Small Craft Advisory 437, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKDT Fri Jun 1 2018

.ANALYSIS AND UPPER LEVELS...
The synoptic pattern remains with a large upper and surface low
moving into the western Aleutians and western Bering Sea with a
potent gale force to low storm force front. A strong jet
supporting this low is rounding the base of the trough. Weak
ridging in advance of this storm is over the eastern Aleutians and
eastern Bering Sea pushing east toward the mainland. Weak low
pressure remains over the Gulf of Alaska and a weak low is over
the southern Yukon Territory advecting moisture and clouds into
the Copper River Basin. Higher CAPE values are driving the
instability across the Copper River Basin into the Talkeetna
Mountains, although the lack of moisture in the Susitna Valley is
preventing the convective growth anticipated. The northeast
steering flow is pushing the convective development along the
Talkeetna Mountains to the southwest.

&&

.MODEL DISCUSSION...
The models are in excellent agreement with respect to the strong
low and front now impacting the western Aleutians and western
Bering Sea and its expected progression through the weekend. The
models have a good handle on the magnitude of the wind associated
with this storm with storm force wind south of it and a large
area of gales in advance of the front.

&&

.AVIATION...
PANC...VFR conditions and light diurnal winds are expected to
persist through early Saturday afternoon. Southerly winds are
expected to increase later Saturday afternoon and continue into
the evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Saturday
and Sunday)...
A few isolated showers and storms have developed this afternoon
along the Talkeetna Mountains. A light northeasterly steering
level wind is moving the showers off the mountains and into the
Matanuska Valley. This will continue for that area into the
evening hours. It appears unlikely at this time that any of that
activity will survive the trip down to Anchorage, but it cannot be
ruled out. Either way, the slow moving nature of the storms
should give those outside plenty of time to seek shelter until the
showers and storms pass. The showers are more plentiful further
east into the Copper River Basin, along the western Susitna
Valley, and across Kodiak Island as well. This activity will
diminish later this evening, once the sun gets low enough in the
sky to no longer be an effective energy source for the convection.

On Saturday, we will have a repeat day as far as shower and
possible isolated thunderstorm activity goes for many areas from
the Matanuska Valley north. With the upper level ridge axis right
over Southcentral, the flow should be very light, and thus any
storms that form along the mountains should be nearly stationary,
or drift northeastward. Coastal ridging is expected to develop
across the north Gulf Coast during the day. That combined with
the thermal trough over interior Alaska will invigorate the
Turnagain Arm and Knik River Valley winds. The thermal trough
should allow the Turnagain wind to move into south and west
Anchorage. Gusts of 35-45 mph are possible along Turnagain Arm and
higher elevations, and gusts up to 35 mph possible through south
and west Anchorage, and through the Knik River Valley and into
Palmer Saturday afternoon. The Turnagain Arm wind is expected to
persist through the overnight/twilight hours and diminish Sunday
morning.

For Sunday, the upper level ridge will persist overhead and
continue to strengthen. This warming of the upper levels should
inhibit most shower and storm activity except right along the
Alaska Range, though the typical daily cumulus field may develop
in the areas where there is more moisture. Otherwise, winds will
remain nearly calm with lots of sunshine expected. An easterly
wave moves into Southcentral Monday, which may touch off more
organized thunderstorms along the Talkeetnas and Alaska Range Monday
afternoon. However, this will depend on the timing, which is
still questionable at this time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level ridge building into SW Alaska will yield warmer
and drier weather over the region through Saturday. Showers with
some isolated thunderstorms will continue to develop over the
Middle Kuskokwim River Valley this evening and slowly drift south
towards the Greater Bristol Bay area before dying out later
tonight. As the ridge axis moves over SW Alaska tonight, a
persistent marine layer currently lingering over the Eastern
Bering Sea will advect south and east towards the Kuskokwim Delta
and Bristol Bay Coasts, bringing periods of low stratus and fog
in the morning, especially along the Northern AKPEN coastline.

Diurnal convection will redevelop Saturday afternoon, but looks
to be confined primarily to areas along the Western Alaska Range
as subsidence associated with the ridge shifts a little farther
east. A rapidly weakening Bering Sea front will reach the coast by
Sunday, providing a chance for stratiform rain over the Delta and
coastal Bristol Bay. The front looks to stall out and shear apart as
it runs into the ridge to the east, keeping the eastern half of
SW Alaska mostly dry.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An unseasonably strong 965 mb low pressure system will continue
to track NE across the Western Bering tonight before steadily
weakening through the day tomorrow. A warm front out ahead of the
system will sweep east across the Aleutian Chain bringing a quick
shot of southerly storm force winds to the Western Aleutians this
evening accompanied by heavy rain from moisture laden air
advecting up from the sub-tropics. A cold front racing rapidly
behind the warm front will bring another shot of storm force winds
overnight to the Western Aleutians, aided by decent cold air
advection. The long fetch winds associated with this system will
lead to rapidly rising seas, with swells increasing to nearly 30
ft by tomorrow morning on the Pacific side of the Western
Aleutians.

The frontal system will rapidly weaken Saturday night as it runs
into a stubborn area of high pressure to the east, with the front
shearing out by the time it reaches the eastern Aleutians Sunday
morning. A more showery regime will then develop from west to east
in the wake of the front as the low weakens and drifts towards
northern Kamchatka.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A broad mid level trough of low pressure across the southern
Bering spans across the Gulf for most of next week. A nearly zonal
jet stream remains just south of the Aleutian Chain to the Dixon
entrance. The jet will act as a conveyor belt to move waves of
energy from the Kamchatka Peninsula to the Gulf. A few easterly
waves will be sent across the southern mainland on the north side
of the low. This energy will be the focus for showers to develop
inland. With a thermal low over the interior sagging toward the
coast, a few thunderstorms will be possible during the late
afternoon and evening. Only minor changes were made overall using
a blend of the GFS/ECMWF operational models and the ensemble
solutions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 177 178.
 Gale Warning 170 172>176 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...KH