Small Craft Advisory 225, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Sat Mar 7 2020

.ANALYSIS AND UPPER LEVELS...

Broad southwest flow encompasses much of our forecast area.
Two prominent features are embedded in this flow. The first being
an upper level low located over the Bering Sea that is moving
quickly west in this zonal flow pattern. It is being reflected at
the surface as a weak low pressure system in the Bering Sea. Also
in the Bering Sea, there is onshore flow producing light snowfall
for southwest AK. Over Southcentral AK, a shortwave trough at
500 MB is producing light snow at the surface as it moves
through.

&&

.MODEL DISCUSSION...

Models are in good agreement in our western forecast area over the
Bering Sea and southwest AK. They have been locked on the pattern
for a few days now, so forecast confidence is high in those areas.
The eastern side of our forecast area is a different story, as
models are struggling with handling the light snow events
forecasted for this weekend. Some models are trying to show a
break in the action with southeasterly flow downsloping Anchorage,
while others show continuous snowfall. So moderate confidence in
the forecast for Anchorage Bowl going into Sunday. We are still
confident that the Susitna Valley will see significant snow
through the weekend.

&&

.AVIATION...
PANC...light snow will continue to fall through the TAF period.
Intensity of snowfall and ceiling height will vary as upper level
features passing through will enhance precipitation. Right now it
looks like heaviest snow will be tomorrow morning.

&&

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

Light snow is currently falling across across a large portion of
Southcentral. This will generally continue through the evening. A
front will track northward through the Gulf overnight bringing
gusty southeasterly winds across Turnagain arm, which may briefly
inhibit accumulating snowfall in the Anchorage bowl early Sunday
morning. It will redevelop thereafter under an upper level
trough, continuing into Sunday evening and tapering to flurries by
Monday morning.

A Winter Weather Advisory is in effect for heavy snowfall in the
Susitna and western Matanuska Valleys through early Monday
morning. Snow totals will significantly increase north and west
of Wasilla. Scattered snow showers will persist in these regions
through Monday in a cold, northerly flow, potentially allowing for
isolated higher totals.

Other notable areas that will see significant snowfall include the
Seward Highway between Moose Pass and Seward as well as along
Prince William Sound including Whittier and Valdez. There remains
model disagreement regarding how long snow will linger across
eastern Prince William Sound, however it is highly likely to
continue through the day Monday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Weak upper level pieces of energy in general zonal flow aloft will
continue to propagate through Southwest Alaska today into tonight,
leading to continuation of light snow with light snow accumulations.
A second elongated stronger piece of upper level energy will move
through Southwest Alaska tonight into Sunday morning with
westerly flow. This will enhance the snowfall intensity across the
Alaska Range due to upslope flow as the air is lifted and
condensed. Areas of Southwest Alaska will begin to dry out during
the morning Sunday from west to east as cold air advection begins
to move into the area with some clearing later in the day Sunday.

A digging trough Sunday night into Monday will bring a return of
arctic air to the region. Wind chills will also return with this
blast of arctic air, however, winds and the arctic air won't
align for advisory level criteria.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

An elongated strong piece of upper level energy will exit rapidly
across the eastern Bering leaving a stationary front at the
surface connected with a weak surface low in the central Bering.
As a result, current weather conditions will persist across the
Pribilof Islands and central Bering.

The weak surface low over the central Bering will drift slowly
southward beginning this afternoon and continuing through Monday
afternoon. As this low moves southeastward, a stronger pressure
gradient will develop between Dutch Harbor and Adak allowing gusty
northerly winds to develop as colder air moves in.

An upper level trough digging southward across the eastern Bering
will bring northerly flow to the eastern Aleutians and southern
Alaska Peninsula. Along with this digging trough will be cold air
advection moving down from the north. A stronger pressure
gradient at the surface due to the departing low and building
surface high pressure will allow for the strong gap winds through
the normal areas of the eastern Aleutians and Alaska Peninsula
Monday through Monday night.

&&

.MARINE (Days 3 through 5)...

A low will track east through the Gulf into Tue. This, combined
with cold air advection in anticyclonic flow over the Bering will
bring gale force northwest winds over the Barren Islands and south
of the AKPEN. Expect elevated offshore winds out of the Copper
River Delta. Winds over the Gulf will diminish Thu. A gale force
low will approach the western Aleutians Wed into Thu, bringing
southerly gales to the Aleutians and Bering Sea.

&&

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

A building Arctic high will continue cold, mostly dry weather with
clearing skies over the Southern Mainland Tue and Wed. Meanwhile,
a shortwave trough will traverse the western to central
Aleutians, bringing a quick shot of precipitation. With cold high
pressure inland, expect increased winds out of the usual gaps and
passes over Southcentral: Valdez waters, Copper River Delta, and
over the Barren Islands. Moving forward, a broad low will approach
the western Bering Thu. This will transition Southwest AK to a
southwesterly flow regime, and Arctic air should get shunted
eastward toward the Alcan border. Expect a warming trend to
initiate from west to east Thu into Fri. The associated front will
traverse the Aleutians and reach the Southwest Mainland Sat,
ushering in moisture from the south and potential for
precipitation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145
MARINE...Gale 174 175 176. Heavy Freezing Spray 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
MARINE/LONG TERM...MM