Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 161357
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Mon Jan 16 2017
.ANALYSIS AND UPPER LEVELS...
Active weather is moving into Southcentral as the right entrance
region of a 75 kt jet streak moves across the area. The associated
divergence is drawing a North Pacific low northward toward Prince
William Sound this morning. Elsewhere...an expansive upper level
longwave trough has overspread western Alaska and nearly all of
the Bering. This setup is keeping very cold north flow going
across the entire region.
Southcentral...Snowfall rates in Whittier have been increasing
throughout the night, and visibilities and ceilings have been
lowering in Valdez as well. The band of heavier precipitation on
Middleton Island radar continues to push westward across Prince
William Sound. Light snow is also being reported at stations
across the Anchorage Bowl. Infrared satellite shows cooling cloud
tops from Anchorage eastward indicating developing snow-producing
clouds as the low over the Gulf approaches. Snow is also falling
over much of the Copper River Basin with visibilities now below 2
miles at Gulkana. The consolidating deformation band from
Anchorage east is leading to associated subsidence west of
Anchorage. This is also confirmed on IR Satellite by rapidly
warming cloud tops and/or clearing over the Kenai Peninsula and
Susitna Valley. As a result, snowfall activity has largely ceased
in that area for now.
Out West...A dome of bitterly cold Arctic high pressure is in
place across Southwest Alaska this morning. Most locales away from
the coast and the Alaska Range are under clear skies. However, the
tradeoff is that the clear skies, light winds, and snow cover have
resulted in near ideal conditions for radiational cooling. Stony
River leads the way with temps falling through the -30s. The
aforementioned upper level trough is keeping a constant supply of
very cold air moving southward through nearly all of the Bering.
The cold air moving over the relatively warm waters is causing
snow showers over nearly all of Bering, an area encompassing
hundreds of square miles. As a result, Cold Bay and St. Paul both
had their lowest temperatures of the season so far observed over
the past 24 hours.
Models are in good agreement with the Gulf low, but lose
consistency with the low south of the Aleutians. The forecast
follows the Gulf low using the NAM/GFS solution, but because of
run to run consistency, uses the EC/GEM out west. Confidence
continues to increase with the Gulf low, but remains moderate in
the west. One thing the models are still struggling with regards
to the Gulf low, are the finer details on when the heavier snow
over Southcentral will start and end. 00Z and 06Z model runs for
the GFS and GEM are showing the snow persisting slightly longer,
keeping snow showers over the cook inlet region and heavier snow
over the Mat Valley through tonight. The NAM however shows the
heavier snow tapering off during the early evening for these
areas, which could essentially lead to lower snowfall totals.
Currently there is higher confidence in the slower GFS/GEM
solutions with regards to when the snow will end, but the faster
NAM solution can`t be completely ruled out.
PANC...Snow will begin to pick back up this morning and into the
afternoon producing primarily MVFR vis/ceiling. There may be
short periods of IFR conditions, most likely Monday morning
between 15z and 21z when the heaviest snow is expected. Cold air
advection and lingering light snow may cause lower conditions to
linger through Monday Afternoon or perhaps even longer.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong surface low will move into Prince William Sound this
morning and then move inland into the Copper River basin this
afternoon. Precipitation from this low is over most of
Southcentral Alaska this morning. This precipitation will taper
off from the southwest later today and tonight as the low moves
inland. Fairly decent snow amounts are expected with this storm
due to the track of the low and the favorable dynamics, including
good cold advection from the west. The associated upper level
trough over southwest Alaska will slide eastward, crossing south
central Alaska tonight into Tuesday. This will usher in a very
cold air mass...with temperatures steadily dropping during the day
Tuesday and through Tuesday night.
.SHORT TERM FORECAST SOUTHWEST ALASKA...
Cold temperatures will continue through Wednesday as an arctic
airmass has settled in over southwest Alaska. The coldest
temperatures will be in inland valleys with temperatures dropping
to around -35F. Winds look to stay light for inland locations
which should diminish any threat of wind chills.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Snow showers will linger over the Bering and Aleutians into
Wednesday under an unstable environment. The low south of the
Central Aleutians will bring intermittently heavier snow to the
Central Aleutians beginning this afternoon. The low will stay
relatively stationary until Wednesday when it pushes off to the
east. There does remain some discrepancy amongst the models on the
location and track of the low with some models bringing the low
into the Bering, but the more consistent models keep it south of
.LONG TERM FORECAST (Days 3 through 7)...
The Southcentral/Southwest Alaska will be influenced by northerly
to northwesterly flow through Saturday. This allow cooler air to
filter in the aforementioned regions. The coldest temperatures
looks to occur over the Lower Kuskokwim Valley, Bristol Bay
interior, Susitna Valley, and the Copper River Basin. Look for
the temperatures to range from minus 20 to upper 30 below range
creating possible wind chill issues for the Southwest Alaska
region from Thursday through the weekend. While the Anchorage
Bowl temperatures will likely drop in the single digit range
during the same time-frame. This cold air advection combined with
increased pressure gradients will produce gusty gap winds through
the Barren Islands, and the Whittier region.
Meanwhile, the models continue with discrepancies with the upper
level closed low over the Bering. While the GFS has a 962 mb low
just west of Saint Paul Island, and the NAM/ECMWF depicts this as
a 983 mb low just south of the Central Aleutians by Wednesday. By
Saturday models are in better agreement, and place this feature in
the North Pacific Ocean south of the Kodiak Island near 52n 153W.
By Saturday afternoon a complex low pressure system approaches
the Western Aleutians. The associated front begins to swing
through the aforementioned region bringing abundant moisture with
warmer temperatures accompanied by gale force southerly winds. The
models struggle with timing and placement of this storm force low
as it tracks through the Western Aleutians in the Bering heading
into Monday. Therefore, the forecast confidence is below average
for the long term forecast period.
PUBLIC...Blizzard Warning 131. Winter Weather Advisory 101 111 125
MARINE...Storm Warning 130.
Gale Warning 119 120 131 132 138 150.
Heavy Freezing Spray Warning 121 130 138 139 141 160 180 181 185.
SYNOPSIS AND MODEL DISCUSSION...TP/JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK