Small Craft Advisory 990, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKST Sun Dec 22 2019

.ANALYSIS AND UPPER LEVELS...
A cold upper level trough extends from the Arctic, across the
interior and to the Bering Sea this afternoon. At the base of
this trough, just a little south of the Pribilof Islands, is a
strong surface low which is the source of the strong winds across
the Aleutian Islands and Alaska Peninsula. The front associated
with this low is lifting northward across Bristol Bay and up Cook
Inlet this afternoon. A Triple point low is developing near the
Barren Islands and will become a factor in the weather for
Southcentral Alaska as it will prevent a surge of warmer air from
moving inland and push it eastward into the eastern Gulf of
Alaska.

&&

.MODEL DISCUSSION...
Models are in very good synoptic agreement into the middle of the
week. The NAM had been tracking a little farther north of the
other models across Bristol Bay tonight, but the 18Z run adjusted
southward which increases confidence in the forecast and that
more southerly track is being used in the forecast.

The details on how the low moves into the northern Gulf and Prince
William Sound region will have a large impact on the timing of the
snow that will move into Southcentral Alaska Tuesday through
Thursday.

&&

.AVIATION...

PANC...VFR conditions should persist until snow moves into the
airport area around midnight. While this sow is expected to be
light, it will bring MVFR and possibly IFR conditions to the
terminal into the morning hours. A MVFR ceiling is likely to
linger after the snow stops and then improve in the early
afternoon Monday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A warm front associated with a low north of Kodiak Island is
bringing areas of snow to much of the Kenai Peninsula. As the
surface low shifts eastward this evening, precipitation will
diminish along the western Kenai Peninsula. A secondary low will
form near Prince William Sound, and amplify precipitation in the
Prince William Sound/northeast Kenai Peninsula area. A Winter
Weather Advisory remains in effect through early Monday morning
for blowing snow from Turnagain Pass to Portage Valley. An upper
trough over Southwest Alaska will steer a series of weak waves
from south to north over Southcentral this evening into Monday.
Initially, expect downsloping in the Anchorage Bowl, but by early
Monday morning some long-awaited snowflakes should make it to the
ground...likely around an inch. Tuesday will be a transition day
to a snowier pattern along the North Gulf Coast, Anchorage Bowl
and points north. The big questions are timing and amounts. Models
have been bouncing around regarding the initial precipitation,
from early to late Tuesday. In any case, confidence is building
that as per usual Anchorage will have a white Christmas (snow on
the ground).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Blowing snow conditions across interior Bristol Bay will improve
tonight as the winds and snow start to weaken as a surface low
moves inland over the Alaska Peninsula by Monday afternoon. There
could still be some coastal locations that experience reduced
visibility with lingering snow showers, but this should also
improve throughout the night. Areas across Bristol Bay stretching
up to they lower Kuskokwim Valley could see an inch or two of snow
tonight, with any new snow accumulations on Monday remaining
south and east of Dillingham.

As this low begins to move out of the forecast area, snowfall
begins to taper off across Southwest Alaska on Tuesday through
Wednesday afternoon. However, areas along the Alaska Peninsula
will see an increase in winds and precipitation (mostly snowfall)
on Tuesday afternoon when a North Pacific low lifts north across
the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The strong Bering low centered east of the Pribilofs will
continue to track eastward through tomorrow afternoon.
Scatterometer imagery shows an area of storm force winds
surrounding the central and eastern Aleutians this afternoon,
which will weaken overnight. Widespread gale force winds will
persist along the southern Bering and Aleutians through Monday
morning when they begin to diminish.

The next low feature will move into the area on Tuesday morning,
which is tracking much farther north across the central Aleutians
than previous forecasts. There is still some model disagreement on
exact placement of the low center, with the GFS and EC staying
farther north and the NAM to the south. This will be the
difference between storm force winds (GFS solution) entering the
coastal waters or low end gale force winds (NAM solution) moving
into those areas south of the central Aleutians. The placement of
this low will play a key in whether areas stretching from Dutch
Harbor to Cold Bay receive strong winds and snowy conditions for
Tuesday afternoon.

