Small Craft Advisory 992, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 AM AKST Mon Dec 23 2019

.ANALYSIS AND UPPER LEVELS...

On satellite imagery there are two mature lows with well defined
frontal boundaries impacting the state of Alaska. There is an
occluded front over Cook Inlet and it extends into Prince William
Sound. The second occluded front is east of Adak and it produced
lightning strikes south of Nikolski overnight. The radars at King
Salmon, Kenai and Middleton Island have each been pretty active
overnight. There is a 120 knot jet core in the Gulf Of Alaska and
180 knot jet core south of Attu Island and Shemya which are
steering the two frontal systems.

&&

.MODEL DISCUSSION...
The run to run continuity with the models has not been stellar.
The GFS, NAM, ECMWF and Canadian have different QPF projections
with both lows this week for the Anchorage Bowl. For
initialization purposes, compared the actual precipitation on the
radar on Kenai at 9z to the models QPF projections for 9z. The
GFS and the Canadian had the best orientation for the
precipitation shield that set up over Cook Inlet. Looking at the
Middleton Island radar and the surface observations, all of the
models were overly aggressive in Prince William Sound area but did
reasonably well or at least similar enough in the Gulf Of Alaska.
The performance of a model in one region doesn't necessarily
translate to a similar performance in another locality which is
often the case in the state of Alaska.

&&

.AVIATION...

PANC...On the Kenai radar there is precipitation over Cook Inlet.
At the onset of the forecast period expect VFR conditions.
However, expect the cigs to drop at PANC as the day progresses.
MVFR conditions expected and possibly IFR conditions for the
aerodrome this TAF package. Amendments will be made as needed.

&&

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

An active weather pattern continues across Southcentral Alaska
through Tuesday. A weakening surface low is moving slowly into
western Prince William Sound this morning. Light snow should
spread across the Anchorage Bowl today and continue across much of
the Gulf, eastern Kenai Peninsula, and the Southern half of the
Copper River Basin. The weak upper short wave associated with this
feature moves rapidly east tonight, so snow should diminish
across the Anchorage Bowl and Kenai Peninsula, but may increase
some over the Copper River Basin. A stronger negatively tilted
upper trough moves into Southcentral Tuesday while its associated
surface low heads toward western Prince William Sound. If this
scenario plays out, we should see widespread snow spreading inland
across the Anchorage Bowl and MAT-SU Valley. That said, there is
still some model uncertainty, so stay tuned.

&&

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

The once powerful low in the Bering continues to weaken as it
tracks generally east. The low is taking a slightly more
southerly track than forecasted yesterday at this time, so the
second round of snowfall for areas around Dillingham is no longer.
With the more southern track, areas of the Alaska Peninsula near
King Salmon will pick up a few inches of snow this morning but
that's about it. This shift in track also means generally quiet
weather for southwest Alaska with just some snow showers around.
Colder temperatures will continue for interior areas.

&&

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

Low pressure continues to work its way east across the Bering Sea,
weakening as it does so. Satellite shows the structure of the low
becoming more diffuse, indicating it is on the down swing. Satellite
derived winds also show the low has weakened considerably. It will
exit the area later today, though strong northerly winds - driven
by cold air advection - on the backside of the low will continue
through tonight for the eastern Aleutians. The Aleutians will not
catch much of a break as the next low is already moving into the
western parts of the island chain now. This low will also move
east but stay south in the north Pacific. It's northern edge will
just scrape the Aleutians as it goes. Expect gale force winds
along the islands with some rain with this system.

&&

.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...Flood Advisory 145.
MARINE...Storm 173. Gale 352 132 411 413 150 155 170 171 172 174
175 176 177. Heavy Freezing Spray 181.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BJB
MARINE/LONG TERM...PD