Small Craft Advisory 100, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKST Mon Jan 30 2017

.ANALYSIS AND UPPER LEVELS...
A large and complex upper level low stretches across the Bering
and Aleutians this morning with a number of embedded troughs and
smaller upper level lows spinning through it. An associated
frontal system is currently pushing onshore over western Alaska
bringing snow and blowing snow. An additional developing wave
further south along the front is pushing east across the Alaska
Peninsula and bringing a mix of rain and snow as well as strong
winds.

Further east, an upper level ridge centered along 150W extends
from the north Pacific across the Gulf and into Southcentral
Alaska. The leading edge of the front to the west is currently
pushing up the upstream side of the ridge, beginning to spread
clouds into southcentral Alaska and light rain and snow over
Kodiak Island and into the western Gulf.

&&

.MODEL DISCUSSION...
Models remain in fair synoptic agreement with the smaller scale
differences in the array of shortwave troughs and smaller upper
lows the main challenge. Agreement is solid on the timing of the
front pushing from Southwest Alaska across into Western Gulf,
providing good confidence in the timing of the onset of the
precipitation. Models, however, diverge rapidly in how they
handle a low swinging into the western Gulf Tuesday morning.
Confidence, therefore is very low regarding how far north into
southcentral Alaska the precipitation will push Tuesday and how
much snow the area might receive.

&&

.AVIATION...
PANC...Southeasterly gap winds strengthening through Turnagain
arm today are expected to bend into the Anchorage area this
afternoon with gusty winds surfacing and increasing at PANC from
22Z through 02Z and then persisting overnight. The downsloping
effects of the strengthening southeasterly winds aloft pushing
over the Chugach mountains are expected to keep PANC mostly dry
and VFR through tonight. Some light rain or snow showers may
develop after around 15Z Tuesday as the cross mountain flow
weakens somewhat and additional moisture spreads into the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
South to east winds will increase across much of the region today
as a strong frontal system approaches from the Bering Sea. With
the parent low headed northward to the far northern Bering, the
front will elongate and slow as it makes its way into the western
Gulf tonight. In fact, the front will completely stall somewhere
over the western Gulf on Tuesday, but continue to weaken. With a
shallow layer of cold air in place and unstable conditions right
above this expect stronger winds to mix down to the surface in
many places, from the Kenai Peninsula to Anchorage and the Mat
Valley. East to southeast gap winds will also be in full force,
with the Turnagain Arm wind expected to bend into south to west
Anchorage later this afternoon through tonight.

With such strong low level southeast flow, the lee of the Kenai
and Chugach mountains will initially see downslope drying.
However, as the surface front weakens tonight through Tuesday,
downsloping winds will weaken and snow will likely move into the
population centers. The wild card in the precip forecast is a
frontal wave which all the models now depict developing into a
closed low somewhere over the western Gulf on Tuesday. Forecast
confidence in the track of this feature is very low, which is
critical to determining where it will snow and how much will
fall. There is potential for significant snowfall along a
deformation band extending northward from the low center. For now,
have used a fairly broad-brush approach to the precip forecast.

Despite the deep southerly flow out ahead of the front, the air
mass will remain cold enough for snow in most locations, with the
exceptions being Kodiak Island and the immediate Gulf coast. As
snow first develops today out ahead of the front, there will be
areas of blowing snow. Have issued a special weather statement to
highlight the likelihood of low visibilities along the Seward
Highway corridor on the Eastern Kenai Peninsula. Fortunately,
snowfall will initially be light and by the time it picks up
temperatures will warm to near or slightly above freezing,
limiting the ability of the snow to blow around.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The occluded low pressure system located just west of Saint
Matthew Island this morning will move into the Chukchi Sea by
early Tuesday morning. The associated warm front moved northward
through the Kuskokwim Delta late Sunday night. This brought in
abundant moisture combined with gusty southeasterly winds
resulting in snow and blowing snow for the Kuskokwim Delta, and
will continue through this early morning. The worst conditions
continue along the Kuskokwim coastline with the Hooper Bay and
Chevak surface observations continuing to report snow with
visibilities as low as one half of a mile at times this morning.
By mid-morning the gradient will weaken, and coincides with the
expiring of the Winter Weather Advisory for Blowing Snow for the
Kuskokwim Delta. Meanwhile, as the upper level trough moves
through the Kuskokwim Delta look for additional 1 to 3 inches of
snowfall before the snowfall diminishes from west to east by this
evening.

The Bristol Bay region seems to be on track for still getting
snowfall amounts ranging from 4 to 8 inches north and west of
Dillingham today. The radar imagery is showing the bulk of the
precipitation echoes along the Western Capes, and along the
Ahklun Mountains this morning. The Winter Weather Advisory for
Snow will expire late this evening with the majority of snowfall
to occur through this afternoon before tapering-off by this
evening. The Bristol Bay interior along the Alaska/Aleutian Range
from King Salmon northward will continue to downslope resulting in
drier conditions. Meanwhile, look for the upper level trough to
swing through before exiting the Lower Kuskokwim Valley. This
region will get its share of snowfall mainly from Sleetmute
westward along the Kuskokwim Mountains with the aforementioned
trough passage.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main feature impacting Bering is the large upper low
containing the remnants of the occluded low in the northern
Bering. The associated cold front is draped along the eastern
Bering, and extending through the Eastern Aleutians this morning.
This is bringing gusty southerly flow accompanied by warm air
advection. Meanwhile, as this system tracks northward, the Bering
will be influenced by westerly to northwesterly flow resulting in
a showery regime. By Tuesday an upper level ridge builds in the
central Bering/Aleutian with possible fog developing under the
high pressure. Further west a storm force low tracks southwest of
Shemya with an associated front approaching the Western Aleutians.
By Tuesday evening gale to storm force southeasterly winds stream
into the Western Bering with a triple point low developing just
south of aforementioned region.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Ridging over the eastern Bering mid week will expand over mainland
Alaska where it looks to reside through much of the weekend. This
setup will keep much of the active weather south of the mainland
over places like the Aleutians and Bering. The systems moving over
the Aleutians and Bering during this time will come from the south
bringing warmer air and rain to the area.

The biggest question mark is with the system moving over the
Western and Central Aleutians Tuesday into Wednesday. Even with
the warm air advection associated with this system there is still
potential for winds to reach near warning level as the front
passes. The most critical time will be late Tuesday night/early
Wednesday morning as a shortwave moves across the islands.
However, at this time there is too much disagreement amongst
models and not enough support for winds to reach warning level.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory...155 161.
MARINE...Storm Warning...150 155 177 178 411.
         Gale warning...130 131 132 138 139 160 165 175 176 180 185
            352 412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PLD
LONG TERM...DEK