Small Craft Advisory 904, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
355 PM AKST Mon Nov 27 2017

.ANALYSIS AND UPPER LEVELS...
The low that has been the main feature the past two days is
rapidly dissipating as it moves across the Alaska Peninsula to just
south of Kodiak Island. The new major feature is a low tracking
into the central Aleutians today which is the source of the high
wind warning in that area. This storm is not as strong as the
previous one but will still have some areas of hurricane force
winds.

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.MODEL DISCUSSION...
Models are handling the synoptic situation well with the biggest
uncertainty with the rapidly developing low south of the
Aleutians that will merge with the main Bering Sea low Tuesday.
The main issue is how far east and north the secondary low moves
as it wraps into the main and as this will impact the winds and
precipitation around the Alaska Peninsula and possibly into
Bristol Bay. The higher resolution NAM is doing probably the best
job tracking a low in the northwestern Gulf and its movement
toward the Kenai Peninsula. This is not a large feature, but its
appearance on satellite imagery suggests there could be some
higher winds and enhanced precipitation near it.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Overall expect warmer temperatures, decreasing winds with snow
along the coast. The pattern remains active for the area,
however, most of the action is along the north gulf coast. A weak
front has pushed inland, leaving the coast temporarily in a
showery pattern for the late afternoon and evening. A compact
disturbance is approaching the coast and will turn snow more
steady along the coast for a short period tonight before turning
back to showers during the day Tuesday. Another stronger front
affects the area on Tuesday night, with rain at sea level and snow
not far off the surface. These fronts and disturbances will
mainly affect the coast. Interior areas will remain dry. There is
a chance for snow showers on Wednesday, further inland, as upper
level temperatures become more conducive for showers while some
weak disturbances come through the flow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A few showers are expected to develop along a weak upper level
trough which propagates north across the region tonight. Broad
east to southeasterly flow in the lower levels will cause
downsloping on the lee side of the ranges; therefore, snow
accumulations will be light across valleys. A few locations near
coastal Bristol Bay warm enough for precipitation to fall as
light rain. Temperatures will moderate slightly warmer through mid
week as low pressure across the Bering brings another warm surge
on Tuesday. There is a slight chance for freezing rain through
Wednesday as the surface freezing line orients east to west along
Bristol Bay. Only trace accumulations are expected since the
system bringing precipitation lacks organization.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A trough along the eastern Bering is shoved eastward tonight as
the next low organizes across the southern Bering. This system
originated as a North Pacific frontal system, and slows its track
as it crosses into the Bering into Tuesday early morning. A
northern wave moves along the back side of this low and anchors
the low just south of the Pribilof Islands on Tuesday. The core
rapidly deepens as cold air advects in from the western Bering on
the backside of the low. The high wind warning was continued for
the Central Aleutians as winds are expected to quickly increase as
the low deepens tonight. There is still a bit of uncertainty with
this storm system as it spans over 700 miles in a data void area
over the North Pacific.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning late Wednesday, the weather over most of the South
Mainland will become much quieter as the low over the Eastern
Bering cross the AK pen and dissipates over the North Pacific,
leaving generally cold and clear conditions. Guidance has also
trended much further south with this low on Thursday, meaning that
without a strong pressure or thermal gradient along the North Gulf
Coast, the persistent outflow winds that have blown through the
area recently are not expected to develop again. Over the Western
Bering, a gale force front entering the area late Wednesday will
slowly spread through the area bringing rain/snow and gusty
southerly winds that will be weaker than the high wind events
that moved through the Aleutians since the Thanksgiving holiday.

This front will then enter the Southwest Mainland late Friday,
bringing snow to that area before creating a triple point low in
the Gulf sometime on Saturday. The models continue to struggle
with the timing and intensity of this development, however they
all create a triple point sometime on Saturday which will track
into the North Gulf. The exact details of the impacts of this low
will need to be determined as model agreement improves, but at
this point it appears that there will be increasing chances of
snow along the North Gulf coast and Southcentral this weekend,
along with the potential for gap winds through the usual areas
such as Turnagain Arm the Matanuska Valley. Afterwards model
agreement begins to break down with the track/intensity of what
looks to be a very active pattern for the Gulf late this
weekend/early next week as a blocking ridge sets up over the
Pacific NW, leaving the Gulf exposed to a deep southerly jet. The
general impact of this pattern change will be to shift the most
active weather further east into the Gulf region for early next
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 187.
MARINE...Hurricane Force Wind Warning 174>177.
 Storm Warning 132 150 155 170 172 173 178.
 Gale Warning 130 131 136>139 160 165 171 179>181 185.
 Heavy Freezing Spray 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...DEK