Small Craft Advisory 231, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKDT Mon Mar 9 2020

.ANALYSIS AND UPPER LEVELS...
There is a north to south oriented upper level trough over Alaska
that is slowly moving eastward. There is a somewhat flat ridge
over the central and western Bering, with another trough near the
Kamchatka Peninsula. At the surface there is a low along the north
Gulf coast, with high pressure over mainland Alaska. There is weak
offshore flow over Southcentral, with stronger northerly flow over
Southwest Alaska. A southward moving surface trough is just south
of the Alaska Peninsula and eastern Aleutians, with brisk
northerly flow to the north of that trough. Some snow and snow
showers are occurring over Southcentral ahead of the upper trough
axis. There are also some showers over the Alaska Peninsula and
eastern Aleutians.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Wednesday
afternoon. There are some minor differences on moisture and
precipitation for the early portion of the forecast. Forecast
confidence is higher than normal today.

&&

.AVIATION...
PANC...Occasional MVFR conditions are possible this evening in
lingering snow showers, otherwise VFR conditions are expected.
Northerly winds will increase this evening, then diminish late
tonight.

&&

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

An upper trough over western AK continues to slowly push east,
while a deformation zone on the backside of an associated surface
low near Prince William Sound continues to bring areas of light
snow to Southcentral. Snow will taper off tonight in the
Anchorage Bowl and Mat-Su Valleys, and ramp up a bit in the
Copper River Basin as the low steers east. As the parent upper
trough dives southeastward tonight into Tuesday, expect a
transition to cold, dry conditions from west to east. Expect
strong offshore winds out of the usual gaps and passes driven by
a strong pressure gradient between cold dense air inland and lower
pressure offshore. Winds will initially ramp up tonight over the
Barren Islands, then gusty winds will set in in Whittier/Passage
Canal by early Tuesday morning. Valdez waterways and Thompson Pass
winds will ramp up during the day Tuesday. Expect gusts around 60
mph through Thompson Pass Tuesday afternoon, causing visibility
reductions from blowing snow. Winds will diminish Wednesday as the
Arctic high becomes centered over the Alcan border and the
surface low in the Gulf exits to the east. Upper ridging will
build in over the Bering, maintaining dry, quiet weather over
Southcentral Wednesday into Thursday.

&&

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

Quiet weather will persist for Southwest AK the next few days. It
will be clear and cold through midweek with northerly drying flow
in place. Wind direction will switch Thursday to the southwest, which
will warm temperatures and bring in cloud cover from the Bering Sea.
This is due to a front approach in the central Bering. However, high
pressure to the east will hold off any weather from reaching the
coast, keeping weather quiet. We are confident in this forecast.

&&

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

The next couple of days the big weather story is northerly cold
air pushing through the Eastern Aleutians creating gusty winds
through bays and passes. This is also creating heavy freezing
spray in areas of gusty winds. The eastern Bering then quiets
Wednesday into Thursday. During this time a low pressure system
enters the Bering from the west. Model runs showed this low coming
in stronger today and setup looks favorable for storm force
winds, so we did increase this low to a storm force today.
Winds will likely need to be adjusted over the next couple of
days, but anticipate it to stay storm force strength. High pressure
over Interior AK will prevent this low from working eastward to
the coast. We have moderate to high confidence in the forecast.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

Gusty northerly gap winds along the Alaska Peninsula and the Gulf
Coast are expected for much of Wednesday. Some of the stronger
winds will be around the Barren Islands, Resurrection Bay, and the
Copper River Delta. Winds will taper west to east Wednesday midday
through the evening hours as high pressure builds in and pressure
gradients weaken. A strong front enters the western Aleutians
Wednesday afternoon and is expected to bring storm force winds to
the western Bering and the western extremity of the Aleutians
Wednesday night. Winds diminish some by Thursday midday, but an
area of storm force winds west of St Matthew Island and north of
56N is expected through that time. As this front weakens Thursday
evening, a period of relatively quiet weather is expected through
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7/Wednesday through Sunday)...

Wednesday will see the start of a quiet weather period with much
of Southcentral Alaska seeing their first sunny skies in a bit.
Clearing skies and northerly winds will bring back colder
temperatures to end the work week. High temperatures will rebound
over the weekend as high pressure dominates southern Alaska.

Weather across much of the Bering will be relatively quiet to
start the period. A storm force front will bring some strong winds
to the Western Aleutians Wednesday night and spread across the
Western Bering, before diminishing early Friday morning. A north
Pacific low will approach the eastern Aleutians Thursday night,
but model confidence is low on its impact to the area. Aside from
this system, weather for the remainder of the weekend will remain
quiet in and around the Bering Sea.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 121 127-132 138 150 155 172 178
FIRE WEATHER...NONE.

&&

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