&&

.MARINE (Days 3 through 5)...
(Wednesday through Friday)

...Bering Sea and the Aleutian Coastal Waters...

We start out the mid-week period with a ~970 mb low tracking
just outside the southern coastal waters of the eastern Aleutians
and AKPEN (Alaska Peninsula). Sustained storm force winds and
seas of 15 to 20 feet will proceed this feature, affecting the
region Wednesday and Thursday, before moving into the Gulf for
Friday.

Further north and west, a gale force front will move into the
western Aleutians and Bering on Wednesday, continuing slowly
eastward on Thursday, with seas building into the 15 to 20 feet
range. Conditions look to improve some for Friday, as the front
weakens.

...Gulf of Alaska...

An active period is in store for the region, as a ~970 mb low
enters the southwestern portions of the Gulf Wednesday afternoon,
reaching the Valdez Narrows by Thursday evening. Sustained gales
and seas of 15 to 20 feet will proceed the lows arrival, with
northwesterly high-end gales following the lows passage, with seas
building into the 20 to 30 feet range, especially for the outer
waters.

&&

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

...Southcentral...

Our recent stretch of quiet weather is about to end for southcentral
Alaska, as a fairly strong area of low pressure tracks from south
of Kodiak Island towards the Valdez Narrows. This is a favorable
track for widespread snowfall to develop for most (if not all ) of
Southcentral, with a few inches of snow possible for Christmas
day, highest near the coast. This system then very slowly moves
inland while prolonging the snowfall potential through Thursday
for the Kenai Peninsula north to the Mat-Su Valleys, while
purloining it further into Friday for the Copper Basin. Then,
another fairly strong low makes a run at Yakutat, with an inverted
trough extending back towards Prince William Sound. This allows
snowfall potential to continue through the weekend for coastal
locations for the sound and into the Copper Basin. Depending on
the track of the low, some of this could affect portions of the
Kenai Peninsula through the Matanuska Valley.

...Southwest Alaska...

Much colder air is poised to invade the region from the north, as
a low passing through the Gulf helps induce a northeasterly flow
pattern, with cold air from central portions of the state sinking
south. Aside from some snowfall near the Alaska Range and Alaska
Peninsula on the backside of the Gulf low, this will be the main
feature to watch, as it drops south over the latter half of the
week. Currently, the 850 mb progs (~4400 feet) show temperatures
of -20 to -30 deg. C moving into the region. This would translate
to highs in the -5 to +5 range, and (depending which model you
believe) and lows of -10 to -30 for most of the area for Friday
through the weekend.

...Bering Sea south through the Aleutians...

A fairly strong area of low pressure passing south of the eastern
Aleutians will bring snow to the region on Wednesday. As the
system passes by, much colder air stretching from Siberia to
Central Alaska will plunge south, reaching the Saint Matthew
Island and Pribilof Islands area by Friday, continuing south
through the weekend. This looks to be some of the coldest air of
the season, and given the warm sea surface temperatures, which are
running a good 5 deg F above normal, the temperature difference
between it and the colder air aloft should foster snow shower/snow
squall development. The colder air looks to most affect locations
east of a Nikolski to Saint Paul Island line.

Further west, a Pacific low approaches the western portions of the
Aleutian chain, while staying south of it. This system will bring
widespread periods of rain and snow Wednesday evening and
Thursday, before a more showery type pattern resumes for Friday
through the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 125.
 Ice Jam Flood Advisory 145 (Willow Creek).
MARINE...Storm Warning 155 165 170 172>174.
 Gale Warning 119 120 125 127 130 132 139 150 171 175>178 181 185.
Heavy Freezing Spray 140 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...MM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...PD