Small Craft Advisory 202, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 PM AKST Thu Mar 8 2018

.ANALYSIS AND UPPER LEVELS...
The triple point low center has developed south of the Alaska
Peninsula and is rapidly intensifying as it heads toward Kodiak
Island. Moderate to heavy snow associated with its weather front
is evident across the Kenai Peninsula and across eastern Bristol
Bay. Warmer temperatures resulted in rain on Kodiak Island.
Strong pressure rises in advance of the front have produced strong
and gusty winds across the North Gulf and eastern Kenai
Peninsula. Strong cross Barrier flow across the Chugach range has
mitigated snowfall across the Matanuska Valley and Anchorage Bowl.

Cold advection snow showers along with gusty winds were evident
across much of the Bering Sea and Aleutians.

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.MODEL DISCUSSION...
Models are struggling some with the triple point heading toward
Kodiak Island. Generally will be leaning toward the NAM and ECMWF
and doing some hand edits.

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.AVIATION...
PANC...May see some MVFR to IFR conditions in snow during the
morning hours Friday. Expect LLWS to continue into Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Fri and
Sat)...
Well the full-force of the well-advertised storm is being felt
across Southern AK, especially the Kenai Peninsula. The surface
low will continue to deepen this evening as cold air wraps in on
its backside and a potential vorticity anomaly adds upper-level
support. Another 1 to 2 feet of snow are likely from Seward up
through the Turnagain Pass and into Portage. The western Susitna
Valley also does very well with snow in this pattern and is
expected to accumulate another 6-12 inches of snow. Gusty winds
will continue as the low deepens, making for blowing snow and
significantly reduced visibilities at times. Winds will shift to
a northwesterly direction and markedly increase for Kodiak Island
tonight as well. A host of winter weather advisories and warnings
are in effect to address the impacts with this system.

The surface low itself will track inland nearly directly over
Seward through Fri as it finally starts to weaken late in the
day. This will allow winds to slowly subside. However, moist
onshore flow will keep snow falling through Sat along south facing
coastlines. This is especially true for the Eastern Prince William
Sound into the Eastern Turnagain Arm as a secondary vorticity
maximum coupled with orographic lift will support several more
inches of snow through the weekend. On the other hand, southeast
flow is keeping much of Anchorage and the Matanuska Valley dry.
The best chance for any snow in these areas will be late tonight
as the abundant moisture associated with the system and the upper-
level shortwave itself work together to overwhelm any cross-
barrier flow. However, accumulations will be light with only as
much as an inch in most places. Another snow threat will move
through Cook Inlet with the secondary shortwave late Fri night
into Sat. But again, we are not expecting much at all in the way
of accumulations.

Onshore flow, cold air, and weak pieces of energy will keep snow
chances in the forecast for inland areas as we head through the
weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A rapidly deepening low south of the AKPen will push into the
Western Gulf late this afternoon, leaving gusty NW winds and areas
of moderate to heavy snowfall over the southwest mainland through
Friday. Heaviest snowfall concentrations remain focused over the
Bristol Bay area (From King Salmon East and North), and with gusty
winds persisting through early Friday morning, we expect
visibilities to remain fairly low in areas of blowing snow and/or
heavier snowfall. The Winter Storm Warning currently in place over
the above mentioned area will remain in effect through Friday
morning. The area of low pressure will weaken as it pushes
northward and further inland Friday, with wrap around snow showers
persisting through the period.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Cold air continues to filter in along the western periphery of the
two low pressure systems currently driving the weather over much
of the eastern Bering and mainland. This colder NW flow will aide
in the likelihood of snow showers and gale-to-storm force gusty
winds persisting over much of the Bering and Aleutians through
Friday. The best corridor of jet energy and instability type
squalls looks to be from Dutch Harbor eastward, with snowfall
totals ranging from an inch or so in some areas, to 3-6 inches
between Dutch Harbor and Cold Bay. A long fetch length will also
allow seas to build into the 25 to 35 feet range overnight tonight
NW of the eastern Aleutians.

This pattern will continue through Friday night, before snow
shower coverage and intensity diminishes into Friday night as the
low becomes vertically stacked and weakens. A brief area of high
pressure builds over the Western Aleutians/Bering Friday morning,
but is quickly pushed southward over the north Pacific as the
next system tracks into the area from the west friday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast is dominated by a trough of low pressure
that stretches from northwestern Alaska, to well southeast of
Kodiak Island. This trough will eventually get pinched off as a
weak wave slides in from the west. A lot of the action and
uncertainty evolve from the jet rounding the base of this trough.
While the small scale details are still uncertain, we can expect
one, or a few lows to move north and along the SE panhandle
Sunday and into Monday. Flow looks to remain supportive for no
precipitation for the Anchorage Bowl through this time. Chances
for snow may increase again next week as a front moves through the
area Monday/Tuesday. Due to a large spread in the models, WPC
guidance focused entirely on ensemble means, with no influence
from any operational members as we move into the middle of next
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 125.
 Winter Weather Advisory 121 145 161.
MARINE...Storm Warning 119 125 130 132 179.
Gale Warning 120 131 136 137 138 140 150 155 160 165 170 171 172
173 174 175 176 177 178 180 181 185.
Heavy Freezing Spray Warning 160 180 181 185.

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SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...MTO/KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...SS