Small Craft Advisory 218, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 AM AKDT Wed Mar 14 2018

.ANALYSIS AND UPPER LEVELS...
Another disturbance near the Prince William Sound has brought a
fetch of moisture into Cordova and the Copper River Basin. The
radar at Middleton Island (PAIH) has a band of precipitation north
of Montague Island and an additional band that has passed through
Cordova. A decaying frontal boundary has moved into the Alaska
Range and appears to be hung up. All the while, there is a storm
south of the Chain. The latest scatterometer pass has pinged into
gale fore winds over the North Pacific with this system. Over the
Pribilof Islands there is a high but near northeast Russia there
is another low.

&&

.MODEL DISCUSSION...
The models are in decent agreement with the synoptic features in
the short term and the midterm. Looking at the GFS ensemble
members there is a tight clustering with the low that is
anticipated to move from the North Pacific to the Bering Thursday.
This tight grouping also holds up as the low moves into Bristol
Bay and into Southwest. Upstream over the Western Bering the
members flock in the same general direction through the weekend.
There is more optimism with the confidence level this package
through day 4 than there has been in awhile.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through much
of the night. There is potential for MVFR conditions with snow
Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Precipitation is expected from about Anchorage eastward today as
moisture continues to stream in from the southeast. An upper level
trough over Kodiak Island this morning will slide northeastward
through the day. The trough will push to upper Cook Inlet this
afternoon, then into the Copper River basin tonight. This will
push the moisture eastward, with precipitation ending along the
trough axis. The next system will approach the area from the
southwest. Precipitation will reach Kodiak Island early Thursday
morning. The precipitation will then spread northeast throughout
the day, continuing through Thursday night. For the western Kenai
Peninsula through Anchorage and the Matanuska valley, strong low
to mid level southeasterly flow will limit precipitation due to
downsloping. Also as a result of this, fairly strong winds are
expected through Turnagain Arm and possibly the upper hillside of
Anchorage. For the Susitna valley, upslope conditions will likely
produce a decent amount of snowfall late Thursday through Friday
morning.

&&

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

Weak troughing over SOuthwest is still supporting some light snow
showers and flurries, although the bulk of the snow has moved east
and will gradually dry out region wide by this afternoon. Dry
weather persists through tonight before a much more substantial
and moisture rich low moves into Southwest Thursday through
Friday. This will bring rather widespread snow (with some mixed
rain along the Alaska Peninsula and Bristol Bay coast) along with
blowing snow over parts of the Kuskokwim Delta. It looks to be a
rather long snow event with some parts of the Lower Kuskokwim
Valley, northern Bristol Bay, and surrounding mountains seeing
accumulations of 6-10 inches of snow. Accumulations will be much
lower along the Bristol Bay coast and Alaska Peninsula where warm
air will change snow to rain during the day Thursday.

&&

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

Banal and quiescent weather over the Bering will become more
active as a North Pacific warm front and associated gale force low
move into the central and eastern Bering Sea late this afternoon
before shifting east towards Southwest Alaska through Friday.
Meanwhile, a gale force front will move toward the far western
Bering Sea/Aleutians this afternoon then stall and weaken through
Thursday night before another gale force front moves in Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast begins Friday with a broad upper level
trough situated over the Bering and a surface low over Bristol Bay
moving East/Northeast into Southwest Alaska. A large swath of
precipitation will spread across the southern half of the state
into Friday bringing warmer temperatures and snow to inland
locations with rain expected along the coast. For Southcentral,
southeast winds look to develop Friday leading to downsloping off
the Kenai and Chugach Mountains limiting the amount of
precipitation for typically downsloped areas. A shortwave ridge
builds in between systems over Southwest then Southcentral Friday
night into Saturday bringing a short break of decent weather. A
front, with parent low well west over NE Russia, will sweep
across the Bering and move into SW Alaska Saturday night before
elongating and falling apart as it goes over the Alaska Range.

Looking out into next week, long range models are showing less
uncertainty than the last couple days and are hinting at the
continuation of the upper level trough over the Bering through the
first half of the week, and hence a persistent unsettled pattern.
There are signs of a pattern change as we head into the latter
half of next week, with both the GFS and EC showing an upper level
ridge building in over the Bering/Western Alaska. This would lead
to the development of dry off shore flow over most of the state,
though depending on how far east the ridge axis sets up, could
bring down a good shot of cold air from the Arctic. Stay tuned for
details.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 414 170 173 174 178.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...KVP