Gale Warning 742, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKST Wed Dec 30 2020

.ANALYSIS AND UPPER LEVELS...

An upper trough centered over the Kenai Peninsula is producing
widespread cloud cover to the southern mainland, but with very
dry low levels for interior Southcentral, very little in the way
of precipitation is reaching the ground. Further south towards
Homer, there is enough moisture to support rain and snow showers,
but with minimal impact. A weak ridge of high pressure over the
Southwest coast allowed for the fog/stratus development from
Nunivak Island southward into Bristol Bay today, which reached
far enough inland today to bring periods of dense fog into King
Salmon. Over the Bering, the nearly stationary trough that has
been persistent over the western Bering is slowly moving to the
east and weakening in advance of a developing low approaching the
Western Aleutians. This low will continue strengthening into the
extremely deep, and potentially record breaking low as it moves
into the Western Aleutians late tonight.

&&

.MODEL DISCUSSION...

Model agreement in the main synoptic scale features is good into
Friday with both the approaching major Bering Sea storm and
placement of smaller scale features in the Gulf/Southcentral.
Agreement continues to improve with the intensity of the Bering
low, leading to improving confidence on the impacts of this
system through Friday. By late in the week, confidence begins to
waver both with the track of the Bering, and a much weaker low
through the Gulf. This is leading to lower confidence in the
Bering Sea and north Gulf Coast by the end of the work week.
Forecast confidence however is much higher for inland areas of the
southern Mainland.

&&

.AVIATION...

PANC...VFR conditions with light winds are expected to continue
into Thursday. There is an outside chance of a stray very light
snow shower this evening, but this is not expected to produce any
accumulation.

&&

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

A chance of an additional round of very light snow persists
across Anchorage Bowl, and over the eastern Kenai Peninsula and
Prince William Sound tonight. This is due to an upper low over the
Kenai Peninsula which is approaching the Anchorage Bowl this
evening. Both NAM and GFS differ slightly on the placement of the
upper low as it curves eastward to Prince William Sound, but the
potential for snow exists over the region as the upper level
pattern brings some energy combined with colder environmental
temperatures aloft to produce snowfall tonight. Once this upper
low phases out Thursday afternoon, drier conditions will move in
as general offshore flow sets in and local gap flows strengthen.
For now, will keep a close eye on the aforementioned low as it
could regain strength before retrograding westward.

&&

.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. With time, the upper level trough will
become a closed low and push towards the Western Alaska Range
bringing a fresh blanket of snow to interior areas. 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
heads 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 actually bring 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 40 degrees. On the latest
satellite imagery the moisture fetch associated with this system
extends to the northern Mariana Islands. This modified tropical
moisture will combine with strong cold air advection from Siberia
to help the low become so strong.

Thursday night and Friday the front will begin to weaken, but
the wind 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 Sat. 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 sweeps into the eastern Gulf by Mon. 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 Mon.
Weak high pressure moves into the Western Gulf for Mon.

Aleutians/Bering:
A strong low tracks in a circle over the western Bering through
Mon. 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 Monday. 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 Mon. The low will continue with gale force winds and seas
to 20 feet over most of the Bering through Monday.

&&

.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 southern
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: 175-178 411 413.
Storm Warning: 170 172-174 179 185 412 414.
Gale Warning: 130 155-165 171 180 181 352.
Heavy Freezing Spray Warning: 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...CMB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...MF/DEK