Small Craft Advisory 105, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKST Tue Jan 31 2017

.ANALYSIS AND UPPER LEVELS...
There is a sharp trough with its axis stretching from southwest of
Kodiak Island north to Norton Sound. This trough is moving very
slowly eastward and weakening. An associated front on the eastern
side of the trough is oriented north-south and is pushing moisture
and precipitation into south central Alaska. There is a large
vertically stacked upper low centered to the southwest of the
western Aleutians with a 958 mb surface low. A storm force front
from this low is moving from the western Aleutians into the
central Aleutians. An upper level ridge is building ahead (to the
east) of this low/front over the eastern Bering and eastern
Aleutians as well as the Alaska Peninsula and the west coast of
the mainland.

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.MODEL DISCUSSION...
The numerical models are in good agreement for the short term
portion of the forecast (through Thursday afternoon). The one area
they are struggling with a bit is with how they handle the
precipitation over Southcentral into Thursday evening. Therefore
the forecast confidence is near to slightly above normal.

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

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.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The frontal system currently extending from the Gulf northward
into the Susitna Valley will remain fairly stationary through
Wednesday morning, while gradually weakening. Rain and snow
associated with this system will persist along the Northern Gulf
coast and Cook Inlet region through this evening, then becoming
more showery overnight tonight as the system weakens. The Winter
Storm Warning for heavy snow remains in effect over parts of the
Susitna Valley through tonight, with persistent southerly upslope
flow and ample moisture feeding in from the Cook Inlet region.

A strong ridge will begin to build over Southcentral from the
north Wednesday afternoon, leading to development of a weak
outflow wind event along the coast. As a result, showery
conditions will taper off over Southcentral by Wednesday
afternoon, with cooler and drier weather settling in across the
region by Wednesday evening.

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.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Fog and low stratus will be the main concern in the short term.
Lingering low-level moisture, diminishing winds, and a building
ridge overhead will promote fog/stratus development and thickening
of areas already in place. Visibility will likely drop to a
quarter-mile or less in places with the Kuskokwim Delta being the
most likely. The Bristol Bay area will also see fog, less
widespread though as weak offshore flow will develop overnight.
Offshore flow will continue to strengthen through the day tomorrow
into Thursday further reducing the fog threat. High pressure will
control the forecast for the next couple days with no chance of
precipitation and relatively light winds.

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.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The storm force front will continue eastward overnight before
stalling over the central Bering/Aleutians on Wednesday. The front
slowly weakens in place through Thursday. High pressure and
offshore flow will stay in place over the eastern Bering while
another gale force front approaches the western Aleutians on
Thursday.

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.LONG TERM FORECAST (Days 3 through 7)...
There remains good agreement that a blocking pattern setting up
over the mainland late this week will bring a period of quiet
weather to much of the Mainland through this weekend. A weak ridge
associated with this blocking pattern has shifted a little further
to the east due to a stronger shortwave trough rotating around
the western edge of it. The result of this is a slightly warmer
forecast from Southcentral into the Copper River Basin, along with
weaker outflow winds than initially expected. This setup will
also put Cook Inlet closer to the ridge axis, which would be more
favorable for fog/low stratus development. This was left out of
the forecast however as the down inlet flow (while a bit weaker)
should be enough to limit significant fog development, but this
will need to be monitored in the coming days. Over the Western
Bering little has changed as stationary frontal boundary will
bring nearly persistent gale force winds with rain to much of the
area from late this week into the weekend.

Forecast confidence then quickly diminishes by early next week as
significant model disagreement in the track and intensity of both an
upper low moving developing over the Mainland and a polar low over
the Beaufort Sea. The exact track of these features will has a
significant impact in the forecast looking into early-mid next
week, especially with regards to temperature.

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.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 145.
MARINE...Storm 175 176 177 178 Gale 170 172 173 174 179 185.

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SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PEPE
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...DK