Small Craft Advisory 912, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 AM AKST Mon Nov 19 2018

.ANALYSIS AND UPPER LEVELS...
A deep upper level trough extends across most of mainland Alaska
this morning as a broad upper level ridge extends over the
central/eastern Bering Sea and Aleutians. A 100+ kt southerly jet
at 300 mb is located on the eastern side of the upper trough,
pumping subtropical air north into eastern Alaska. This jet is
supporting the development of a surface low just to the NE of
Kodiak with an occluded front pushing north across the Gulf Coast.
This front is bringing widespread rain across the coast with
precipitation spilling over into the Copper River Basin. Surface
temperatures are again mostly above freezing with the exception of
locations near the center of the Copper River Basin, where
freezing rain has been reported since yesterday afternoon. The
upper ridge over the Bering is moving west leading to increasing
northerly winds dragging colder air south across the eastern
Bering/Aleutians. Further west, another surface low supported by a
strong zonal jet is pushing a frontal system east across the
western Bering.

&&

.MODEL DISCUSSION...
Models are in decent agreement this morning through the short term
forecast period. Models are a bit slower with the northward
progress of the front pushing across the Gulf Coast and
Southcentral this morning, thus precipitation should taper off
more rapidly today. Models all show the surface low in the Gulf
slowly track northeast through Tuesday, though the EC brings the
low center into the Prince William Sound while other solutions
keep the low out over the open Gulf. The low tracking east across
the western/central Bering is also being handled well by the
models, with no major discrepancies noted.

&&

.AVIATION...
PANC...Conditions should remain VFR today as the front and
associated precipitation have already pushed north past the
terminal this morning, with cloud cover diminishing behind the
system. Some patchy low clouds may linger in the Anchorage Bowl
this morning but are expected to diminish and not impact the
terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Mon and Tue)...

A fairly strong front moves rapidly north across Southcentral this
morning bringing more showery conditions across many areas today.
A lower 980s mb vertically stacked low near Kodiak Island slowly
weakens as it moves to the eastern Gulf coast Tuesday afternoon.
This center will produce gales across the Gulf today which should
hold into the late afternoon hours before weakening. A fairly
tight pressure gradient on the backside (west) should be enough
to continue westerly gales over the western Gulf/Barren Islands
through tonight, then weakening on Tuesday. Brisk to strong gap
flows across Turnagain Arm/Portage Valley will diminish this
evening. A combination of a broad upper level trough and some low
level moisture will keep the threat of showers across
Southcentral into Tuesday. The coast however will be the main
recipient of the moisture. The main body of cold air looks like it
will hold on the interior side of the Alaska Range keeping the
milder airmass with a nudge to cooler conditions across
Southcentral. So in general mixed showers will likely be the main
precipitation types.

&&

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

A weak disturbance in the mid levels of the atmosphere is
currently moving ashore, with a decent batch of snow showers
developing immediately ahead of it across the Kuskokwim Delta.
This feature will slowly trek eastward through the day, spreading
the snow inland, before washing out overnight. Another impact from
this disturbance will be continued cold advection. This will cause
a rain snow mix to develop for coastal locations north of Bristol
Bay, and snow inland. Further south (mainly east of the Kilbuck
Mountains and south of the Nushagak Hills), this region will be
further removed from the cold air, with mainly a rain to at times
a rain/snow mix scenario developing.

Then for your Tuesday, a much stronger disturbance of Siberian
origin will move to Hooper Bay by early afternoon. A surface low
associated with this disturbance will send a very strong cold
front towards the Kuskokwim Delta and western AKPEN by mid
afternoon, with widespread precipitation developing along it.
Given the strength of the cold air rushing into the region, mainly
snow will be the predominant precipitation type, with a few
inches of accumulation possible. The exception to this will be
coastal Bristol Bay, where it'll take just a bit longer for the
cold air to reach the region. By Tuesday night, everyone will feel
the chill in the air.

&&

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

A strengthening area of low pressure will move from the western
Aleutians this morning to Cape Newenham by tomorrow morning, with
a trailing cold front sweeping eastward. Widespread precipitation
will develop along and ahead of both of these features, with
sustained winds increasing to small craft criteria north of the
chain, and gales further south. High pressure will develop in its
wake, with cyclonic flow around the low and anti-cyclonic flow
around the high tightening the pressure gradient for the eastern
half of the Bering Sea and Aleutians. This will result in
northwest winds advecting in colder air all the way down to the
chain/AKPEN, with rain showers changing over to snow showers with
time.

&&

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

The extended time frame starts with troughing over southcentral
AK, ridging for the central and eastern Aleutians, and troughing
over the Western Aleutians. This is similar to the solution
yesterday for midweek, so confident in the pattern. This means
active weather for southcentral AK and the western Aleutians.

Models are still showing ridging working its way east later next
week with the ridge moving over southcentral AK. There is some
difference in the strength of this ridge so remains to be seen how
much influence it will have, but generally a cooling and drying
trend can still be anticipated later next week for southcentral AK.
As with the past few days, models diverge significantly going
into next weekend. This means a middle of the road approach in the
forecast until things can get straightened out with model consistency.
The western Aleutians are also a mess by Thursday of this week
with the forecast. With confidence being so low in the long range,
not much point in trying to nail down a solution at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...141 Winter Weather Advisory.
MARINE...Gales 119 120 130 131 132 174 176 177 351 352.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...BB