Small Craft Advisory 957, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKST Mon Dec 3 2018

.ANALYSIS AND UPPER LEVELS...

A trough which brought rain to the Anchorage Bowl and advisory
level snows to the MAT-SU Valleys is heading quickly east and has
just about cleared the Copper River Basin this afternoon.
Abundant low level moisture left in the wake of this system
combined with subsidence lead to fairly extensive stratus and
patchy fog across the MAT-SU Valleys. A strong upper vortex was
located south of the central Aleutian Islands. At the surface a
couple of 980s mb low centers were located in the same general
area south of the ALeutians. The associated frontal trough wrapped
around the low complex south of the Aleutians, then extended
eastward to the eastern Aleutians. Very strong winds were evident
on the leading edge of the front with storm force winds reported.
Dutch harbor airport reported easterly peak winds of around 64
knots. Very strong northerly winds with were peaking around 58
knots near Shemya this afternoon. Widespread rain was located
along and in advance of the frontal system.

&&

.MODEL DISCUSSION...

Models are in fair synoptic agreement with the pressure fields
through Wednesday afternoon. The NAM QPF fields for the Susitna
Valley appear to be somewhat overdone for Tuesday. Model agreement
becomes very poor overall Wednesday night through Thursday as
they struggle with the timing of the next wave to head toward the
western Gulf. So for now will limit editing of grids during this
timeframe until better model agreement.

&&

.AVIATION...
PANC...Generally light surface winds are expected. However, a
strong southeasterly flow aloft will develop on Tuesday. This will
create low low wind shear in the vicinity of the airport complex.
Fog and stratus is possible tonight and Tuesday morning.

&&

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

A gale force warm front will rapidly advance towards Kodiak and
the North Gulf Coast tonight and Tuesday bringing coastal rain
with rain and spotty freezing rain to interior portions of
Southcentral. Before that, tonight will see only some high clouds,
so fog has already developed in favorable areas and may become
locally dense around Cook Inlet, the Copper Basin, the Susitna
Valley, and the Mat Valley as well as Valdez. This front will also
bring a chance of a wintry mix to interior locations with mix of
rain/freezing rain possible, although this system will have quite
a bit of downslope drying so interior precip amounts will be very
light. The front clears and weakens Tuesday night leaving only
some spotty coastal rain with some higher elevation coastal snow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon night
through Tue night)...
A challenging forecast for both precipitation and wind is
expected through late Tuesday. The storm system along the Central
Aleutians slowly crawls eastward tonight, nudging the surface
front across the Alaska Peninsula this evening. Gusty gale force
winds and rain develop along the boundary. The front moves to
Bristol Bay and Kuskokwim Delta coasts after midnight.
Precipitation types are expected to change over to rain across
Bristol Bay; however, pockets of freezing rain are possible as an
off shore flow re-enforces a shallow cold pool at the surface.
Areas along the Kuskokwim Delta coast will experience mostly snow
as the front aloft moves ahead slightly faster.

Early Tuesday morning, the front picks up speed moving north and
east as upper level waves propagate ahead of the main low. This
brings increasing chances for freezing rain during the day Tuesday,
as freezing surface temperatures climb later during the afternoon.
Issued a Freezing rain advisory for the Kuskokwim Delta and
Valley with blowing snow expected along Delta. There will be some
limitations for precipitation accumulation/ice accumulation
amounts, with downsloping easterly winds quite strong with this
front. Gusty flow tapers off late Tuesday, with easterly flow
backing to the south. Showery conditions are expected on Wednesday
as upper level waves continue to ride north ahead of the parent low
over the Aleutian Chain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Mon night
through Tue night)...

An unsettled pattern continues across the Bering through Wednesday
as low pressure across the Central Aleutians slowly tracks east.
The occluded frontal boundary pushes north through the northern
waters by early Tuesday with the center of low tracking to the
eastern Chain on Wednesday. The center weakens and fills to 985 mb
on Wednesday with precipitation becoming less organized in response.

&&

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

A high degree of uncertainty will make for an interesting forecast
for the end of the week and into early next week. The models are
in very poor agreement (to put it mildly) with regard to a Bering
Straight trough that tries to dig down into the eastern Bering
and Southcentral, but gets deflected to our north due to a series
of vort maxes on the backside of a full latitude trough that
stretches from the Bering Straight south through the central
Aleutians and into the Pacific. As these pieces of energy round
the base of said through, cyclogenesis commences as they initially
head north before curving northwest into the Prince William Sound
region. Added to the mix will be a deep tropical connection with
the Integrated Water Vapor Transport (IVT) from the GFS model
showing values some 400 to 600 percent above normal.

However, the GFS/ECMWF/Canadian Global height fields and surfaces
pressures are all over the place, making the spaghetti plot truly
look like spaghetti. About the only thing they do agree on is the
colder air associated with the disturbance near the Bering
Straight will shift eastward across the northern half of the
state. So in short it looks like an active and wet period, but the
details are murky due to model disagreement and inconsistency
being far below normal.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155, 161.
MARINE...Gale 130-132 138 139 150-170 172-178 181 185.
		 Storm 179 180.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...PD