Small Craft Advisory 32, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKST Wed Jan 9 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level low over Alaska, centered near Nuiqsut
and slowly sliding southward. A short-wave trough associated with
this low is over the Susitna Valley and moving eastward. This
trough is fairly weak, and producing some clouds and a few snow
showers...mainly over higher terrain. There is an upper level
ridge over the Gulf, extending northwestward along the the
southwest coast and far eastern Bering Sea. There is an upper
level trough stretching from the eastern Aleutians and Alaska
Peninsula through the Pribilofs to the northwest Bering. There is
a strong developing low south of the western Aleutians. At the
surface, high pressure over mainland Alaska is producing offshore
flow and mainly dry conditions.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement over most of the
forecast area through the short term portion of the forecast
(Friday afternoon). The only exception is in the handling of a low
moving into the central and western Aleutians on Friday. There is
some east-west disagreement on where the central surface low will
be, with the ECMWF furthest west and the Canadian GEM furthest
east. The Canadian is also an outlier with a weaker low than the
other models.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Saturday)...

A weak trough continues to slide across the Cook Inlet and Mat-Su
Valleys this evening. As it moves through tonight expect a few
flurries in valley locations and more pronounced upslope snow
showers over the Talkeetna and Chugach Mountains. A second upper-
level shortwave will then follow on the heels of the first for
Thursday, keeping clouds and a few isolated snow showers over the
Chugach. This shortwave will also aid in the development of a weak
surface low over the northern Gulf which will help steer clouds
and moisture north along the Copper River Delta and over the
Wrangells. Colder air in the wake of these systems then begins
to slide south and across much of Southcentral, continuing the
downward trend in temperatures for Thursday through Friday.
Gusty outflow winds will also continue through the overnight
hours along the North Gulf Coast, eastern/southern Kenai Peninsula
before weakening Friday morning.

A change in the weather pattern is then expected for all of
Southcentral beginning late Friday as a series of surface lows
moves from south of the Aleutians east into the western Gulf.
Multiple fronts sweeping north into the northern Gulf will bring
the return of a moist, southeasterly flow to the region with
warming temperatures and areas of snow for the mountains and
interior locations and a mix of snow and rain for the immediate
coast.

&&

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

A strong upper level low over the northern half of the state
continues to move southward, bringing extremely cold air across
Southwest Alaska through the end of the week. While low level
stratus is still in place across Southwest and the northern AKPEN,
satellite imagery shows it quickly eroding as the upper level
trough begins deepening and pushing the cold air southward. The
coldest temperatures will arrive Thursday night, with values as
low as 38 to 40 degrees below zero. In addition to dropping
temperatures, offshore flow will add a windchill factor over the
Kuskokwim Delta, thus a Wind Chill Advisory will be in effect
Thursday evening through midday Saturday.

&&

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

An upper level longwave trough over the western Bering continues
to support the development of a strong area of low pressure
currently approaching the western Aleutians. This low currently
has a central pressure of 940 mb and has impressively dropped
nearly 62 mb in the last 24 hours. As the associated front moves
over the western Aleutians tonight, a large swath of easterly
gales will begin to overtake the Bering and will last through the
forecast period. Winds will ramp up to storm force over the
western Aleutians by Thu morning. Meanwhile, a large trough over
the northern mainland plunges to the south. This will provide
ample amounts of cold air which will be advected over much of the
Bering through Friday night. This cold air will enhance winds even
more, producing easterly gales across the entire western Bering.
Additionally, a swath of storm force winds will redevelop in the
southwestern Bering just north of Shemya by Friday night.

&&

.MARINE (Days 3 through 5)...

An active storm track will continue to bring a series of lows and
frontal systems north over the marine waters of the Aleutians,
Alaska Peninsula and western Gulf through this weekend. While the
precise details of timing and track of each system remains uncertain
it is likely that gales will impact much of these waters at times.
For the Bering, the persistent, cold east to northeasterly gales
and possible storm force winds in the southwest Bering on Friday
and Friday night will ease somewhat through the weekend. The
stronger winds will turn more northerly in direction and shifting
further west and north as vertically stacked surface lows begin
pushing further north and stalling out over the east and central
Bering.

&&

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

On Friday and Friday night, upper level ridging will build into
the Gulf and southern Mainland Alaska pushing the cold upper level
trough further north and into interior Alaska. Warming air aloft
will spread across the southern third of the state on Saturday,
however northeasterly offshore flow will keep low level
temperatures cooler with moderate to strong low level inversions
remaining in place. A surface low will lift north to Kodiak Island
on Saturday with a second deeper low lifting north to the eastern
Aleutians. A third larger vertically stack low will be located
south of the western Aleutians on Saturday with a cold upper low
over the northwestern Bering.

On Saturday night and Sunday, upper level ridging will continue
to build over western Canada and mainland Alaska with the surface
low over the eastern Aleutians tracking north across the Pribilof
Islands and into the central Bering. Southeasterly flow will
strengthen and build north into the Gulf with east to northeasterly
flow and overrunning precipitation a potential concern for
Southwest Alaska. In addition to windy conditions, moderate to
heavy rain will spread north across Kodiak Island late Saturday
night through Sunday morning. Rain at the lowest elevations along
the coast and snow inland over the Kenai Mountains will continue
north along the eastern Kenai Peninsula, over Prince William Sound
and to the northern Gulf Coast Sunday through Sunday evening.

A series of additional North Pacific low pressure systems will
track north across the Alaska Peninsula and into Southwest Alaska,
Bristol Bay and eastern Bering through the first half of next
week. The associated frontal system will lift north across the
Gulf and into Southwest Alaska. While each of these systems will
continue to bring an additional round of rainy and windy conditions
to the Gulf coast and gradually warming temperatures further
inland, persistent offshore flow will keep much of Southcentral
Alaska somewhat drier with temperatures persisting at or below
freezing. For Southwest Alaska precipitation type is going to be
an ongoing challenge with the strength of the northeasterly flow
for each system determining whether rain, snow, blowing snow or
freezing rain will be the primary impact.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131 155.
MARINE...Heavy Freezing Spray Warning 121 127 160 165 180 185.
		 Storm Warning 175 176 177 178.
		 Gale Warning 127 170 172 173 174 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/JA
MARINE/LONG TERM...JR