Heavy Freezing Spray Warning 73, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
612 AM AKST Mon Nov 21 2016

.ANALYSIS AND UPPER LEVELS...
The contoured water vapor image aloft looks like a snowman(8) with
a pair of circulations over our forecast area. One is much
deeper, featuring the majority of the active weather, over the
southwestern gulf. The other is a weak remnant from the previous
system, however it does play a role in a developing
deformation zone over the Bristol Bay area. The front associated
with the stronger system is stalling out over the northern Gulf
this morning, featuring a wide band of unstable air aloft. Snow
showers over Kachemak Bay are a result of this air and frontal
proximity.

&&

.MODEL DISCUSSION...
The biggest differences in guidance is on the small scale around
Kachemak Bay this morning. Guidance differs on how much convective
snow to bring over the mountains, as well as how much downslope
drying occurs. The NAM/GEM regional favor a downslope scenario
while the GFS brings more moisture over in general. The GFS seems
to be catching on to the current scenario better than the other
models, it will be favored for the current forecast.

&&

AVIATION...

PANC...VFR conditions expected through at least this evening, with
clearing skies during the afternoon. Fog may become a possibility
beginning around 09z tonight through the remainder of the TAF
period (18z Tue), and this is hinted at with VCFG beginning at
that time. Expect changes as confidence increases toward such
development.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The biggest question for the weather across southcentral today is
the amount of snowfall for the southern Kenai Peninsula. The
decaying front in the northern Gulf of Alaska is bringing a series
of showers through the region. Most of these are remaining on the
windward side of the coastal mountains but they are making it into
the southern Kenai Peninsula and bringing some snowfall. At this
time a total snow accumulation of 1 to 3 inches looks reasonable
this morning for the region around Kachemak Bay. By afternoon the
showers should taper off and those that remain may be mixed with
rain as temperatures warm up some as well. After this system most
of southcentral Alaska should come into a drier and clearer
pattern through the middle of the week. This exception will, as
usual, be Kodiak Island which will get back into some
precipitation Wednesday.

&&

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

A deformation zone has set-up over the Aleutian Range near Iliamna
and King Salmon this morning which has helped to produce some snow
showers. Forecasting how far westward the snow will fall and
exactly how much is quite challenging in this situation as
offshore flow ranging from 10 to 20 kts has been common and has
kept the lowest levels of the atmosphere from saturating.
Nevertheless, the strongest forcing appears to be near Iliamna
which should help squeeze out 2 to 4 inches of snow through this
evening while areas as far west as King Salmon will see scattered
snow showers throughout the day with little or no snow accumulation.
Snow chances will diminish tonight near the Aleutian Range as an
area of low pressure in the Gulf of Alaska shifts southeast and
the deformation zone weakens and shifts eastward as well. Snow and
rain chances will increase across the Alaska Peninsula and and
Bristol Bay Area south of King Salmon on Tuesday as a potent front
moves into the eastern Bering Sea. Temperatures will continue to
run slightly below normal through the forecast period as a feed of
cold air from the arctic continues.

&&

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

A gale force front currently located between the Western and
Central Aleutians will continue to propagate eastward through
Tuesday morning before falling apart off the west coast of Alaska.
Gusty winds ranging from 35 to 45 kts have also been seen across
the Alaska Peninsula near Cold Bay, False Pass, and King Cove as
northwesterly winds aided by cold air advection help push winds
through gaps in the terrain. These gusty winds should shut off
this afternoon as flow shifts to a more westerly vs northwesterly
direction ahead of the aforementioned frontal boundary. On Monday
night into Tuesday, the area of low pressure responsible for the
aforementioned frontal boundary will move into the far western
Bering Sea. This will allow an interesting scenario to play out as
winds behind the frontal boundary will be westerly and a nice jet
streak accompanying the low as it moves into the Bering Sea. This
will produce a nice dynamic fetch which will allow seas to possibly
increase to near 40 ft near the Western Aleutian Islands tonight
and Tuesday morning. This is fairly uncommon with winds expected
to only range from 50 to 55 kts so mariners should make sure to
keep an eye on this situation.

&&

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

From previous discussion...

The jet stream continues to remain in a zonal flow along the Aleutian
Chain as it moves farther eastward before digging into the base of the
longwave trough positioned along the Northwestern coastline near Washington
State. This synoptic pattern will keep the storm track south of the
Southwest/Southcentral region heading into next weekend. While the
Bering/Aleutians receive a showery weather regime as a storm force
low tracks into the Western Aleutians/Bering by Tuesday. The
associated weather front with this system tracks becomes elongated
from a west to east orientation in the Bering as it falls apart by
Wednesday. By late Wednesday afternoon the next storm force low
moves into the Kamchatka Peninsula with associated weather front
moving through the Western Aleutians. The front tracks on same
path as previous, except this time a wave develops with a surface
low developing in the eastern Bering by Friday. This system tracks
across the Alaska Peninsula into the northern Gulf by Saturday.
This results in the majority of the precipitation to occur along
the northern Gulf, coastal communities. Look for the
Southwest/Southcentral regions to remain in offshore flow with the
coldest temperatures to remain in the Copper River Basin, and the
Lower Kuskokwim Valley through this week. The models are in decent
agreement with the track of these synoptic systems, but have
discrepancies in the placement of the surface lows. Therefore, the
forecast confidence remains average as we head into next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...119 120 130 131 139 150 155 170 172-176 179 185 351 411-414

&&

SYNOPSIS AND MODEL DISCUSSION...ML
AVIATION...CC
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...PD