Small Craft Advisory 175, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Thu Feb 20 2020

.ANALYSIS AND UPPER LEVELS...
There is a negatively tilted upper level trough extending from
Norton Sound through Kodiak Island into the Gulf. A fairly strong
surface low associated with this trough is centered just north of
Kodiak Island and is moving slowly towards the northeast. Showers
are occurring over much of southern Alaska and the Gulf in the
cyclonic flow. There is also strong cold advection to the west of
the trough and surface low. Cyclonic flow aloft continues westward
through much of the Bering Sea and Aleutians with another trough
over the eastern Bering, and yet another one over the western
Bering.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement on the synoptic pattern
through Saturday afternoon. There are some differences on the
locations and movements of several surface lows in the Bering on
Saturday. However, the differences are not large and the lows are
fairly weak. Therefore the forecast confidence is a little above
normal today.

&&

.AVIATION...
PANC...MVFR conditions in snow are expected to develop by this
evening. Visibilities should drop to IFR late tonight, and
continue through Friday afternoon. Light winds will persist.

&&

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

A low south of the Kenai Peninsula will continue to weaken as it
moves north across Southcentral through Friday afternoon. Aloft, a
negatively tilted upper level trough will close off to form a
closed low that will move up Cook Inlet. These features, combined
with weak waves of energy, will provide the forcing necessary for
widespread snowfall across Southcentral. Heaviest amounts will
fall in the Susitna Valley and across coastal and higher
elevations. Lighter snow amounts will fall across the western
Kenai Peninsula, Anchorage Bowl, and Matanuska Valley, where a
general 1 to 4 inches of accumulation expected. As the low moves
into the northern Gulf by Saturday afternoon, precipitation
amounts will taper off as we look at a cooler and drier pattern,
with the development of northerly outflow winds and a return to
colder temperatures by the end of the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tonight
through Saturday)...

A low over the northern Gulf coupled with an upper level low is
causing snow shower activity across portions of Southwest Alaska.
The effects of the snow are the most intense along the Kuskokwim
Delta coast. In this area, a combination of previously fallen snow
that accumulated on the sea ice, strong northwesterly winds, and
falling snow from snow bands have resulted in blizzard conditions
along the coast. Since the strong winds are expected to continue,
even as the falling snow gradually weakens, the Blizzard Warning
for the coast remains in effect until noon on Friday. Elsewhere,
snow showers are also occurring around King Salmon, where the
winds have shifted to the west, and the onshore flow is upsloping
along the nearby mountains. Tonight, the same areas of snow will
generally continue, as upslope snow showers spread northward along
the foothills of the Alaska Range. Upsloping is also a component
of the snow occurring around Bethel, and that too is likely to
continue through the overnight hours. The colder air moving in
along the coast tonight will also cause wind chills to drop to
between 25 below and 35 below. At those wind chill temperatures,
frostbite is possible on exposed skin in 10 minutes.

On Friday, the moisture will gradually shift eastward, as a cold
air mass deepens from west to east across the rest of Southwest
Alaska. This will lead to falling temperatures with many areas
dropping into the teens below zero on Friday night. As high
pressure builds overhead on Saturday, the winds will diminish, and
should finally clear, allowing for cold sunshine for the entire
day.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Tonight
through Saturday)...

A large low over the northern Gulf is causing an Arctic air mass
to pour southward across the Bering. This is producing widespread
snow showers over much of the southern Bering for all areas east
of Adak. This will continue into Friday as an area of high pressure
builds over the eastern Bering and causes the winds to taper off.
A new North Pacific low will approach the central Aleutians on
Saturday. It will bring rain and wind to those islands. The low
center will stall well south of the islands, so the northern
periphery of the storm will be what impacts the islands from Adak
almost to Shemya. The low will slowly shift south away from the
islands Saturday night.

&&

.MARINE (Days 3 through 5 Sun through Tue)...
A low south of the central Aleutians Sunday morning will move
eastward across the eastern Aleutians Sunday and into the Gulf on
Monday. Gale force winds are expected with this low. Gale force
northerly winds are possible in the central and Western Aleutians
Monday night and Tuesday. Confidence in the forecast for Monday
night and Tuesday is fairly low.

&&

.LONG TERM FORECAST (Days 4 through 7 Mon through Thu)...
On Monday morning the pattern will consist of an upper level
trough over the Copper River basin extending south into the
northeast Gulf of Alaska, an upper level ridge over Southwest
Alaska extending into the eastern and central Bering Sea, and
upper level lows south of the eastern Aleutians and south of the
western Aleutians. The trough over the Copper River basin will
slowly slide east into Canada by Monday afternoon. The ridge will
move east and north through Tuesday. The ridge will then split,
with one half moving into Canada and the other half retreating
westward into northeast Russia. The eastern Aleutian low will
move east and a bit north through Wednesday, undercutting the
ridge over southern Alaska and settling in the Gulf Wednesday into
Thursday. The western Aleutian low has a more uncertain future,
possibly staying nearly in place or it could move into the Bering.
Model spread is fairly large for the extended portion of the
forecast and confidence is fairly low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 155.
MARINE...Gales 130 132 138 150 351 352 NONE.
FIRE WEATHER...NONE.

&&

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