 additional lows show signs
of developing. The first is way out over the Western Bering while
the second one is near the Alaska Peninsula/Kodiak. There is still
plenty of model uncertainty with the exact strength and location
of each of these systems, but in general, a GFS/EC blend was
preferred.

By late next weekend, it does appear that things become slightly
more progressive from west to east over the mainland. This could
open the possibility of a Gulf low inching closer to the
Southcentral coast as well as allowing a front from SW AK to make
it over the Alaska Range. For now though, the forecast looks
mostly dry.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 175-178.
Gale Warning: 130,173,174,179,411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BG
MARINE/LONG TERM...MK/MSO