Small Craft Advisory 915, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKST Thu Nov 30 2017

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough remains centered over Southern Alaska
and the Gulf of Alaska today. The surface low center, quickly
weakening and moving southeast, is currently south of Kodiak
Island. Just east of the Kamchatka Peninsula, a large upper low is
carving itself out and moving east into the Bering Sea with an
associated gale force warm front moving into the Central
Aleutians. Sandwiched in between these two troughs is a shortwave
ridge axis which is quickly moving east across the Bering Sea
toward the Southwest Alaska coast. Locally, the low level flow is
rather weak as the pressure gradient relaxes. Some rain and snow
shower activity continues across parts of the North Gulf Coast and
over the Bristol Bay region, otherwise, conditions over Southern
Alaska are generally dry.

&&

.MODEL DISCUSSION...
The numerical models are now in excellent agreement with the
track of a warm front into southwest Alaska and then the track of
the upper level shortwave trough as it moves into Southcentral.
What we now are confident in is a trough that will eventually
track through Cook Inlet up into the Susitna Valley bringing
widespread precipitation to parts of inland Southcentral by
Saturday (after bringing it to Southwest on Friday (see discussion
below).

The next big weather maker is a powerful cyclone which is expected
to cross the Alaska Peninsula from the North Pacific Ocean late
Saturday night through Sunday and into Monday. The GFS and ECMWF
are in excellent agreement with the track and strength of this
low, so there is high confidence that a significant storm and
major warm-up is likely by Monday (see extended discussion).

&&

.AVIATION...
PANC...VFR conditions and light north winds will persist through
late Thursday evening. Patchy fog may develop near the terminal
after midnight as winds subside and ridging begins to build in
from the west.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The break in the weather continues for Southcentral as weak upper
level troughing moves overhead, flipping the upper level flow
from south over to northwest. This means the shower activity over
the Gulf and coastal locations will diminish as the showers begin
to move towards the southeast away from mainland Alaska. Thus for
tonight, the primary hazard to contend with across Southcentral
will be areas of freezing fog, particularly in the Copper Basin
and near Cook Inlet and Knik and Turnagain Arms. The skies should
clear overnight, allowing the fog to form and sending temperatures
back down into the teens with single digits for the colder spots.

On Friday, portions of Southcentral will continue to deal with fog
through the morning, perhaps persisting into the afternoon along
Cook Inlet until a little offshore wind finally kicks in and
begins to move the fog out. Otherwise, the models are slowing down
the advance of the next storm system to impact Southcentral, with
precipitation now not expected to begin in Kodiak until late
Friday evening. Thus, even Kodiak will enjoy a nice, quiet weather
day on Friday.

On Friday night, the next round of precipitation slowly moves
into Southcentral from the west. It is accompanied by more warm
air, so rain-snow line issues will be a problem once again with
this storm system. Kodiak should quickly turn to rain after
precipitation begins. Homer, Seward, and Prince William Sound
will deal with a rain/snow mix for longer, but all areas should at
least begin the event with snow. Snow will also begin across the
Susitna Valley Friday night.

For Saturday, precipitation will overspread the rest of
Southcentral through the day. Warmer air will continue gradually
encroaching northward, turning Homer, Seward, and possibly Cordova
and Whittier over to rain. Further inland, wet snow is expected
remain the dominant precipitation type, though locally temperatures
potentially getting above freezing may limit snowfall totals.
Another limiting factor for snow totals in Anchorage and the
Matanuska Valley will be some southeast winds with the Turnagain
Jet, particularly Saturday morning. However, some models are
suggesting the wind turns more southwesterly Saturday afternoon
and evening, which would result in at least some snowfall in
Anchorage and the Matanuska Valley. The primary winner for snow
with this event, besides the coastal mountains, will be the
western and northern Susitna Valley. A prolonged period of upslope
southerly flow and the strongest forcing in the upper levels
moving overhead will enhance snowfall totals.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A front moves to the Southwest coast Friday morning and continues
a slow eastward progressing track through Saturday bringing
precipitation across the region along with southerly flow. The
main challenge with this system is precipitation type as, a warm
air mass nudges inland. Precipitation will mainly be snow, however
the western Capes to the Kuskokwim Delta my briefly change over
to rain Friday afternoon and evening. The front clears the region
by Saturday afternoon with lingering light showers behind the
front along weak short waves propagating ahead of the Bering low.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Low pressure over the western Bering pushes a front across to the
eastern waters tonight. Rain is expected along the front with a
mix of rain and snow showers in its wake as the parent low center
anchors over the western Bering. Gusty south to west will be the
predominant flow with the low maintaining its position through
Saturday, however the strong flow diminishes as the low fills on
Saturday.

&&

.LONG TERM FORECAST (Saturday through Wednesday)...
Sunday will see the exit of a weak system from Southcentral
Alaska. On it's heels is another storm system that most of the
models have really locked in on. A strong 170+ kt jet across the
northern Pacific will eject a rather potent shortwave about 500
miles South of Dutch. This storm system will be the primary focus
through the end of the weekend and into the early week. This low
will quickly deepen and track to the north, crossing the western
periphery of Kodiak Island. Guidance has been consistent with
bringing a strong front across the the Pacific and into the Gulf
of Alaska. As the front moves across the coast it will bring quite
the river of moisture and warmth with it. Guidance is also
hinting at the fact that Turnagain Arm and high elevations will
see a strong wind event into Monday. It's looking to not be an
event impacting much of the Anchorage Bowl, as winds exiting the
Arm should turn and head south down the inlet. With the strong
front will come strong cross barrier flow over much of
Southcentral. The winds will warm and dry the lower
atmosphere...leading to warmer than freezing temperatures and the
lack of precipitation across much of the usual downslope regions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 165 170 172>179 181 185.

&&

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