Small Craft Advisory 10, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 PM AKST Wed Jan 3 2018

.ANALYSIS AND UPPER LEVELS...
A weak surface low over Prince William Sound this morning is
headed inland while a negatively tilted upper level short-wave
trough over the northwest Gulf lifts very slowly northward across
the Kenai Peninsula. In the low levels, colder air is slowly
advecting in behind the low changing precipitation from rain to
snow across much of the region. The light winds and weak cold
advection up Cook Inlet is ideal for snow in Anchorage and the
Matanuska Valley, and sure enough Kenai radar imagery shows steady
snow focused over this area. Weak cyclonic flow and low level
instability are producing scattered snow showers over Southwest
Alaska while weak onshore flow is also leading to areas of low
clouds and patchy fog.

Focus for the next couple of days will shift out west, where a
very large mature low is spinning south of the western Aleutians.
The upper jet is well to the south, leaving the main low center
in weak flow, tracking slowly northward toward the Aleutians. A
triple point low has formed farther east and is also lifting
northward. All this is leading to strengthening winds and precip
along the Aleutians. Precip started as snow this morning, but as
winds strengthen temps will quickly rise and lead to a change to
rain.

&&

.MODEL DISCUSSION...
Models finally have a good handle on upper level short-wave trough
lifting across Southcentral this through tonight, so can now
fine-tune snow amounts with some confidence. Meanwhile, guidance
has converged on a common solution for handling of multiple lows
out west, with juts minor differences in track and progression of
frontal boundaries toward the mainland. Thus, overall forecast
confidence is above average.

&&

.AVIATION...
PANC...Expect a period of IFR conditions in the heaviest snow this
afternoon, but quickly reverting back to MVFR as snow lightens up
during the evening. There is a chance that enough warm air could
mix precipitation for a short time before the snow ends as well.
Conditions may improve further as snow ends, but think this would
be short- lived. The fact that there never is an upper level
trough passage and low level flow goes slack overnight favors a
stratus deck setting up in the entire Cook Inlet region. If there
is a stratus deck, that will at least limit fog development.
Forecast confidence is low, but have indicated a downward trend in
ceiling overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
The slow moving upper level wave that is pushing through
Southcentral Alaska will continue to bring snow through the
evening before tapering off early Thursday morning. High pressure
will then build in bringing colder temps back to the area. On
Friday a front will approach from the south and bring warm air and
rain to Kodiak before tracking further north.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed and Thu)...
A quick break in the weather is still in the forecast Thursday for
Southwest Alaska, with a return to more seasonable temperatures. A
bit of fog was around this afternoon, but as we move into the
evening and the flow becomes more offshore, this should be less of
an issue. Thursday evening winds will begin to pick up along the
coast as a front approaches from the west. The main front will
remain offshore, so no significant weather is expected until the
second front approaches on Friday. This second front brings more
precipitation to Southwest Alaska, but it looks like temperatures
will warm up rapidly with the front, so snowfall is expected to be
minimal for the outer coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Wed and
Thu)...

Weather in the Bering Sea is expected to get increasingly stormy
through the forecast period. Although temperatures in the northern
Bering are still cold enough for snow, an approaching front in
the Central Aleutians is already pumping warm air up from the
south. As this front moves into the Bering, winds across the
region will increase to gale force and seas will increase to 15
feet in some regions. This front moves into the northern Bering on
Thursday and is just a precursor to the front associated with the
triple point low moving in on Thursday afternoon. This next
system will increase the winds from gales to storms in the eastern
Bering. More precipitation is also expected with this front, but
with increasingly warm air, it is expected to fall as rain. The
triple point low will move westward Friday and tap into the cold
air coming off Russia, which will allow the low to intensify. This
won't have too much effect on the front that is in the eastern
Bering by that time, but it will allow cold air to bring gale
force winds and snow showers to the Western Aleutians Friday
afternoon.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The most impactful weather this weekend will be centered around
the Gulf as a weak low near Kodiak Island moves northward and
dissipates near the Eastern Kenai Peninsula. This is a feature
that the models have consistently been trending weaker with since
Monday, and is now expected to remain below gale force and bring
precipitation generally to the Gulf Coast. Depending on the exact
low track, there is an outside chance snow could move into
Southcentral on Sunday as downsloping no longer is expected to be
significant along the Chugach Mountains. For now this will remain
as just a chance of snow, however if the models begin trending
further west (such as the latest GFS) these chances may need to be
increased.

Much more quiet weather is then expected to move over the Southern
Mainland for early next week as confidence remains very good in
widespread ridging, producing clearing skies and cooler
temperatures. This ridging will move over the area as storminess
returns to the Aleutians/Bering, beginning with the gale force low
projected to move into the area late Sun-Mon. This pattern is
expected to continue into the middle of next week as a broad
trough sets up over the Bering with ridging over the South
Mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale: 150 160 171 174 175 176 177 178 179 180 181 185
.............. 411 412 413
........ Storm:155 165 170 172 173 414

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF
LONG TERM...DEK

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1101 PM AKST WED JAN 03 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1000 PM     SNOW             1 W GLEN ALPS           61.10N 149.72W
01/03/2018  M6.0 INCH                           AK   TRAINED SPOTTER

1050 PM     SNOW             PALMER                  61.60N 149.11W
01/03/2018  M4.4 INCH                           AK   PUBLIC

1050 PM     SNOW             RABBIT CREEK            61.08N 149.82W
01/03/2018  M4.6 INCH                           AK   PUBLIC

0550 PM     SNOW             1 NE RABBIT CREEK       61.09N 149.80W
01/03/2018  M4.5 INCH                           AK   NWS EMPLOYEE

&&

AHILL