Small Craft Advisory 796, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 PM AKDT Tue Oct 20 2020

.ANALYSIS AND UPPER LEVELS...
High surface pressure is over Southwest mainland Alaska with a low
in the eastern Gulf of Alaska. This pattern is producing the
mostly clear skies over mainland areas with gusty winds through
channeled terrain, especially across the northern Gulf. The jet
stream is running well south of the Western Aleutians, bends
sharply around a trough located near 165W, and then increases
again as it heads south of the Gulf of Alaska. This pattern is
setting up the next low which will impact the region in the Bering
Sea. This area south of the Aleutians has a strong jet max of
around 170 kt at 250 mb. This is allowing the surface low in the
left exit region (near 47N 175E) to spin up rapidly, and that
process has already begun this afternoon.

&&

.MODEL DISCUSSION...
Models remain in good agreement for much of the remainder of the
week. The biggest issue at this time revolves around the progress
of the warm front into the northern Gulf later this week. The NAM
and GFS have both settled on the solution bringing some storms
into parts of the Bering Sea Wed night into Thursday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. There is a
chance for fog Wednesday morning with clearing skies, but the
light northerly winds should help keep it at bay. There is also a
chance for some stratus to develop in Knik Arm overnight and then drift
over the airport.

&&

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

Clouds continue to linger across from the Chugach Mountains east
to the Copper River Basin as an upper-level shortwave drops south
across the eastern half of the Southcentral mainland. A cold,
northerly flow has set up over the region. Gap winds are
increasing over favored coastal locations as a pressure gradient
tightens between a ridge building over the interior of Alaska and
a surface trough over the northern Gulf. These winds will peak
overnight tonight across the eastern Kenai and by tomorrow
afternoon over Prince William Sound and the Copper River Basin.
Wind gusts will be enhanced overnight due to a combination of cold
air advection from the interior strengthening a thermal gradient
and a second upper-level wave moving across Southcentral.

The main forecast challenge will be overnight low temperatures as
the increased gap winds mix the air at near the surface and limit
radiational cooling. This includes locations such as Seward,
Whittier, and Valdez where gusty winds tonight and tomorrow night
should keep overnight temperatures from falling into the 20s.
Overnight lows will likely stay in the upper 20s for Palmer and
Wasilla as a Mat Valley wind sets up tonight. Elsewhere, overnight
lows, especially Wednesday night into early morning Thursday,
could be the coldest thus far this season.

A front enters the southwest Gulf late Wednesday night into
Thursday bringing rain and high elevation snow to Kodiak Island
along with southeasterly gales across the western Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tuesday
afternoon through Friday afternoon)...

One last day of mostly clear skies and seasonal temperatures
across the Southwest mainland are expected for Wednesday before a
strong front moves into the region. The frontal system is entering from
the North Pacific and is expected to reach the far southern
reaches of the AKPEN Wednesday morning. The parent low is quite
weak and is expected to be around 995 mb initially. However, the
strong gradient along the front between the low and the retreating
high over the interior will create strong winds and moderate to
heavy rain for the AKPEN Wednesday and Thursday as the front moves
north. By Thursday morning, the front is expected to reach the
Bristol Bay coastline where it will slowly move inland and weaken
by Friday afternoon. Residual cold air at the time of the frontal
arrival may produce a few flakes before precipitation rapidly
transitions to rain. Strong southeasterly winds are also likely as
the front moves through. Snow, or at least a wintry mix, is
likely over the higher elevations including the Kilbuck
Mountains. Rain should be dissipating over the lower elevations of
the mainland by Friday afternoon, and the AKPEN will see another
round of rain on south facing slopes as the cold air wraps around.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Tuesday
afternoon through Friday afternoon)...

High pressure and calm winds will occupy the Northern Bering
Wednesday while two strong frontal systems impact the Aleutian
Chain Wednesday morning. Storm force winds along the eastern of
the two fronts are expected over the Eastern Aleutians on
Wednesday, extending into the Eastern Bering Wednesday night. The
western front is associated with the stronger of the two lows and
will bring gale force winds and rain to the Western Aleutians on
Wednesday. The two fronts will then combine Wednesday night to
then bring a broad region of gale force winds and rain to the
Bering on Thursday. By this time the two lows will have
conglomerated into one near Adak, creating a broad, strong, occluded
low. Cold air wrapping around the southern flank of the low on
Friday should bring another round of gale force winds and rain
from Dutch Harbor to the pribilofs as winds and waves slowly
subside across the region.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Gulf of Alaska:

A well developed front moves to the North Gulf Coast by Sun. The
track and development vary considerably, but widespread
southeasterly gale force winds and waves will occur across the
Gulf through Sat. The area of gale force winds then retreats to
the Barren Islands area Sat, leaving small craft winds over the
Gulf slowly diminishing over the Northern Gulf through Sun.

Bering Sea:

A strong low and front moves into the Bering through Sun. Track
and development of the system vary considerably, but overall area
coverage looks reasonable, with gale force winds and waves
wrapping around the low as it spreads over the Bering. High end
gale winds expected in Bristol Bay near the Alaska Peninsula
channeled areas for Fri. The wind-wrapped low brings a second
round of gale force winds across the Eastern Aleutians Fri and
Sat. The entire system slowly diminishes to small craft winds
through Sun.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

By Friday, a front associated with a large low across the Bering
Sea will have made its way inland on the southwest coast, across
the AKPEN and Kodiak Island. This will bring precipitation across
these areas with the front propagating into Southcentral Saturday
and Sunday. Gale force winds are expected with the low over the
Bering until it begins to dissipate on Saturday. The Anchorage
Bowl looks to remain relatively dry until Monday before drying
out again Tuesday. Precipitation will continue for the Bering, the
Southwest, higher elevations and coastal locations through
Tuesday. Precipitation should be a wintry mix at lower elevations,
but temperatures aloft should stay warm enough to not turn all
precipitation to snow. Expect snow at higher elevations through
the forecast period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING 150,155,160,165,170,172,180. GALE WARNING
127,128,130,131,132,138,351,171,173-179,181,185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...MK/CK