Small Craft Advisory 388, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...RESENT
National Weather Service Anchorage AK
447 PM AKDT Thu May 7 2020

.ANALYSIS AND UPPER LEVELS...

A large scale low pressure system over the North Pacific was
centered near 50N 152W. From this low, an occluded front extends
northward and curves westward across Kodiak and the Gulf of
Alaska. Wind data from local sensors and maritime vessels reported
high end gales, with higher gusts, across much of the Gulf waters.
Light to moderate rain and clouds continue to spread across the
Gulf through Kodiak and northern portion of the Alaska Peninsula,
including King Salmon. Farther north, a mid to upper-level ridge
stretches across the mainland of Alaska, bringing mostly clear
or partly cloudy skies to much of Southcentral and Southwest
Alaska. However, high clouds from the occluded front over the
Gulf have moved over Anchorage and are headed inland across
Southcentral.

Over the central Bering Sea, an upper low remains slow-moving,
with clouds and fog moving across the Bering Sea, Pribilof
Islands and along the Aleutian Chain.

&&

.MODEL DISCUSSION...

Models remain in good agreement with evolution of large scale
features over the next few days and forecast confidence is
generally high. The primary forecast challenges include the
strength of localized gap winds in Southcentral and Southwest
Alaska in response to the storm system currently over the Gulf
as well as probability of seeing precipitation in downslope
areas.

&&

.AVIATION...

PANC...VFR conditions will persist. The wind forecast will remain
a bit of a challenge, as Turnagain Arm winds remaining nearby.
Lower pressure over the Gulf favors down Inlet flow (with the
Turnagain Arm jet bending southward). However, inland heating is
lowering pressure over the Mat-Su, which favors some southerly
flow in Anchorage. The key factor appears to be stability. The
Anchorage soundings yesterday and early this morning are weakly
unstable with a well mixed boundary layer. This should allow some
southeast winds to move into the terminal this afternoon through
evening. It looks like a repeat on Friday afternoon/evening with
a similar set-up.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (through Sunday night)...

The feature of interest for Southcentral is a front in the Gulf
that is slowly moving onshore. Most of the rain with the front
will stay confined to the mountains and coastal areas though there
is a chance that some very light precipitation could make it over
the mountains. Otherwise an uptick in gap winds is the only other
impact from this weather system. Normal gap winds through
Turnagain Arm and Knik Valley will continue this evening before
tapering off late tonight into Friday morning. The front will
dissolve along the coast on Friday, while the cloudiness
associated with it will linger through mid-day Saturday before
beginning to clear out. On Sunday, high pressure will try to nose
back into the area from the Yukon and return drier and warmer
conditions. The Kodiak Island/Western Kenai are will remain under
a little cooler air mass. Any uncertainty during this time frame
will come from a disturbance that will ride between these two air
masses, and could bring a quick shot of wind/rain over the
weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A large area of low pressure in the Gulf of Alaska with high
pressure over the north slope will make for strong east-
northeasterly winds this evening and tonight, especially along the
AKPEN (east of the Shumagin Islands) and in Bristol Bay. Little
to no precipitation is expected due to this east-northeasterly
flow downsloping off the Aleutian Range. Gale force northeasterly
winds in Shelikof Strait and along the Pacific side of the AKPEN
will gradually ease tonight as pressure gradients ease. Low level
wind shear is expected this evening and tonight across the
northern AKPEN and Bristol Bay. Friday and Saturday will remain
dry as the offshore easterly flow prevails and allows for
downsloping to continue across the Kuskokwim Delta and Bristol
Bay. An upper level disturbance will approach the northern AKPEN
and Bristol Bay from the southeast on Sunday. Conditions will
remain dry due to downsloping off the Aleutian Range, though this
disturbance will produce gusty east- southeasterly winds at
times, especially in valleys that are prone to localized
funneling.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

Low pressure located east of the Kuril Islands will continue to
move northeastward and strengthen. Scatterometry (ASCAT) data from
this morning shows Gale force east-northeasterly winds along an
occluded front on the north side of this low. This low will bring
widespread Gale force winds and storm force gusts to the western
Bering/Aleutians tonight through tomorrow night. Widespread rain
associated with this low is also expected across the western and
central Bering/Aleutians tonight through tomorrow night. There is
good agreement among guidance that these winds will near storm
force at times. Large long period swell is expected on the Pacific
side of the western Aleutians tomorrow and tomorrow night due to
the widespread gale force winds and ample fetch for this swell to
build and lengthen in period. Conditions will ease on Sunday as
the low moves into Siberia.

&&

.MARINE (Days 3-5 Sunday through Tuesday)...

The general pattern will consist of low pressure over the Bering
Sea and Aleutians, and high pressure over mainland Alaska and the
Gulf. Winds are expected to stay below gale force, with small
craft advisory winds limited to the Aleutians and Bering.

&&

.LONG TERM FORECAST (Days 4-7 Monday through Thursday)...

The general pattern for the extended portion of the forecast will
consist of a ridge over mainland Alaska and a trough over the
Bering Sea and Aleutians. The pattern will have a negative tilt,
with the interface between the ridge and trough roughly over
southwest Alaska. The main forecast challenge is the spatial
extent of clouds and precipitation over southwest Alaska (and
possibly Southcentral by Thursday) due to the interface between
the trough and ridge. Temperatures will mostly be above normal.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 155 (Kuskokwim River at Kwethluk)
Flood Advisory 155 (Kuskokwim River Akiachak to Napakiak).
MARINE...Gale Warning 119 120 130 131 132 137 138 139 150 160 173
175 177 178 351 411 412 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CB/SEB
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...BL