Heavy Freezing Spray Warning 11, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKST Thu Dec 31 2020

.ANALYSIS AND UPPER LEVELS...

A ~200 kt upper level jet is deepening a 500 mb low in its left
exit region over the far western Aleutians. Upper level heights
are expected to fall to an impressive 466 Dm. Likewise the
associated surface low will drop to a potential North Pacific
record-breaking 920 mb. Hurricane-force winds have been observed
over Shemya along its associated front. These winds will gradually
weaken as the front tracks farther east today and Friday.
Moderate to heavy rain is likewise occurring along the front as
the stream of associated upper level moisture extends all the way
to the Philippines. Conditions are much more quiet ahead of the
front in the eastern Aleutians under a weak upper ridge. This,
combined with weak surface flow is leading to areas of dense fog
across the Southwest Mainland, although it will gradually
dissipate through the morning as offshore winds will develop.
Farther east, an upper level low is leading to scattered snow
showers over the Kenai Peninsula and Prince William Sound. Patchy
low stratus is also shown on satellite over portions of the
Susitna Valley and Copper River Basin. Skies are expected to clear
over much of Southcentral by afternoon.

&&

.MODEL DISCUSSION...

Synoptic model agreement remains good with the potentially record
setting low in the western Bering as well as the development of
relatively stronger high pressure over northwestern Canada leading
to gradually increasing offshore flow over the Mainland. There are
some slight differences Friday into Saturday with the interaction
of two Gulf lows. However, outside of possible light snow in
Kodiak, no impacts are expected.

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

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

A fairly persistent area of snow showers across the Kenai
Peninsula and Northern Cook Inlet will slowly diminishes through
today as the responsible upper low weakens and tracks south.
After that, easterly flow across the Gulf will continue the threat
of showers (snow or mix) over Kodiak Island through Friday night.
General offshore flow with embedded gap winds will set up tonight
and strengthen some Friday. The strongest/gustiest areas will
likely be along the North Gulf Coast near the Copper River Delta
and Thompson Pass. Although weaker, gusty northeast winds are
expected to develop across the Matanuska Valley later today and
continue into Friday night. Temperature forecasts for the Copper
River Basin could be somewhat challenging. 850mb temps over the
Basin continue to drop, however, low clouds will likely persist
across the area at least into Friday afternoon. So how cold does
it really get? The MOS guidance shows clearing. For now kept
temperatures tonight a little above MOS guidance then lowered
temps in the Basin near MOS values (-20s) Friday night in
anticipation of clearing.

&&

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

There is an area of troughing across Southwest Alaska which is
present at the surface and at 500 mb (middle atmosphere). This
will bring areas of light rain and snow to the Bristol Bay and
Kuskokwim Delta area. As time elapses the upper level trough will
actually become a closed low and push towards the Western Alaska
Range bringing a fresh blanket of snow to interior locations also.
A ferocious low will move into the Bering with an estimated
intensity of 922 mb - 925 mb. While the Bering will see the lions
share of the winds and precipitation, late Thursday and Friday
winds will increase as the front approaches Western Alaska.

&&

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

There is a low with a magnitude of 947 mb southwest of Attu
Island this afternoon. This low will continue to deepen, as it
moves towards the western Aleutians tonight. This storm could end
up being a record-breaker. This low pressure system is expected
to bottom out Thursday morning around 922 to 925 mb based on
current model guidance. This will rival the Bering Sea storm from
1977 which had an observed pressure of 924 mb. The front
associated with this low will bring a band of storm and hurricane
force winds. The models indicate 50 foot seas north and south of
the chain.

Winds will be near warning levels for Shemya with the initial
front Wednesday night and Thursday morning but should stay below
75 mph threshold so will not issue a high wind warning. Blowing
snow should be limited as temperatures warm above freezing fairly
quickly after the onset of snow Wednesday night and some locations
may even see temperatures near 40F. On the latest satellite
imagery the moisture fetch associated with this system extends to
the Northern Mariana Islands. This modified tropical moisture
will merge with strong cold air advection from Russia.

Thursday night and Friday the front will begin to weaken, but
the pressure gradient field is expansive and will still produce
fairly strong winds for the eastern Aleutians with gusts up to 50
knots and a mix of rain and snow.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

Gulf of Alaska:
A barrier jet brings small craft winds over the Northern Gulf
across Kodiak Island and the Western Gulf through Saturday. A
broad area of low pressure stretches across the Gulf through Mon.
A second low pressure center develops along the front over the
Southern Gulf and moves into the Eastern Gulf by Monday. Model
track and development are mixed but confidence is good for small
craft winds and waves with gale force gusts across the southern
Gulf with seas to 20 feet spreading northward over the Gulf
through Monday. Weak high pressure moves into the Western Gulf for
Monday.

Aleutians/Bering:
A strong low tracks in a circle over the western Bering through
Monday. Model tracking and development are mixed but confidence
remains high for this wind wrapped low to spread high end gale
force winds and waves with storm force gusts over the Aleutians
and Bering mostly on Saturday, diminishing through Mon. This
brings seas to 45 feet on the Pacific side and 35 feet on the
Bering side between Shemya and Adak for Sat and diminishing slowly
through Monday. The low will continue with gale force winds and
seas to 20 feet over most of the Bering through Mon.

&&

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

Confidence is fairly low in the specifics of the forecast this
weekend, but there is generally good confidence that any major
systems will remain offshore in the Bering and Gulf of Alaska into
early next week. As a result, the weather for much of the South
Mainland, especially interior areas, should remain fairly benign
and stable with temperatures near to slightly below normal.
Storminess will also continue to slowly diminish in the Bering
Sea/Aleutians into early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning: 187.
MARINE...Hurricane Force Wind Warning: 175-178.
         Storm Warning: 170 172-174 179 185.
         Gale Warning: 150 155 160 165 171 180 181.
         Heavy Freezing Spray Warning: 160 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...MF/DEK