Small Craft Advisory 827, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 AM AKDT THU OCT 20 2016

.ANALYSIS AND UPPER LEVELS...

The early morning upper levels show an amplified longwave trough
encompassing the vast majority of the state of Alaska and the
coastal waters. High pressure both at the upper levels and surface
has begun to push off of Siberia and into the northern Bering Sea
with a second area of high pressure pushing northward from the
northwest Pacific Ocean. These two areas of high pressure are
expected to merge this morning as a stacked low near the Kamchatka
Peninsula deepens and ridging downstream of the upper low (in the
northwest Pacific Ocean) pushes into the southern Bering Sea. The
upper level trough which has been anchored by a closed upper level
low finally shows signs of moving eastward as a decent jet has
begun to develop as ridging begins to build over the Bering Sea.
At the surface, there is not all that much to talk about. A weak
area of low pressure near Kodiak Island is pushing south this
morning and is expected to dissipate later this morning. Some rain
showers have been observed across Kodiak Island and eastern Kenai
Peninsula but inland locations remain on the dry side with persisting
offshore flow.

&&

.MODEL DISCUSSION...

Models remain in excellent agreement synoptically but continue to
struggle a bit with the formation of a deformation band over
Anchorage and the Mat-Su valleys later tonight. Numerical guidance
continues to suggest an area of increased moisture coupled with
increased lift moving over the aforementioned locations and
produce a couple inches of snow. While the actual amount of snow
expected is still in question, confidence continues to increase
for accumulating snowfall. See the Southcentral discussion for
more information regarding the snowfall forecast. With models in
good agreement synoptically, a blend of the NAM and high-res WRF
data was utilized for forecast updates.

&&

.AVIATION...
PANC...VFR conditions with light winds will continue until early
this evening when a RA/SN mix will begin develop, causing
ceilings to fall below 5000 ft. There is some uncertainty as to
when this mix will switch over to accumulating snow, but there is
reasonable confidence that this will happen by late evening,
bringing IFR conditions, a few inches of new snowfall, and sub-
freezing temperatures that will persist into early Friday morning.
Conditions will then improve to VFR with continued flurries on
Friday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Accumulating snow returns to the area tonight, followed by a
colder air mass and gusty outflow winds along the coast. The focus
of the morning forecast continues to be the arctic trough digging
southeastward. Lift from the trough will begin tapping moisture on
our side of the Alaska Range this afternoon, starting
precipitation as rain from the western Inlet northward into the
Susitna Valley. During the late afternoon/early evening hours rain
will turn to snow in those areas as cold air filters through the
Alaska Range while the precipitation propagates eastward toward
the Matanuska Valley, Anchorage Bowl, and southward onto the Kenai
Peninsula. Precipitation will likely start as rain in these places
as well before turning to snow in the late evening hours. Snow
will be a brief shot, likely ending in the early morning hours.
Total accumulations still look to be in the trace to three inch
range for the event, with highest accumulations being on westward
facing slopes of the Chugach and Talkeetna mountains.

A colder air-mass settles in behind this system, which will drive
gusty winds in favored gap locations. Strong northerly winds will
ramp up Friday morning in Seward, Whittier, and Valdez, along with
any gap/passes in the Prince William Sound area. These enhanced
winds will persist through Saturday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cool, dry weather will continue through the end of the week as
upper level troughing allows more cold air to push into Southwest
Alaska tonight and Friday. As the upper level trough digs south
along the Alaska and Aleutian Range Thursday night and Friday and
then into the Southwest Gulf Friday night, northerly winds will
increase across Southwest Alaska bringing brisk but sunny conditions.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The surface low in the southern Bering will dissipate today as its
supporting upper level trough tracks away to the southeast and
into the southwestern Gulf. The next frontal system will approach
the western and central Aleutians from the south tonight bringing
easterly Gales. The front will lift north into the Bering Friday
morning. A following trough will swing through the western
Aleutians Friday afternoon with next developing low approaching
Friday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Cold air advection and outflow winds are expected to impact
Southcentral Alaska through much of the weekend until the ridge
axis moves east of the area. After that the main focus in the
extended timeframe will be out west as there is potential for a
strong system to move into the Bering Tuesday. The EC is by far
the strongest with this system, but even it has gotten weaker from
run to run, lowering confidence in a sub 960 MB low. There also
remains significant differences between the different
deterministic and ensemble runs, also lowering confidence.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130 138 150 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...MMC
AVIATION...DEK
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DK

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
344 PM AKDT THU OCT 20 2016

AKZ101-111-121-125-131-141-145-211600-
ANCHORAGE-MATANUSKA VALLEY-WESTERN KENAI PENINSULA-
WESTERN PRINCE WILLIAM SOUND-NORTHEAST PRINCE WILLIAM SOUND-
COPPER RIVER BASIN-SUSITNA VALLEY-
INCLUDING THE CITIES OF...ANCHORAGE...EAGLE RIVER...INDIAN...
EKLUTNA...PALMER...WASILLA...SUTTON...CHICKALOON...KENAI...
SOLDOTNA...HOMER...COOPER LANDING...WHITTIER...SEWARD...
GIRDWOOD...MOOSE PASS...VALDEZ...THOMPSON PASS...GLENNALLEN...
EUREKA...MCCARTHY...PAXSON...SLANA...TALKEETNA...WILLOW...
CANTWELL
344 PM AKDT THU OCT 20 2016

...WINTER WEATHER COMING TO SOUTHCENTRAL THROUGH FRIDAY NIGHT...

RAIN AND SNOW WILL IMPACT SOUTHCENTRAL BEGINNING THIS AFTERNOON
ACROSS THE SUSITNA VALLEY THEN SPREAD INTO THE MATANUSKA VALLEY
AND ANCHORAGE THIS EVENING. A CHANGEOVER TO ALL SNOW WILL OCCUR
LATE THIS EVENING WITH SNOW MOVING EAST INTO THE COPPER RIVER
BASIN AND TOWARDS VALDEZ FRIDAY. ARCTIC AIR MOVING IN BEHIND THE
COLD FRONT MAY ALSO CREATE SLICK AND HAZARDOUS DRIVING CONDITIONS
FOR THE FRIDAY MORNING COMMUTE FROM THE SUSITNA VALLEY SOUTHWARD
INTO THE ANCHORAGE AREA. SNOW ACCUMULATIONS WILL VARY FROM A 1 TO
3 INCHES...WITH THE HIGHEST ACCUMULATIONS ALONG WEST FACING SLOPES
OF THE TALKEETNA AND CHUGACH MOUNTAINS...AS WELL AS THE NORTHERN
SUSITNA VALLEY AND COPPER RIVER BASIN.

CHECK THE NATIONAL WEATHER SERVICE FORECASTS FOR MORE DETAILS ON
THIS APPROACHING EVENT. THE LATEST FORECAST AND WARNINGS ARE
ALWAYS AVAILABLE AT WWW.WEATHER.GOV/AFC