Small Craft Advisory 257, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKDT Mon Mar 27 2017

.ANALYSIS AND UPPER LEVELS...
The large scale pattern continues to shift as a closed upper level
high over the central Bering Sea slowly weakens and retrogrades
west. An upper level trough is now firmly entrenched over
mainland Alaska and the North Pacific, supported by a robust
subtropical jet. A series of lows are strung out south of the
Aleutian Islands and on up into the Gulf of Alaska. A weak
compact low over the northern Gulf of Alaska is producing light
snow today, primarily in the Prince William Sound region, though
some snow may make it over the coastal mountains. The focus of the
forecast is with the complex low pressure system south of the
Alaska Peninsula. A developing triple point low is headed
north into the Gulf of Alaska with an already established low
over the North Pacific beginning to curve northeast toward the
Gulf of Alaska. These two systems will be the major players in
producing widespread snow across south central Alaska the next
couple days.

&&

.MODEL DISCUSSION...
There has been quite a bit of spread in handling of lows crossing
the Gulf of Alaska. The 12Z model runs are grouped closer
together, with a median track of the next two lows toward western
Prince William Sound. Ensembles also support this general idea.
The NAM is the one notable outlier with a farther east track and
will be ignored. Based on increasing confidence in the general low
track, we will aggressively increase the probability of snow for
much of the area, including the population centers of Anchorage
and the Matanuska Valley. Confidence in the location of heavier
precipitation with these systems is low. Thus, we will remain
conservative with snow accumulation forecast for now.

&&

.AVIATION...
PANC...There is increasing confidence in development of MVFR
conditions as snow moves in on Tuesday. There is still quite a bit
of uncertainty in the exact track of lows crossing the Gulf of
Alaska which will greatly affect the intensity of snowfall. If
heavier snow does develop we could easily see IFR/LIFR conditions
Tuesday Night into Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The current low over the northern Gulf of Alaska will dissipate
as it heads inland across the Kenai Peninsula tonight. The bulk of
snow will remain in the Prince William Sound area this afternoon
through this evening, then tapering off after that. Low level dry
air is the limiting factor for snowfall as moisture advects across
the coastal mountain ranges to the rest of south central Alaska.
We have generally stuck with chance or slight chance for these
inland areas. If snow does fall it will be light with little or no
accumulation.

More significant and widespread snowfall is on tap for Tuesday
through Wednesday. The latest model guidance is beginning to lock
into some very favorable features for snowfall including a series
of fairly weak lows moving up from the Gulf of Alaska to Prince
William Sound area, a strongly negatively tilted upper level
trough, and cold air advection from the Bering Sea to the Gulf of
Alaska and south central Alaska. The question of the day is
"where will the surface lows track to?". This is the key to
determining how much snow will fall and how long it will fall.
Based on model trends, the latest forecast will lean pretty
heavily toward a low track to Western Prince William Sound, which
would bring a long duration snow event to Prince William Sound,
the eastern Kenai Peninsula, Anchorage, and portions of the
Matanuska and Susitna Valleys. This would all change with a small
shift in low track.

Although we have trended up with the likelihood of snow across
the region, we have gone with a conservative snowfall
accumulation forecast until there is more confidence in the low
track. For now, the big message is that the weeks of dry weather
for much of the region are coming to an end and people should
prepare for a return to winter weather and winter driving
conditions.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

The forecast period commences with a surface low 200 miles south
of Dutch Harbor resulting in widespread easterly winds from the
Kenai Peninsula to Attu Island. As time elapses expect the low to
drop south taking the enhanced pressure gradient force winds with
it shifting the strongest gusts to the east followed by a brief
stint of northerly winds. By Tuesday night a new low will enter
the Gulf of Alaska resulting in gale force winds and increased
chance of precipitation.

&&

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

The blocking pattern over the Bering Sea which has been in place
most of March will finally break down over this forecast period.
By day two anticipate stormy conditions to move into the western
portion of the Bering Sea and Aleutians complete with a spike in
precipitable water and easterly gusts.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The extended forecast looks to be quite an active one for much of
Alaska. Starting late Wednesday into Thursday, an upper level
trough extending over the Bering Sea and northern Pacific Ocean
will continue to amplify and elongate as an influx of energy from
the coast of Russia moves into the Pacific Ocean. As the trough
amplifies, a warmer and more moist air mass originating in the
subtropics will begin to be advected north toward the gulf coast
of Alaska. While models have struggled recently with the pattern
change, models are in near lock-step through Friday before some
discrepancies begin to develop.

The big story on Thursday and Friday will be a powerful low
pressure system moving north toward the Alaska Peninsula which
will likely slam the gulf coast with a mixture of rain and snow.
Model guidance shows surface to 500 mb thicknesses increasing and
850 mb temperatures approaching 0 degrees C which indicates a
change to rainfall along the coast. However, interior locations
that see precipitation will likely remain snow. The low track will
be important as this far west track will likely keep Anchorage
and normal downsloped locations on the dry side. Heading into the
weekend, models begin to struggle with the orientation of the
upper level trough as the ECMWF tries to flatten the trough out
which brings all the energy toward Juneau while the GFS keeps the
energy in the Gulf of Alaska. The take-away story here is that
this active pattern looks to be here to stay for the next 7 to 10
days but it's difficult to see as of now which areas may see the
most rainfall/snowfall. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS/DEK
LONG TERM...MC