Small Craft Advisory 734, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKDT Wed Sep 26 2018

.ANALYSIS AND UPPER LEVELS...
A change in the weather can be seen approaching from the south as a
wave rides along the western edge of a ridge over the Northeast
Pacific. So far this wave is only bringing rain to the offshore
areas of the Gulf, however as it continues spreading to the north it
will be the primary driver for rainfall over the South Mainland
Thursday. Otherwise conditions remain very quiet to the east of the
dateline as a broad area of high pressure is present over much of
the area. This ridge is beginning to slide slowly off to the east
and weaken however, allowing for the gale force front currently over
the Western Bering to spread eastward.

&&

.MODEL DISCUSSION...
Model agreement remains good on the synoptic scale features
through Friday, but there are still some smaller scale differences
impacting the overall forecast confidence. The most significant of
which is the track of the shortwave trough that will move over the
southwest mainland Thursday. The NAM especially has trended much
stronger with this wave, bringing significantly more rainfall to
interior Southcentral. On the other hand the GFS has trended
towards a weaker more compact low, resulting in a much drier
solution, especially from Anchorage northward. The GEM regional on
the other hand is a nice compromise of these two solutions, which
was also closest to the current forecast and was preferred for
today's forecast updates.

&&

.AVIATION...
PANC...VFR conditions with light winds are expected to continue
into Thursday morning, with no expected redevelopment of the fog
and stratus experienced at the airport this morning. This is due
to increased offshore flow ahead of a system moving into the Gulf
of Alaska, which will dry out the low levels but also bring
rainfall and otherwise VFR conditions to the terminal for
Thursday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Saturday)...
Cloud cover will rapidly increase from south to north this
evening across the southern mainland as a potent warm front pushes
northward across the Northern Pacific and Gulf of Alaska. A
strong southerly upper level jet is tapping into a long fetch of
tropical moisture and sending it northward towards the southern
mainland. The moisture will ride over the top of a large blocking
high to the east, spreading rain across Kodiak this evening, the
Kenai Peninsula overnight, and into the Anchorage and Mat-Su by
tomorrow morning. The front will produce gale force winds over the
Southern Gulf and Shelikof Strait tonight while heavy rain will
fall across coastal regions, with up to 1-2 inches expected along
the Northern Gulf Coast with lesser amounts inland and in
sheltered locations. Precipitation is also expected in the Copper
River Basin tomorrow, though models have disagreed as to how far
north the precipitation reaches. Have trended towards keeping
precipitation out of the Northern Copper River Basin. There has
been some discussion as to whether snow could be seen in the
Copper River Basin tomorrow, but models have trended warmer, with
850 mb temps dropping barely below 0C. Thus, accumulations along
the Glenn Highway between Chickaloon and Glen Allen are not
likely, though some snow could mix in with rain near Eureka and
Sheep Mountain Lodge in the morning.

Rain will rapidly diminish from west to east over the southern
mainland Thursday evening and night as the driving upper level
trough becomes positively tilted and the upper ridge builds in
over the area. Some residual clouds will linger over the area on
Friday before mostly clearing out by Saturday, setting the stage
for another warm and sunny weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Saturday)
Wet weather will return to southwest Alaska, as a series of lows
moving through the central Bering send multiple fronts across the
region. A warm front associated with the first disturbance will
reach the AKPEN later this evening, then to the Kuskokwim Delta
by lunchtime tomorrow. Widespread rainfall will develop for all of
the region, some of which may be moderate to locally heavy at
times.

For your Friday and the start of the weekend, the additional lows
will take more of a northwesterly track through the central
Bering, with each storm bringing a warm front across the area.
However, these additional fronts will be much weaker with very
little surface convergence. Given the lack of a low-level focus,
and with the better dynamics remaining offshore, the only foci
for rainfall to develop will be via orographic lifting. With a
southeast wind flow, the Peninsula will see prolonged periods of
potential rainfall, where as the remainder of the southwest will see
just broken clouds with very warm afternoon temperatures given the
downsloping component at the surface and bursts of warm advection
aloft.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Saturday)
The calm before the storm(s) best sums up the weather through the
end of the week. A slow-moving frontal boundary will continue to
be a source of widespread rainfall for the western half of the
Bering Sea and Aleutians, as a low meanders across western portions
of the Bering Sea. Meanwhile, further off to the east, multiple
surface lows will lift north to northwest from the central Aleutians
towards Saint Matthew Island. Each system will drag a warm front
northward, bringing periods of steady rainfall, some of which will
be moderate to locally heavy at times.

As for winds and waves, small craft to low-end gales will
continue along and ahead of the front and also near the area of low
pressure through the period. For the eastern half of the Bering Sea
and Aleutian chain, the winds will remain light through tomorrow.
They'll then increase to sustained gales for Friday and Saturday,
as the incoming lows/warm fronts don't begin to arrive until
Friday morning.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning the extended period forecast Saturday night, there will
be a very high amplitude (3.5 sigma+) upper level ridge over the
state of Alaska with a deep upper trough over the Bering Sea.
Mainland high pressure will remain dominant which means dry and
beautiful with warmer than average days through the remainder of
the weekend. Moreover, with the eastern part of the ridge over the
Yukon, this will promote a northerly flow aloft which may enhance
some local offshore winds, thus ensuring a dry and pleasant
weekend.

Heading into Monday, the global numerical models have come into
much better agreement with two important features, the recurving
former Super Typhoon Trami and the breakdown of the upper
blocking high over Alaska. It does seem more likely now that a
low moving into Southwest Alaska will begin to erode the block
sometime late Monday into Tuesday with the front eventually moving
into the Gulf Coast by midweek. This should bring more clouds and
precipitation chances back to parts of Southern Alaska,
especially Southwest, so this forecast will begin trending those
chances upwards. Regarding Trami, there remains quite a bit of
model spread, but increasing likelihood that remnants of Trami
will at least bring some potential gale force winds into the
western Bering Sea Monday night into Tuesday, so this forecast
will also trend that upward to depict this change.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 120 131 132 138 150 155 175>178 185.

&&

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