Small Craft Advisory 836, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 AM AKST Mon Nov 2 2020

.ANALYSIS AND UPPER LEVELS...

A deep 500 mb low centered over Prince William Sound is bringing
very cold temperatures and strong northerly gap winds across
Southern Alaska. A High Wind Warning remains in effect through
mid-morning for the Matanuska Valley and Thompson Pass. Blowing
dust remains seen on satellite out of the Copper River Delta and
Knik River Valley. Likewise. some low stratus is still present
over the Kuskokwim Delta, however most of the mainland remains
clear. Scattered rain/show showers are occurring over much of the
central and western Bering under a series of shortwave troughs.

&&

.MODEL DISCUSSION...

Synoptic models remain in good agreement showing a continued
offshore flow pattern through Wednesday. High-res models remain
preferred to represent the strongest gap winds. As one low
dissipates in the Gulf this evening, another is expected to form
in roughly the same area tonight into Tuesday. The NAM develops
this into a stronger, more compact center by Tuesday evening,
while other models maintain a weaker, more broad circulation.
Although the NAM is currently the outlier, it would not be
surprising if it verified given the unseasonably cold air
remaining in place to the north wrapping around its center causing
it to strengthen. This would increase the pressure gradient near
the coast and thus subsequently increase northerly gap winds,
particularly in the Copper River Delta. However, as the high
pressure over the mainland is only about 1013 mb, these winds are
likely to not be as strong as they are presently even if the
deeper NAM solution verifies. Farther west, a low and associated
front is well-agreed upon to bring periods of heavy snow and
reduced visibility along the coast. A Winter Storm Watch for
potential blizzard conditions is in effect for the Kuskokwim
Delta. Although reduced visibility is highly likely, the snow
rates may not be heavy enough to verify this. However, overall
forecast confidence remains good for both of these aforementioned
regions.

&&

.AVIATION...

PANC...VFR conditions will persist. Gusty northerly winds will
continue through the day, though will very gradually diminish into
Tuesday morning.

&&

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

Outflow winds continue to be the focus in the near term, with a
polar upper level low now in the process of cutting off and
stalling out over the Prince William Sound. The High Wind Warning
for Thompson Pass remains on track to expire later this morning
with gusts very gradually declining as cold advection decreases,
and the surface low in the Eastern Gulf moves inland (reducing the
pressure gradient). That being said, a moderate pressure gradient
will still remain and plenty of strong cold air remains
in place across the interior. So gusty winds are likely to
persist into Tuesday, but they will just be below warning
criteria.

For the Matanuska Valley wind gusts look to peak this morning and
then start to fall below warning levels later today, but gusty
winds and occasional blowing dust will continue into Tuesday as
pressure/temperature gradients slowly weaken. For the Anchorage
Bowl, gusts of around 30-35 mph will continue through the day
today, gradually weakening tonight into early Tuesday.

Also on Tuesday models continue to indicate a compact surface low
developing south of Icy Cape. Depending on the exact track and
strength, this may be enough to spread a few snow showers back
towards eastern areas of Prince William Sound from late Tuesday
into Wednesday. There is uncertainty as to how far west this
moisture will get. For now we have kept some slight snow chances
in, mainly for the higher elevations.

Following the wind, the focus will shift to a shortwave crossing
the AKPEN late Tuesday night into early Wednesday. It appears a
quick burst of snow will be possible for southern Kodiak Island
as this shortwave moves through. There are model disagreements as
to whether the best forcing sets up over the Island, or to its
south. For now we kept the greatest chances for snow across the
southern half of the island.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2): Monday and
Tuesday...

The temperatures at 850 mb range from -10 to -16 Celsius over
Southwest Alaska at the onset of the forecast period. Widespread
northerly flow will persist over the region. Meanwhile, the sea
surface temperatures for the Bering Sea are ranging from +7C to
+4C. The models are advertising a potent shortwave moving across
the Bering Strait and over the Yukon and Kuskokwim Delta on
Tuesday. By Tuesday night this shortwave will be over Bristol Bay
and the AKPEN. This is a fast moving upper level disturbance. The
concern is that cold air over warm water is already unstable but
with the addition of that shortwave, convective snow will develop.
The winds at the surface will be gusty on Tuesday and Tuesday
night. A Winter Storm Watch for zone 155 has been issued for
Tuesday afternoon through late Tuesday night as wind gusts are
expected to exceed 35 mph with visibility of 1/4 miles or less
possible. A few locations may even see gusts up to 50 MPH. The
heaviest snow and lowest visibilities will be near the coast,
though inland areas such as Bethel will also see a period of gusty
winds and falling and blowing snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2): Monday
and Tuesday...

