Small Craft Advisory 418, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 AM AKDT Tue May 19 2020

.ANALYSIS AND UPPER LEVELS...

Two lows are currently phasing in an area of strong upper level
divergence south of Shemya to form a deeper, stronger low which
will gradually become occluded as it tracks to the east. Moderate
to heavy rain and gusty southerly winds will accompany the
associated surface front as is already indicated by limited data
in the western Aleutians. Weak ridging over the central/eastern
Aleutians and Pribilofs is leading to more benign conditions for
these areas this morning. Farther east, a weak upper low centered
over Kodiak Island in combination with a dying front is bringing
scattered showers to much of western and Southcentral Alaska. The
heaviest of these are presently occurring near Prince William
Sound along a weakening barrier jet.

&&

.MODEL DISCUSSION...

Models remain in good agreement and are consistent with previous
runs. Forecast confidence is above average.

&&

.AVIATION...

PANC...Scattered showers will taper off this morning. VFR ceilings
are expected to persist, however brief lower ceilings are
possible early. Partial clearing is expected this afternoon
through Wednesday morning. Light winds will continue through this
morning with southeasterly Turnagain Arm winds again developing
this afternoon. They are expected to be somewhat lighter than
yesterday, gusting around 15 kt.

&&

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

A front over the area this morning will weaken today as the
responsible upper low moves southeast across the Gulf. Showers
are expected over much of Southcentral today. These showers will
decrease later today and tonight. Showers will end in most areas
by Wednesday as high pressure moves into the area. The exception
will be the Copper River valley and the northern Susitna valley
where some isolated showers are possible Wednesday as considerable
moisture remains in those areas. On Wednesday night another
front, associated with a deep low over the southern Bering Sea,
will move into Kodiak Island. This will bring another round of
wind and rain to Kodiak by late Wednesday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A dying front is currently bringing scattered showers across much
of western Alaska. These will gradually dissipate through the day,
but will not completely taper off until an upper ridge builds
early Wednesday morning resulting in partial clearing. This
combined with weak onshore flow in spots and the leftover
residual moisture may lead to brief patchy fog during the early
morning hours. A well-defined front will move onshore Wednesday
afternoon into Thursday morning. Moderate rain and gusty
southeasterly winds are likely, with highest gusts though terrain
gaps in the Alaska Range. The front will progressively weaken
Friday morning, though scattered showers will persist into the
weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A broad low pressure system is currently forming south of Shemya.
Its associated front will track eastward across the Aleutians
bringing moderate to locally heavy rain and gusty southerly winds,
particularly to the Pacific side of the Chain. As the low
occludes, it will stall over the western Bering, however will
maintain its large circulation. Thus widespread showers and
overall unsettled weather is expected to continue across the
entire Bering/Aleutians through the end of the week.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska: There is high confidence that a frontal system
will slowly pass through the Gulf from southwest to northeast
throughout the time period. Accompanying this frontal system will
be an area of small craft advisory winds and waves as the front
moves through. Another low is expected to form along the front
early Saturday which will keep the region unsettled through the
first half of the weekend.

Bering Sea: A low pressure system will meander its way through
the Bering Thursday through Saturday while slowly weakening which
will keep the Bering unsettled. During this timeframe, winds are
expected to remain below gale force winds with small craft
advisory winds the main strength with waves accompanying them. The
main differences are the exact placement of the low but guidance
remains in fairly good agreement on the strength leading to above
average confidence.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Tuesday)...

The long term period looks quite unsettled for the Bering
Sea/Aleutian Chain and Southwest Alaska. A complex low is
expected to meander in the Bering the duration of the forecast
period keeping the weather unsettled. Differences between model
guidance emerge with the strength of the ridge over the ALCAN
border northwest to the North Slope of Alaska. Depending on the
strength of the ridge, multiple upper level waves of energy may
rotate around the low in the Bering. A stronger ridge will help to
suppress the fronts/upper level waves to the immediate coastline
while a weaker ridge would allow for these waves to penetrate
further inland.

With this pattern in place, cooler temperatures and increased
winds are expected through the period especially for Southwest
Alaska and the Aleutian Chain, and Pribilofs. For Southcentral,
the forecast for temperatures becomes a little more tricky as
it depends on how strong the upper level ridge is. The Copper
River Basin is where temperatures may still be above average as
the upper level ridge would have the best hold there. As a result,
forecast confidence is average for this period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 170 172-178 411 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MV