Small Craft Advisory 922, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 AM AKST Tue Dec 3 2019

.ANALYSIS AND UPPER LEVELS...

A weakening low is situated south of Prince William Sound, and
continues to bring light snow to the Anchorage Bowl into the Mat-
Su Valleys. Arctic air continues to push into Southcentral from
the north and west. While the latest ASCAT pass at 7:30Z
indicated storm force winds out of Kamishak Bay and over the
Barren Islands, the surface pressure gradient between the Gulf low
and Arctic high pressure has diminished slightly and winds have
slowly been trending lighter over the last few hours.

Out west, a broad low remains in place over the eastern Bering,
supporting showers over the eastern Bering into the Kuskokwim
Delta Coast. Inland, temperatures have dropped to the single
digits to below zero over much of the Kuskokwim Valley/Delta and
Bristol Bay areas under clear skies and cold air advection. An
occluded low centered roughly 200 miles south of the eastern
Aleutians continues to bring scattered showers to the central and
eastern Aleutians, while the strongest winds over the Aleutians
are just upstream as cold air wraps around the backside of the low
enhancing the pressure gradient. Meanwhile, a Kamchatka low is
pushing a gale force front towards the western Aleutians/Bering.

&&

.MODEL DISCUSSION...

Models are in good agreement synoptically in the short term.

&&

.AVIATION...

PANC...Snow will slowly taper off to flurries this morning, and IFR
to MVFR cigs/vsbys will slowly improve. Though there will be ample
ambient moisture from recent snow, north winds should stay just
strong enough to prevent fog tonight.

&&

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

The broad upper trough over Southcentral and surface low over the
northern Gulf will drift south across the Gulf through tonight.
Light snow/flurries across the region will diminish through the
day as high pressure builds aloft and low level offshore flow
brings drying. That said, some inland areas with light winds may
see some patchy fog tonight. Gales over the Western Gulf and brisk
outflow winds across the eastern Kenai Peninsula and North Gulf
Coast (Copper River Delta) will slowly diminish on Wednesday/Wednesday
night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Snow showers will continue to linger along the coast this morning,
while areas farther inland will see fog in the morning. Precip and
fog will clear in the afternoon making way for clear skies. Fog
looks to redevelop tonight over the Kuskokwim Delta and around
Bristol Bay before dissipating Wednesday morning. Wednesday
evening a front will push in along the coast bringing snow back to
the area. However, there is uncertainty with how much snow will
fall and how warm temperatures will get. This front will also
bring stronger winds creating blowing snow conditions through
Wednesday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A low south of the eastern Aleutians will continue to track to the
southeast today bringing rain and snow to the eastern Aleutians
and Alaska Peninsula into the afternoon. Out west, a front is
approaching the western Aleutians that will push east across the
Bering through Wednesday. It is bringing up a lot of warm air with
it, but areas will still see snow at first before transitioning
over to rain. The front is also bringing stronger winds to the
area with the western Aleutians expected to see storm force winds
today.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

The primary focus for this time frame will be a developing low
over the western Bering. Most of the models keep the center of the
low over the western Bering with only slow movement southeastward
towards the central Aleutians. Gales are expected around this
low Friday and into Saturday. Storm force winds are possible
across the western and central Aleutians as well. The leading warm
front will move into the western Gulf on Friday, which may persist
into Saturday. Gales are expected with the front as well. As the
front impacts the north Gulf coast on Saturday, a storm force
barrier jet may form through the north Gulf coast and into Prince
William Sound.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through Monday)...

On Thursday, a front looks to move into Southwest Alaska. It may
be associated with a weak low, but the models are not in agreement
about that. Warmer air will be moving in behind the front, but
since the air mass ahead of the front will be quite cold, much of
the precipitation away from the immediate coast will fall as snow.
The front will quickly weaken as it moves inland. Meanwhile a
couple lows will be consolidating across the western and central
Aleutians on Thursday. There is understandably significant model
disagreement on how this interaction will turn out, regarding
where the primary low ends up forming. A combination of a strong
Pacific jet well to the south of the Aleutians and the tapping of
a Siberian polar air mass will act to strengthen both lows. The
foreign models favor the north and western-most low, which would
keep most of the strongest winds and impacts over the Bering. The
GFS favors a much more south and eastward track of the center of
the low, which in turn shifts the impacts east over much more land
area. Beyond that, the model disagreement is too large to glean
any meaningful insight into the forecast.

For Southcentral, the long-term will begin with very cold air over
the Copper River Basin causing strong gap winds through Valdez,
the Matanuska Valley, and the Copper River Delta. While there are
some differences in timing, the models agree on a warm front
moving into the area on Friday. By this point it will be slow
moving, so it could result in a return to warmer temperatures, but
also unsettled/precipitation likely weather for the entire
weekend. There are large differences in how much precipitation and
what form it will be in, but it seems likely that there will be
coastal rain, and inland snow or mixed precipitation, along with
areas of strong winds. Thus, the cold snap will be just that,
short-lived.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales: 130 131 132 172 173 174 175 176 177 181 185 351
352 411 412 413
Storms: 178
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MM
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
MARINE/LONG TERM...JW