Small Craft Advisory 863, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKST Thu Nov 12 2020

.ANALYSIS AND UPPER LEVELS...

A broad upper low currently extends across the Bering/Aleutians
and over the Alaskan mainland. An area of low pressure in the
eastern Gulf continues to move northeastward towards Southeast
Alaska. The subtropical jetstream remains south of 50N west of
170W then extends along 50N between 170W and 145W before curving
southeastward towards California. The strongest portion of the
subtropical jet is between 170W and 145W as this is where the
largest height gradient is as a strong upper level ridge persists
to the south while the aforementioned upper low is located north
of the jet.

Closer to the surface, radar imagery shows precipitation moving
northeastward in Cook Inlet. Precipitation is currently in the
form of snow along the northern Kenai Peninsula, across the Anchorage
Bowl into the Mat-Su Valley. Kenai and southward, precipitation
is in the form of rain as temperatures are warmer in these
locations. Radar also shows precipitation across the Kuskokwim
Delta and Bristol Bay. Temperatures are cold enough inland for
snow, though along the coast precipitation is in the form of rain.

&&

.MODEL DISCUSSION...

Guidance is in good agreement with the placement and orientation
of synoptic scale features at both the surface and in the upper
levels through the short term. Forecast confidence remains above
average. High resolution models were favored to resolve local
effects.

&&

.AVIATION...
PANC...Light winds will persist throughout this TAF package.
Conditions will gradually become MVFR through 16z today as
ceilings fall between 1000-3000ft. MVFR ceilings will continue
through 08z on the 13th before rising back to VFR.

&&

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

The main upper level low is edging into Southcentral across the
Alaska Range and into northern Cook Inlet this morning and then
will make it into the northern Gulf of Alaska tonight. A front
extending across Cook Inlet and into Prince William Sound this
morning is the focus for snow showers from Anchorage northward and
rain showers from Kenai Southward this morning.

Colder air aloft is moving in from the west in conjunction with
this low so precipitation should be switching to all snow by
tonight for most of the area. The low moving into the northern
Gulf will also keep strong northwesterly winds over the Kodiak
area and into the Gulf through tomorrow.

This will usher in a bit of a pattern change for the end of the
week as Southcentral will see a cooling and drying trend Friday
and into the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Thursday
morning through Saturday)...

A broad upper level trough with two embedded lows persists across
the area. The main low over Southwest Alaska this morning will
slowly move east over the Alaska Range and into the Gulf, while
the secondary low near St Matthew Island remains stationary today.
These features will continue to bring rain and snow to Southwest
Alaska with the rain mainly along the coast and south of King
Salmon with snow to the north. Expecting 1 to 3 inches of snow
across the Kuskokwim Delta and from areas north of King Salmon
today and tonight. Snow will taper off for most valley areas
tomorrow as another low pressure and cold front moves across the
northern Bering on Friday and moves towards Bristol Bay by
Saturday morning this will bring another chance of snow across the
Kuskokwim Delta Friday afternoon into Saturday morning. This cold
front will reinforce the cold air moving across Southwest Alaska
with temperatures falling each day during the period. Finally
winds will increase today across most inland areas as the pressure
gradient tightens up, and begin to decrease a bit on Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Thursday
morning through Saturday)...

Primarily westerly flow across the Bering today with winds
generally decreasing except for some gales along the Southwest
coast and storm force winds in Bristol Bay this morning. Another
cold front approaches from the north and will track across the
Bering towards the central and eastern Aleutians Friday through
Saturday with a mixture of rain and snow showers. Flow transitions
to north/northwesterly behind the front and will include
significant cold air advection. By Saturday, a ridge begins to
form over the Western Bering, but will likely be short- lived as a
low currently over Russia approaches the Western Aleutians.

&&

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

Gulf of Alaska:

A series of weak low centers moves across the Northern Gulf
through Mon. The track and development of these lows are mixed,
but confidence is good for easterly winds along the North Gulf
Coast and westerly winds over the Southern Gulf, gustiest near
Kodiak Island Sat. A polar front crosses the Alaska Peninsula Sun
with increasing confidence for widespread northwesterly small
craft winds and waves to extend from the Shumagin Islands to the
Barren Island and Kachemak Bay into the Gulf, with local gale
force winds near Kodiak Island through Mon.

Bering Sea:

A polar front crosses the Aleutians and Alaska Peninsula for Sun.
Widespread northerly small craft winds and waves over the Central
and Eastern Bering exit Bristol Bay Sun night. Local gale force
winds with the front across the Central Aleutians through Sat
night. A well developed front approaches the Western Aleutians and
Bering Sun. The track and development of the front are mixed, but
confidence is good for widespread high-end southerly gale force
winds and waves with local storm force wind gusts moving to the
Central Bering by Mon.

&&

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

The long term forecast begins Sunday with the highly amplified
ridge over the Aleutians shifting east in response to a storm
system developing off the coast of Russia. This eastward shift
will help relax the cold, northerly flow across the
Bering/Aleutians with snow showers and gap winds over the AKPEN
both diminishing. This ridge will extend north and east over the
Southern Mainland by Monday, resulting in a continuation of mainly
clear and dry conditions (with the exception of isolated high
elevation snow showers) and gusty, northerly gap winds along the
Southcentral coast.

By Monday, the GFS and EC diverge in their respective solutions.
The EC drops a shortwave southeast from the Bering Strait across
interior Alaska. This solution would result in a weaker ridge
building over Southcentral and keep a higher chance for snow
showers in the forecast, especially along the Alaska Range
through Tuesday. With the ridge not as robust, the EC also brings
the front associated with the aforementioned low to Southwest
Alaska for Tuesday. The GFS, on the other hand, keeps a stronger
ridge in place while slowing and weakening the front as it moves
across the Bering. The GFS finally brings a weaker front to the
Southwest coast by Wednesday, with both long-term models
dissolving the front while keeping the ridge steadfast over the
Al-Can border. Also by Wednesday, both models have the entire
upper-level trough moving over the Bering with unsettled weather
continuing through mid-week for the Bering and Aleutians. As for
the Southern Mainland, it looks like clear and dry conditions will
prevail, especially for Southcentral where the northerly flow
will persist.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 131 132 136 137 138 150 155 165 180
181 351 352 414.
Storm Warning 160.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...MK/TM

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
927 AM AKST THU NOV 12 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0851 AM     SNOW             RABBIT CRK/BEAR VALLEY  61.08N 149.72W
11/12/2020  E4.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            CORRECTS LOCATION.

&&

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
931 AM AKST THU NOV 12 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0929 AM     SNOW             2 ESE POTTER HILL       61.08N 149.78W
11/12/2020  M1.5 INCH        ANCHORAGE          AK   TRAINED SPOTTER

0851 AM     SNOW             RABBIT CRK/BEAR VALLEY  61.08N 149.72W
11/12/2020  E4.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            CORRECTS LOCATION.

&&

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1134 AM AKST Thu Nov 12 2020

AKZ145-150100-
Susitna Valley-
Including the cities of Talkeetna, Willow, and Cantwell
1134 AM AKST Thu Nov 12 2020

There is a freeze up ice jam on the Talkeetna River which is
currently causing flooding of two residences in East Talkeetna
along Mercedes Drive. Be alert for changing conditions as water
levels will fluctate as the ice jam shifts.

In the event that flooding becomes worse, notify the Talkeetna
Fire Chief and/or the Matsu Borough.