Area Forecast Discussion - SouthCentral and SouthWest


FXAK68 PAFC 221427
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKST Wed Nov 22 2017
.ANALYSIS AND UPPER LEVELS...
The next incoming surface low is rapidly deepening as it crosses 
the 180 degree longitude line and speeds to the northeast across 
the Bering. The associated rather strong warm/occluding front is 
pushing across the Pribilof Islands early this morning with the 
trailing cold front moving into the central Aleutians. Further to 
the east, a shearing apart upper level trough is tracking east 
across southcentral Alaska while a transient ridge builds in 
behind it.

&&
.MODEL DISCUSSION...
Model agreement continues to improve with regards to the strong
low tracking towards Nunivak Island this afternoon/evening, 
increasing confidence in the various high wind and winter weather 
impacts expected across Southwest Alaska, the eastern Bering and 
Alaska Peninsula. Additionally there is good agreement with the 
trough and surface low rotating into the Gulf of Alaska Wednesday 
night and Thursday and the resulting increasing offshore flow.

&&
.AVIATION...
PANC...While there are a few patchy low clouds drifting around this
morning VFR conditions are expected to prevail.

&&

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

A sharp shortwave trough is moving through the area this morning.
This feature is dragging cold air with it (12z Anchorage UA
sounding shows -12C 850 hpa temperatures) and is pushing this cold
air through the gaps along the North Gulf Coast. Winds will peak
midday then begin to weaken as the pressure gradient weakens with
the eastward progression of the trough. Light snow today will
mainly be over the Alaska Range, Talkeetna Mountains, and Wrangell
Mountains, but some light snow will continue across the Copper 
Basin.

Attention quickly turns to a fast moving front which will move
into the Gulf of Alaska, bringing a quick shot of accumulating
snow to western Kodiak Island before turning to a mix of rain and
snow. The front will reach the southern Kenai Peninsula tonight, 
bringing accumulating snow to the area through morning before the
main low tracks south. Temperatures aloft will be quite cold, so
offshore winds look to really ramp up Thursday night into Friday.
The Matanuska Valley, Thompson Pass, Valdez, Kodiak, Copper 
Delta, etc. will all blow hard as a reinforcing upper trough digs
in.

&&

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

The forecast remains largely on track with few changes made this
forecast package. A strong area of low pressure will reach Kipnuk
around midnight tonight, before dropping southeast into Bristol 
Bay. An attendant cold front will sweep westward across the 
Bering through the afternoon before reaching the coast of 
southwest Alaska around sunset. Widespread precipitation will 
develop along the front. 

Taking a closer look at the model data, widespread snow will
develop ahead of the front with periods of moderate to heavy 
snowfall likely. The front will eventually stall near a Bethel to 
King Salmon line. Where this front stalls is important as warmer 
air will push inland following frontal passage for a few hours, 
before colder air advects into the region from the northwest as 
the low moves along the coast. This has impacts on how much 
snowfall will be seen near the coast, as a slower frontal 
progression could allow for mesoscale banding to develop in the 
precipitation, thus putting out higher snowfall totals. This is 
something we'll keep an eye on as the system evolves. 

Otherwise, look for snow to change to a rain/snow mix along the 
coast, with perhaps some of the western most portions of the coast
briefly seeing all rain. Snowfall accumulations look similar to 
the previous forecast package. Strong southerly winds ahead of the
front may also bring some coastal flood concerns, and the 
previous headlines for this were unchanged as the models remain 
consistent with this potential. Drier air will advect into the 
region by Thursday night. However, gusty northwest winds will keep
blowing snow concerns in our picture.

&&

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

Abrupt changes are coming over the next 24 hours as a strong cold
front sweeps eastward from the west central Aleutians this 
morning to eastern portions of the AKPEN tonight. Look for 
widespread precipitation to develop along the front before 
northwesterly winds move in behind the front. These winds will 
rapidly increase in strength with some hurricane force gusts 
likely. Only minor temporal changes were made to the headlines for
the Pribilof Islands, while upgrading the watch to a warning for 
zone 181 along the peninsula while adjusting the timing for both 
locations. The winds will gradually diminish for all areas from 
west to east for the Bering and the Aleutian chain by Friday 
morning.

&&

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

Beginning the extended period forecast on Friday, there will be a
longwave trough over the state of Alaska, extending into the Gulf
of Alaska, with a transient ridge over the Bering Sea. Deep cold
air and a strong offshore pressure gradient over Southcentral will
continue a strong gap wind event over that region. Otherwise, most
of Southern Alaska will be dry, and rather cold, through Saturday
night into Sunday. 

The forecast quickly becomes complicated as a large and very cold
trough over Eastern Siberia pinwheels a series of shortwaves into
the Bering Sea for the weekend. The first of these looks to
develops over the Bering late Saturday into Sunday then move a 
front into Southwest Alaska sometime on Sunday. This will bring a 
mix of rain and snow threats to Southwest before the energy moves 
into the Gulf of Alaska Sunday night through Tuesday. Meanwhile, a
series of powerful lows looks poised to move through the western 
Bering Sea and North Pacific along the Aleutian Chain, but huge 
uncertainty develops from Monday onward. It does seem the net 
effect of this pattern shift, alluded to in all the global model 
guidance, is that the deep northerly flow and cold pattern over 
the state will wane (at least for Southern Alaska) Monday through 
midweek.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181 195. 
         Coastal Flood Advisory 155 161.
         Winter Weather Advisory 155 161.
MARINE...Gale 120 127 130-132 150 160 173-178 351 352 411
         Storm 155 165 170 171 172 179 180 181 185 412-414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...Ahsenmacher
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...P Doll
LONG TERM...Ahsenmacher