Small Craft Advisory 198, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
417 PM AKST Mon Mar 5 2018

.ANALYSIS AND UPPER LEVELS...
There is a weak upper level trough over Southcentral Alaska this
afternoon. This trough is producing a few snow showers, mainly
over higher terrain from the Kuskokwim basin through the Copper
River basin. An associated surface low (also weak) is over the
northern Gulf, centered south of Middleton Island. An upper level
ridge is briefly building into coastal southwest Alaska. A
stronger upper level trough is moving eastward through the central
Bering and Aleutians.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through the
short term portion of the forecast (Wednesday afternoon). There
are some differences in the location of a surface low that
develops in the north Gulf Wednesday morning that moves eastward
through Wednesday afternoon, with the Canadian GEM a bit of an
outlier with a further south solution.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. There is the potential for some fog and/or low stratus
later tonight through Tuesday morning. Snow is expected to develop
some time Tuesday afternoon, bringing conditions down to MVFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Areas of fog may re-develop tonight. The LEO Polar orbiting
satellite has been detecting fog and low stratus over portions of
Cook Inlet and Knik Arm. The models have a shortwave clipping
Southcentral tonight, but in some instances the low level
moisture doesn't always get scoured out by such weak shortwaves.
Then the models bring a weak ridge in the wake of the fast moving
shortwave. Out of an abundance of caution, left patchy fog in the
grids tonight. The very same upper level shortwave does however
have the potential to bring snow showers to the Talkeetna
Mountains and the Copper River Basin but significant accumulations
are not expected. A second shortwave will enter the domain
Tuesday which will bring diffluent flow aloft and southwest winds
up Cook Inlet. Anticipating widespread light snow across the
Anchorage Metro Area and the Mat-Su Valleys beginning Tuesday
morning. The NAM even brings the surface low directly up Cook
Inlet which if this solution pans out, anticipate the development
of snow bands across the Kenai. High confidence that Southcentral
will get a fresh blanket of snow on Tuesday/Wednesday but
generally expecting 1 to 3 inches.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

While shortwave ridging aloft is moving over Southwest, there
remains a lot of onshore flow with snow showers over parts of the
Kuskokwim Delta and the Lower Kusko Valley. Fog is present over
the Bristol Bay coast and will remain in place until the next
front arrives tonight. The occluded front will quickly move east
Tuesday, spreading widespread snow across the region. Amounts will
not be significant, but they will range from 2 to maybe 4 inches.
Broad westerly onshore flow behind the front will keep snow
showers over the mainland through Tuesday night before another
front moves inland Wednesday and Wednesday night. This will spread
widespread inland snow with rain mixing along the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A very active pattern will remain in place for the next three days
as a series of lows tracks across the Bering Sea, bringing
widespread gale force winds and rain/snow showers. The first
occluded front will cross the Bering Sea tonight. Broad westerly
flow and showers will continue behind the front before another
more substantial low moves into the Bering Sea Wednesday and lasts
through Thursday. By Thursday afternoon, broad and strong
cyclonic flow and cold air advection will encompass the entire
Bering Sea.

&&

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

The beginning of the mid term forecast on Wednesday evening marks
the point where it gets challenging. Synoptically speaking, most
models are in agreement with having a parent low over the Northern
Bering and an intense jet rounding the base of a positively
tilted trough across the western Aleutians. Models have been
having a difficult time honing on the details of how quickly a
shortwave rounding the trough base will help to eject out a triple
point low in the western Gulf of Alaska. The GFS remains
steadfast on its path of shooting out a low north of Kodiak and
moving it into Prince William Sound. The NAM and the ECMWF have
both been farther east in previous runs, but continue to sharpen
the trough and shift the low farther west with time. This means
that they are converging on the GFS solution. The details here are
what matter most, as the low placement shifts by 50 or so
nautical miles each run. That means the difference between
stronger Turnagain Arm winds across the area and a low snow
event, or weak winds/cross barrier flow, with greater snowfall
amounts. Even with the uncertainty, we should see some snow during
this time and we did bring the forecast closer to likely PoPs in
that period. However, it is too early to estimate true snow
potential Wednesday afternoon into the evening hours with
reasonable confidence. On the heels of this storm system is the
next that will move in at the end of the week. Again, this is all
from the trough that helped to kick out the triple point low mid
week. The upper level jet will intensify on the western periphery
of the trough as it taps into cold Arctic air. As the trough
tilts negatively, another low is expected to fire out ahead of it.
Again, the details matter the most, but longer range guidance is
showing low pressure on the order of 960 mb within the vicinity
of Prince William Sound, bringing the chances for more snow this
weekend. Despite March and April being the driest months of the
year (climatologically), Mother Nature is giving that statistic a
run for its money.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 173 175 177 178 179 411 412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
AVIATION...BL
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...SS