Small Craft Advisory 214, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKDT Mon Mar 12 2018

.ANALYSIS AND UPPER LEVELS...
Weak ridging built over Southcentral this morning after a weak
trough moved through the area. However, a band of snow
showers/snow with upper level forcing continues over western Cook
Inlet/Susitna Valley. Weakening low pressure over the Southwest
Mainland is continuing to produce clouds and some light snow
across the area. Gusty winds with some areas of blowing snow were
still observed over the Bristol Bay zone. A surface low under
favorable upper jet support is developing along a trailing front
over the ALeutians and is moving eastward. Strong westerly winds
and rain associated with this feature was observed over the
western and central Aleutians.

&&

.MODEL DISCUSSION...
Models overall are in loose agreement synoptically speaking. The
big difference comes in the details. They struggle some with the
placement of the next low moving into the southern Gulf tonight,
but are in fair agreement on the timing and coverage of
precipitation moving into Southcentral tonight and Tuesday. So
confidence is fairly high with precipitation potential through
Tuesday. Models then struggle with timing of the upper wave and
associated surface low/front moving east across the Bering Sea
and into the Southern Mainland Tuesday and Wednesday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. MVFR to IFR conditions in snow are likely beginning
around Midnight. There is quite a bit of uncertainty in arrival
time of the snow, so this could shift one way or the other. Weak
cross mountain flow will develop Tuesday morning and the best
moisture/lift will shift north and west, so high confidence in
returning to VFR conditions for much of the day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The latest front approaching the North Gulf Coast will bring
a fresh round of snow to the area tonight, with several
additional inches likely in the Valdez area and lower
accumulations for western areas of Prince William Sound. Cordova
should once again stay mostly rain due to warmer temperatures
along the immediate coast. A shortwave trough which had
previously been progged to lift into the Cook Inlet region tonight
now looks to be sheared apart over the western Alaska Range. This
will allow the precipitation shield associated with the front to
extend as far west as Anchorage by late tonight into early Tuesday
morning, aided by favorable upper level jet support. This could
result in another quick 1-2 inches of snow for Anchorage and
surrounding areas before snow dissipates late Tuesday morning.
Meanwhile, snow will continue over eastern Prince William Sound
into Wednesday morning as another weak disturbance lifts
northward through the Gulf, with rain continuing to mix in at sea
level. An upper trough will then move across the region from the
southwest on Wednesday. Models are still struggling with the
timing of this feature, but it should bring a brief break in
precipitation to much of Southcentral by late Wednesday as it
departs to the northeast through the Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A decaying low over the southwest coast continues to product some
light rain and snow showers over the area this evening, and will
briefly taper off tonight through tomorrow morning before the next
system approaches the coast Tuesday afternoon. By Tuesday night,
another round of light snow is expected to fall over much of the
southwest mainland through Wednesday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A low along the Bristol Bay coast will quickly fall apart this
evening, with the next in a series of lows already moving through
the Central Aleutians/Bering. Ahead of this system is an already
decaying warm front producing some rain and snow over the Eastern
Aleutians/Bering and the Alaska Peninsula. This front will
continue to weaken as it approaches the southwest coast late
Tuesday morning. By Wednesday morning, the next front associated
with a low over the north Pacific will track over the central and
eastern Aleutians/Bering, bringing another round of precipitation
and gusty winds to the area. Further west on Wednesday, another
front will push into the western Aleutians, bringing rain and
gusty winds to that portion of the chain as well.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Models are coming into better agreement at the start of the long
range forecast period beginning on Thursday as a system develops
near the central Aleutians and tracks NE into mainland Alaska. A
longwave trough will dig south across the Bering as a strong
southerly jet helps support the formation of a surface low with a
decent tap of sub-tropical moisture. Models are generally showing
this system moving across SW Alaska and into Southcentral Thursday
afternoon through Friday morning. Based on the track of the low,
snow could be heavy along the Bristol Bay coast and up into the
lower Kuskokwim River Valley on Thursday, though a more northward
track would allow warmer air to push up past the coast changing
precip over to mostly rain. Precipitation will spread east into
Southcentral by late Thursday night or Friday morning, depending
on the model solution, bringing snow to inland locations and a
mix, or all rain, to the coast. Due to better run-to-run consistency
over the past couple of days, the forecast for days 3 and 4 were
hedged more towards the slower Euro/Canadian solutions, which
would bring precipitation into southcentral late Thursday/early
Friday morning.

Past Friday, the models again show a wide range of solutions,
though generally an upper level low over the NW Bering Sea will
continue the active pattern into the weekend and at least early
next week. Therefore, the forecast incorporated mostly ensemble
means after day 4.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 181.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...KVP