Small Craft Advisory 955, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 PM AKST Thu Dec 10 2020

.ANALYSIS AND UPPER LEVELS...
A very large vertically stacked low is nearly stationary along the
western Aleutians. A strong occluded front is oriented east-west
across the central to western Bering, then takes a sharp southward
turn near the Pribilof Islands and continues down to the eastern
Aleutians (Dutch Harbor). Surface observations and satellite-
based wind estimates show widespread storm force winds ahead of
this front, along with 25 to 30 foot seas across the heart of the
Bering Sea. As southeasterly winds channel through the mountains
of the eastern Aleutians, isolated peak winds as high as 85 mph
(74 kts) have been observed in Dutch Harbor. Meanwhile, the
combination of falling snow and 60 mph winds has been producing
blowing snow with half mile or less visibilities across the
Pribilof Islands. Temperatures are slowly warming above freezing
as warmer air moves up from the south, leading to improving
visibilities this afternoon. Strong ridging out ahead of this
storm over Southwest Alaska is preventing the front from making
much eastward progress. However, the east-west oriented portion of
the front is lifting northward, leading to elongation and gradual
weakening of the north-south oriented portion of the front.
Bottom line, the front the bulk of precipitation and strongest
winds will remain along the Pribilofs to eastern Aleutians
corridor.

A high amplitude trough south of the Aleutian low is digging
across the North Pacific, pulling a deep vertically stacked low
down around 48N/156W northward toward the Alaska Peninsula. This
will be a key player in the forecast for Friday/Friday night.

The strong upper ridge over Southwest Alaska is building north
and east into the Gulf of Alaska and Southcentral. Just ahead
of the ridge, a couple of short-waves are exiting Southcentral.
There is some light snow over the northern Susitna Valley.
Otherwise dry weather dominates the southern mainland of Alaska
under the influence of the ridge. Areas of fog have also settled
over portions of Southcentral, most widespread from the western
Kenai peninsula up to Anchorage. With a low level inversion in
place (as seen on the 12Z Anchorage sounding) and the upper ridge
building overhead, it looks like this fog is going to hang around
for a while.

&&

.MODEL DISCUSSION...
Models are in much better agreement (but still not perfect
agreement) with low approaching the Alaska Peninsula on friday.
Prefer the GFS/ECMWF solutions which depict a deeper low center
and match well on the low track. Model differences continue with
the next deep low tracking from the northeast Pacific toward the
eastern Gulf Saturday through Sunday. These surface differences
are all related to upper level differences in the
strength/orientation of ridging over the Gulf and southern Alaska
as well as handling of a trough over northern Alaska. The NAM
seems like a clear outlier, while the GFS/ECMWF/Canadian
solutions are more similar and preferred. This will primarily
impact the wind forecast over the Gulf this weekend, but by early
next week these differences grow larger and affect the forecast
for Southcentral as well.

&&

.AVIATION...
PANC...There will be a slight increase in north-south pressure
gradients the next 24 hours as a deep low approaches the Alaska
Peninsula. This should produce a little bit of low level northerly
flow, particularly west of the terminal over northern Cook Inlet.
However, it does not look like enough wind to erode the fog. Thus,
expect periods of fog, with IFR or LIFR conditions, to persist
through at least Friday morning and possibly beyond. There may
be just enough wind during the day Friday to at least see
improvement in the ceiling/vis.

&&

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

Light snowfall remains over the Susitna Valley, which should
continue to taper off tonight. High pressure and cooler
temperatures will prevail across Southcentral through Friday.
Expect increased local gap winds across the north Gulf Coast,
eastern Kenai Peninsula and western Prince William Sound. Fog and
low stratus will continue to be observed for valley locations, the
Knik Arm, Turnagain Arm and Anchorage Bowl. A Dense Fog Advisory
has been issued for Anchorage through noon on Friday with
visibility one quarter or less at times. Chances for rain and snow
will return for Kodiak Island Friday as a weather front
approaches from an area of low pressure south of the eastern
Aleutians.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Friday
through Sunday)...
A negatively tilted upper level ridge is expected to remain over
Southwest through Saturday morning. This should keep the weather
calm while upper level clouds and lee side subsidence warm the
surface. The pattern will then shift Saturday as a strong, compact
low moves over the AKPen. Brisk northeasterly winds are expected
as the low moves over the eastern Bering. Snow is also expected
Saturday from Dillingham north into the Kilbuck Mountains. By
Sunday the surface low should be near St. Matthew Island, which
will bring the wrap around cold air to coastal Southwest. This
combined with moisture advection aloft from a low south of the
Gulf should bring more widespread snow to coastal Southwest.
Lingering high pressure over the interior should keep inland areas
on the drier side.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Friday
through Sunday)...
The storm force front that impacted the central Bering should be
out of the forecast area by Friday morning although gales will
linger near St. Matthew Island through the forecast period. Our
attention quickly shifts to the next low expected to move over the
AKPen early Saturday morning. There is still some uncertainty in
the low track as it moves over the AKPen and into the eastern
Bering on Saturday. Current thinking is there will be storm force
winds over Bristol Bay and the eastern Bering Friday night.
Accompanied with this storm will be rain for the AKPen Friday
night through Saturday. There will be a brief break late Saturday
before the next round of moisture returns to the AKPen on Sunday.
Precipitation type should switch back to snow late on Sunday but
the exact timing of this transition remains uncertain.

Weather over the western Bering should remain fairly calm through
Saturday as the main low center north of Shemya fills in. Winds
and waves will peak Saturday afternoon as persistent cold
advection to the south of the low moves over the western
Aleutians. These winds subside through Sunday as the next low
approaches from the west.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...
A deep low over the Northeast Pacific will lift northward toward
the Gulf of Alaska on Sunday, but looks like it will stall just
south of the Gulf. Right now it looks like winds will top out at
small craft levels across the central to western Gulf waters, but
if the low does end up tracking a little farther north then would
expect gale force winds over the central Gulf.

Forecast confidence for the Monday through Tuesday timeframe is
low. The pattern looks favorable for fast moving deep lows, but
with a huge spread in model solutions it's impossible to say where
the lows will be located. Bottom line, gale force winds are likely
during this time period with a chance for storm force winds in
the vicinity of deeper lows.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)...
The weather pattern next week looks quite dynamic, with a strong
subtropical jet stream over the North Pacific and a deep Arctic
low/trough somewhere over the Yukon and Mainland Alaska. There
will certainly be interaction between the northern stream and
southern stream energy. This pattern favors active weather across
the entire forecast area, with fast moving deep lows likely across
southern areas (the Aleutians eastward to the Gulf of Alaska). As
far as details on individual storms, it is impossible to say due
to the shear number and complexity of features. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 101.
MARINE...Gale 132 138 150 155 170-172 179.
		 Storm 160 165 180 185.
		 Heavy Freezing Spray 160 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...SEB