Small Craft Advisory 343, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 AM AKDT Fri Apr 27 2018

.ANALYSIS AND UPPER LEVELS...
An upper level low anchored over the Bering Strait is leading to
broad cyclonic flow over the eastern Bering and western mainland
Alaska. A surface low rotating around the base of the upper low is
moving SE towards the SW coast with an associated front extending
out from the low center currently moving ashore. A large swath of
precipitation out ahead of the front is showing up on both the
Bethel and King Salmon radars and is spreading east across the
Kuskokwim Delta, Kuskokwim River Valley, and the Bristol Bay
coast, with most stations in the area reporting snow. Southwest
onshore flow is continuing to promote scattered rain showers over
the upper Cook Inlet area north across Anchorage and the Mat-Su,
though returns from the Kenai radar suggest a brief decline in
shower activity across southcentral. Out west, a deep low
pressure system over the Kamchatka Peninsula is spreading a front
across the western Bering and Aleutians bringing gale force winds
and rain to Shemya and Adak.

&&

.MODEL DISCUSSION...
Models are in lock step through Saturday regarding the synoptic
scale features moving across the Bering and mainland Alaska. By
Sunday, the models begin to diverge as another low moves into the
Bering Sea. The NAM/GFS track the system east across the central Bering
and into mainland Alaska by Sunday night while the EC stalls the
low out over the northern Bering before finally coming ashore on
Tuesday, not before rapidly weakening. For now, went with the GFS
solution which brings a bit stronger front across mainland Alaska
sunday afternoon through Monday morning.

&&

.AVIATION...
PANC...VFR conditions will persist through most of the day though
southerly winds will be periodically gusty. A front approaches
this afternoon spreading rain across the Anchorage Bowl. Initially
the Turnagain Arm winds should keep the terminal dry and VFR but
eventually by the early evening hours enough moisture from good
mid level southwest flow combined with weakening downslope winds
will allow rain and MVFR conditions to work their way into the
terminal. Enough cold air aloft may also mix some snow in with
rain but is not expected to significantly decrease the flight
category.

&&

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

The active weather pattern across Southcentral continues. A
vigorous trough currently over western Alaska moves rapidly east
today and will bring fairly widespread showers, especially to the
coasts. Showers will tapper off across the Cook Inlet to MAT-SU
corridor through the evening hours as the trough races east.
Showers will likely linger over the Copper River Basin until
trough passage later Saturday morning. Showers will be mainly in
the form of rain along the coasts. Inland areas will be somewhat
more interesting with low wet bulb heights. Current thought is
that mainly lower elevations across Cook Inlet to MAT-SU will see
mixed precipitation types today with mostly snow at higher
elevations and northern valley sections. The brunt of
precipitation across the Anchorage Bowl/Mat Valley will likely be
delayed until the cross barrier flow across the Chugach diminishes
later this afternoon/early evening. Generally gusty winds and
locally strong gap flows will accompany the trough, but should
diminish overnight.

A weather front, associated with the low moving into Southwest
Alaska will bring another round of generally windy conditions with
strong gap flows (Turnagain, Knik, Copper River, etc.) Saturday.
The air will be warmer and more stable with this system with
precipitation being more stratiform and a nod toward rain. The
brunt of the precipitation will be confined to coasts as cross
barrier flow over the Chugach will limit precipitation inland.
However, the northern and western Susitna valley will likely see
steady precipitation with upslope flow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The shortwave pushing through southwest Alaska will continue to
bring snow to the area through the morning before it tracks off to
the east. Weak ridging will then move in over the area this
afternoon but will be short lived as a front will move in over
southwest Alaska this evening. This system will bring a strong
push of warm air with it that will transition the snow to rain for
many locations, especially along the coast. Showers will then
linger over the area through Saturday ahead of another front that
will make it to southwest Alaska Saturday night bringing another
push of warm air and more rain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
There is an active pattern over the Bering/Aleutians that will
see several systems track across it through the weekend. The
shortwave over the eastern Bering will quickly exit to the east
while a stronger system over the western Bering will continue to
move east bringing a strong push of warm air with it. This system
will make it to the eastern Aleutians by this afternoon and move
out of the area to the east tonight.

Ridging will build in behind the system before another front
tracks over the western/central Aleutians Saturday morning from
the North Pacific. This system will also bring up warm air
bringing primarily rain for the Aleutians and will then push
through the Bering and Aleutians making it to the southwest coast
Saturday night. Cold, westerly flow will follow behind the system
before a weaker system pushes in over the western Aleutians Sunday
night.

&&

.LONG TERM FORECAST (Days 3 through 7)...

An active pattern continues at the start of the long range
forecast beginning Saturday night. This period starts out with a
weakening front slowly tracking west to east across the southern
mainland with another storm system swiftly moving across the
Bering to the Southwest coast. This pattern keeps higher
precipitation chances across the southern mainland and
surrounding waters through the weekend. Yet another low moves
across the Aleutians on Monday staying on a more southerly track,
but pushes across the southern mainland and gulf by Tuesday.

The jet stream pattern shifts from zonal flow along the Aleutians
and Gulf to an amplified which should change up the speed and
track of the storm systems heading into the middle of next week.
Models continue to struggle with the extended forecast leading to
lower confidence and conservative changes for Sunday through
Wednesday. A blend of the operational models and ensembles were
used to make any updates necessary.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 132 150 155 165 170 172 173 174 175 176 177 178 352
413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK/TM
LONG TERM...KH