Small Craft Advisory 948, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 AM AKST Tue Dec 8 2020

.ANALYSIS AND UPPER LEVELS...

Broad longwave trough with weak surface reflection lingers over
most of the state through Thursday afternoon. As the trough exits
to the north, an upper-level ridge will set up over Southern
Alaska through the end of the week. Heaviest precipitation will be
along the Southcentral coast and will be mostly in the form of
snow or a rain/snow mix. Additionally, drier conditions across
Southwest by Wednesday night and Southcentral by Thursday night as
the ridge pushes inland.

Out west, a storm force front enters the Western Bering on Tuesday
night. The parent low will remain south of the Aleutian Chain
until Thursday, crossing over into the Southern Bering on Thursday
afternoon. Multiple embedded surface lows rotate cyclonically
around the parent low and across the Central/Western Bering.
Ahead of the front, temperatures should be cold
enough to keep precipitation as snow. Warmer air will encompass
much of the Bering behind the front through the end of the week,
resulting in mostly rain or a rain/snow mix.

&&

.MODEL DISCUSSION...

Models are in great overall synoptic agreement through the end of
the week. On the smaller scales, models diverge slightly on the
embedded lows within the storm force low west of the Aleutian
Chain. All models solutions show a series of surface lows spinning
up along the front where the temperature gradient is greatest,
but differ on speed and location.

&&

.AVIATION...
PANC... IFR conditions starting this morning as a band of light
snow passes over Anchorage today with lowered cloud ceilings and
visibilities. Improvement to marginal conditions by this
afternoon/evening.

&&

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

Broad upper level cyclonic flow with embedded short waves will be
the main upper level features driving the weather today/tonight.
Things get more complicated in the lower levels as a surface low
heads toward western Prince William Sound today and cold air
advection spreads north up Cook Inlet. Snow will generally spread
north across Southcentral this morning with cold advection
and as the low to mid level flow become more southerly. This will
bring measurable snowfall to the Anchorage Bowl and MAT-SU
Valleys today. Snow will diminish across these areas this evening.
The North Gulf Coast will generally see rain or a mix of rain and
snow today and snow showers tonight as cold air finally moves
over the area. Gusty southerly winds are expected across Cook
Inlet and near Seward with pressure rises as the surface low lifts
north Prince William Sound.

On Wednesday, the upper low over Bristol Bay today lifts north
over Southcentral Wednesday afternoon/night. This will likely
bring increasing chances of snow/snow showers across the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tuesday
morning through Friday morning)...

A vertically stacked low over Bristol Bay will gradually track
eastward through Wednesday. Gusty northerly winds are still
blowing along much of the coast, especially along the Kuskokwim
Delta coast. Snow and blowing snow will gradually taper this
morning as the low weakens and the majority of the precipitation
dissipates. Some snow showers are likely across the interior
Bristol Bay area, the Lower Kuskokwim Valley, and the foothills of
the Alaska Range this evening as residual moisture tracks
eastward with the low. Meanwhile for the Alaska Peninsula, snow
and blowing snow conditions are still expected through the day
today as gusty northerly flow persists. By Wednesday, clearer
conditions are expected as a much colder airmass arrives from the
north and an upper level ridge builds eastward through Thursday. A
pattern change is expected by Friday with a return of warm
southerly flow as a front associated with a storm force low in the
Bering approaches the southwest coast.

&&

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

A developing storm force low south of the western Aleutians is
visible on satellite imagery this morning. As this low tracks
eastward, its associated front will extend across the Aleutian
Chain, bringing storm force winds across the Bering waters through
Thursday as it travels east. Model solutions seem to consistently
agree in a rapid occlusion of this front, allowing precipitation
to initially fall as a rain/snow mix over the western Aleutians
tonight with a change to snow as colder air fills in on the
backside of the low. Given the strong temperature gradient along
the front, model solutions are suggesting several smaller lows
will develop along the front as it reaches the central Aleutians
by Wednesday night. Meanwhile the parent low will have evolved
into a broad area of low pressure centered over the western Bering
by Thursday. This system will allow active weather to persist
through the end of the week.

&&

.MARINE (Days 3 through 5 - Thursday through Saturday)...

Bering: A well-developed Western Bering low merges with a North
Pacific low, forming a new low that moves to the Central Bering
by Sat. Its front sweeps to the Northeastern Bering by Sat. While
model track and development are mixed, widespread high-end gale
force winds and waves wraps the low and front through Sat. The
rear flank winds over the Western and Central Aleutians moves
South and dissipate Sat as the low weakens.

Gulf of Alaska: A weakening low moves across the Northern Gulf
through Thu night. Model track and development vary, but lingering
gap winds bring northwesterly small craft winds and waves over the
Barren Island into the Western Gulf diminishing Thu. A front
extending across the Alaska Peninsula into the North Pacific moves
into the Gulf through Sat. A well-developed North Pacific low
moves along the front to the extreme southwest corner of the Gulf.
Widespread high-end southeast gale force winds move into the Gulf
becoming easterly across Kodiak Island and the Gulf by Sat.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Models are in decent agreement on a broad vertically stacked low
near the western Aleutians early on Friday with a weakening front
draped across the central/eastern Bering and Aleutians.

On Saturday the Aleutians/Bering storm continues to gradually
weaken, with a new storm system lifting out of the North Pacific
towards the Alaska Peninsula. There is significant disagreement as
to whether this low will bring a front towards the Northern Gulf
Coast, or keeps its impacts primarily just limited to Kodiak and
the AKPEN.

On Sunday this new low looks to retrograde and merge with the
broader low centered in the Bering, with a fairly quiet pattern
setting up for Southcentral which will persist into Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 181
MARINE...Storm: 411, 413, 175, 176
		 Gale: 412, 414, 132, 155, 172, 173, 174, 177, 178, 180
 		 Heavy Freezing Spray: 160, 180, 181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...KZ/BG

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
253 PM AKST Tue Dec 8 2020

AKZ125-091300-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
253 PM AKST Tue Dec 8 2020

...HEAVY SNOW LIKELY ALONG THE SEWARD HIGHWAY FROM MOOSE PASS TO
SEWARD FOR WEDNESDAY...

Snow is expected to develop Wednesday morning across the southern
Kenai Peninsula with bands of locally heavy snow at times through
late Wednesday afternoon. Snowfall rates of 1 to 2 inches per
hour are possible along the Seward Highway from Moose Pass to
Seward in the heavier snow bands with visibility dropping to as
low as one quarter mile. Up to one foot of snow, with locally
higher amounts, is possible before the snow tapers off late
Wednesday evening.

For the latest road conditions call 5 1 1 or visit
511.alaska.gov.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
342 PM AKST TUE DEC 8 2020

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

0341 PM     SNOW             NWS ANCHORAGE           61.16N 149.98W
12/08/2020  M2.4 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            SPOTTER REPORTED TOTAL SNOW ACCUMULATION OF
            2.4 INCHES.

&&

MV

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
343 PM AKST TUE DEC 8 2020

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

0313 PM     SNOW             4 WNW PALMER            61.62N 149.21W
12/08/2020  M2.0 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

0216 PM     SNOW             SUTTON                  61.72N 148.88W
12/08/2020  M1.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

0341 PM     SNOW             NWS ANCHORAGE           61.16N 149.98W
12/08/2020  M2.4 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            SPOTTER REPORTED TOTAL SNOW ACCUMULATION OF
            2.4 INCHES.

&&