Small Craft Advisory 169, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
523 PM AKST Tue Feb 18 2020

.ANALYSIS AND UPPER LEVELS...
There is a strong and nearly vertically stacked low in the Gulf
moving rapidly northward. Storm force winds are occurring along
the north Gulf coast ahead of this low. There is also a
significant amount of moisture associated with this low, with
areas of moderate to heavy snow over much of Southcentral. There
is a vertically stacked low centered over the Yukon Delta that is
moving northward. Some showers and breezy conditions associated
with this low remain over southwest Alaska. Flow aloft over the
Bering Sea is westerly and flat, with some showers evident. An
east to west oriented front is bringing some winds and
precipitation (mainly snow) to the central and western Aleutians.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Thursday
afternoon. One exception to this is with the low currently in the
Gulf, with the NAM faster bringing it north compared to the other
models.

&&

.AVIATION...
PANC...IFR conditions in snow are expected into the early morning
hours Wednesday. Conditions should improve to MVFR during the late
morning, and then to VFR in the early afternoon as the snowfall
diminishes then ends.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The frontal zone that brought snow to much of Southcentral Alaska
has shifted to the west over the Kenai Peninsula this afternoon. A
surface low is currently developing along this front and will
bring a gale-force front to the north Gulf Coast this evening.
That system will continue to shift the frontal band to the west
while lightening the snow over the Anchorage Bowl and Matanuska
Valley. As that surface low moves into Prince William Sound, cold
air will come up Cook Inlet and return a final burst of snow
through Wednesday morning before tapering off for most inland
locations. Snow showers will linger along the Gulf Coast through
Wednesday evening.

Yet another front enters the Gulf on Wednesday night. The gale-
force front will again ramp up gap winds in favored areas, but
look to be weaker than the previous system. Conditions are not as
favorable to get winds to come down the Hillside again, but
Turnagain Arm should still be very gusty. Overall, expect near
normal to slightly above normal temperatures and chances for snow
inland with a mix of rain and snow along the coast.

&&

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

A low that has been impacting Southwest Alaska for the last couple
days is slowly moving up the west coast this afternoon, and is
approaching Norton Sound. The southwesterly onshore flow continues
across much of Southwest Alaska as the precipitation associated
with the low weakens. This should make for favorable conditions
for fog and low stratus to develop across much of the area
tonight. Otherwise quiet weather will persist through most of the
day on Wednesday.

The next low will approach the area Wednesday evening. There is
some disagreement in the track of the center, but most of the
models bring it up into Bristol Bay Wednesday night. This low will
bring another round of snow to much of Bristol Bay. Depending on
those differences in track, portions of the area could become
windy enough, especially north and west of the low track, to
experience blowing snow overnight. Currently the greatest threat
is around the Dillingham area. A Winter Weather Advisory may be
required for blowing snow if model consistency remains good. The
low center will shift into the Gulf on Thursday, and stall out
there. This will keep snow showers ongoing over the area both on
Thursday and Friday.

The next big weather story will be the influx of much colder air
into Southwest Alaska during the day on Thursday. This will result
in falling temperatures through the day on Thursday, with below
zero temperatures returning to the Kuskokwim Delta Thursday night,
and the cold air moves into the rest of Southwest Alaska by
Friday.

&&

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

The main weather stories across the Bering will be a new low
tracking along the Aleutians, and the cold air it brings south
into the Bering behind it. The low is currently moving into the
central and western Aleutians, then will move across the islands
Wednesday morning, then tracks along the north side of the
eastern Aleutians and Alaska Peninsula Wednesday afternoon, then
moves into Bristol Bay Wednesday night. This low will bring a
round of rain and snow to the islands ahead of it, then all snow
is expected behind the low. Since the low will be fast moving, no
one area is expected to pick up more than a couple inches of snow
in a brief hit as the low moves by. The bigger story will be the
much colder air moving in behind the low. This will likely return
nearly all of the Bering to widespread snow showers. The core of
the coldest air will shift eastward into Southwest Alaska by
Friday evening. Meanwhile across the western Bering, compact lows
spinning up in the cold air will produce brief periods of gusty
winds and snows as they move by Thursday and Friday, but exact
tracks of those lows remain highly uncertain.

&&

.MARINE (Days 3 through 5 Fri through Sun)...
Friday night through Saturday night, Gale force west to northwest
winds are expected over the Gulf and south of the Alaska
Peninsula. A storm system moving into the western and central
Aleutians on Saturday will produce gale force winds, which will
slide into the eastern Aleutians on Sunday.

&&

.LONG TERM FORECAST (Days 4 through 7 Sat through Tue)...
On Saturday morning the pattern will consist of an upper level
trough over southwest Alaska extending southeastward into the Gulf
of Alaska. The trough will very slowly slide northeast across
southern Alaska through Sunday then exit into Canada Sunday night
as the main upper low retreats northward. A weak ridge will then
build over southern Alaska Sunday night, sliding eastward on
Monday. An upper level low just south of the western Aleutians on
Saturday will move eastward through Sunday, moving north into the
central Aleutians on Monday, then into the southern Bering Tuesday.
The exact path and timing of this feature is somewhat uncertain
at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 101 111 121 145.
MARINE...Storms 119. Gales 120 125 130-132 150 155 170 172-178 351
                           352 413 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...BL