There is a surface high between Attu Island and Kamchatka this
morning. This high will push east today and tomorrow. In its wake,
there is a 984 mb low. The pressure gradient between the high and
the low will bring strong winds to the western Aleutians on
Tuesday. Additionally, this system is being driven by a 150 knot
jet core. Gales and storm force winds will accompany this system.
A hefty amount of moisture is expected with the frontal boundary.
However, the leading edge of the precipitation shield will not
hit Shemya until Tuesday night.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

Northwesterly gales and seas 10 to 15 feet will persist across
the Bristol Bay and AKPEN marine zones Wednesday before
diminishing Wednesday night. There could be storm force gusts
along the south side of the AKPEN.

A Kamchatka low will track a front with southwesterly storm-
force winds and seas to 30 feet over the western Bering/Aleutians
by Wednesday. These winds will move east along with the front to
the AKPEN and eastern Bering/Aleutians by Thursday with storms
diminishing to gales and seas of 15 feet. Westerly gales will
continue behind the front over the Bering through Thursday before
diminishing to widespread small-craft level winds and waves by the
end of the week.

For the Gulf, northwesterly gales with seas to 10 feet are likely
to linger through Thursday from the Barren Islands to Shelikof
Strait. Gusty northerly outflow winds will also likely continue
into Thursday along the immediate Southcentral coast. These winds
are expected to diminish by the end of the week.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

The long term forecast begins Thursday with an eastward
progression of the upper-level trough-ridge pattern that has
dominated the weather over the past few days. This shift is due to
a deep arctic trough digging south across west Asia amplifying
the pattern and shifting the ridge that has been over the western
Bering/Aleutians toward the AKPEN and the trough over the state
toward Canada. At the surface, this translates to the cold surface
ridge sitting along the AL-CAN border slowly tracking into the
Yukon and Northwest Territories by the end of the week. As it
does, the pressure gradient between it and the trough over the
northern Gulf will relax allowing the strong outflow winds across
coastal Southcentral to diminish. The decrease in northerly flow
will also allow temperatures across the Southern Mainland to
slowly warm back up.

Farther west, a storm-force Kamchatka low looks to track a front
toward the AKPEN and Southwest coast by Thursday. An increasing
southwesterly flow will usher in warmer air that will allow
precipitation along the front to transition quickly from snow to
rain along the coast. Farther inland, precipitation looks to
remain all snow or a wintry mix as the front pushes onshore and
quickly weakens by early Friday. Models are also indicating the
potential for a triple-point low to spin up along the now weakened
occluded front somewhere near Kodiak Island. This solution would
help enhance a moist, southerly flow over the Gulf and spread
light snow across Southcentral Alaska for Friday as the surface
low quickly tracks east over the northern Gulf.

A second trough looks to move over the Kuskokwim Delta by
Saturday with another round of light mixed precipitation for
Southwest Alaska as the southwesterly flow continues due in part
to the now stationary aforementioned Kamchatka low. Models then
begin to diverge more significantly by Sunday regarding a north
Pacific low moving toward the AKPEN. Previously, guidance
indicated the potential for this system to lift due north over the
AKPEN into the eastern Bering. Recent runs, however, have moved
away from this solution toward one that would bring this system
closer to the AKPEN then drive it east over the Gulf. This
solution would still bring some rain, snow, and stronger winds to
the AKPEN and Bristol Bay late Saturday into Sunday, especially as
it interacts with the trough moving over the Kuskokwim Delta. The
greatest uncertainty is centered around the track of this low as
it enters the Gulf and whether any precipitation and/or gap winds
develop over the southern half of Southcentral Alaska by the end
of weekend and into early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...

PUBLIC...High Wind Warning: 111 131.
         Winter Storm Watch: 155.
MARINE...Gale Warning: 120 121 127 128 130-132 139 140 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...BG/MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...TM