Small Craft Advisory 123, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 PM AKST Wed Feb 8 2017

.ANALYSIS AND UPPER LEVELS...
The upper level and surface low near Nunivak Island is moving east
southeast pushing moisture over southwest Alaska this morning. A
weak upper level and surface low is over the North Gulf Coast.

A weak ridge is between these two lows with a strong low level
inversion resulting in stratus and fog across the Cook Inlet
region and Prince William Sound.

A gale force front is now sweeping across the western Aleutians
bringing warm temperatures and rain to the region. The front is
elongating to the east southeast as a triple point low is apparent
and forming along the front well south of the Aleutians.

&&

.MODEL DISCUSSION...
Models remain in good agreement through the short range forecast
into Saturday morning. The main issue is with respect to the
details of the spread and coverage of precipitation across south
Central Alaska over the next two days. The differences in the
locations of the low over the north Gulf Coast tonight and again
Friday while minor, will have a big impact on where or if the snow
will fall over the Cook Inlet region to Anchorage and Palmer. If
the low does track further east as suggested by the SREF and GFS
ensemble, little if any snow will fall in Anchorage and Palmer. At
this point we are leaning to the operational NAM, GFS and EC.

&&

.AVIATION...
PANC...
Primary concern is the uncertainty with snow development tonight
and Thursday. Only flurries are expected, and predominantly VFR
conditions, however updates may be needed if snow is heavier and
thus restricts VSBY and lowers CIGS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and Thu)...
A few quick shots of snow are likely across South Central over the
next few days. The ridge that has brought us dry weather and the
patchy fog for the past week or so is finally moving away from the
area. It will be replaced by an a fairly strong upper level low
dropping in from southwest Alaska. As this low drops in, it will
develop a surface low near Prince William Sound. This should allow
some wrap around moisture to spill over the Cook Inlet and up
through the Matanuska Valley. This moisture when combined with
some weak-moderate lift will allow very light snow to fall
overnight into Thu morning. Accumulations will be severely limited
with this system due to the dry air that is in place and the lack
of any strong forcing. The best chance for a couple of inches will
be from the Cordova area through Eastern Prince William Sound.

As the pattern continues to shift, another round of snow will
likely set-up on Thu night into Fri morning. The primary, stronger
polar trough will dive down into the area. This will cause two
things to happen. First, more cold air will bleed into South
Central. Second, it will reinvigorate the surface low in the
north Gulf. If the low stays far enough north and west, it could
bring a better chance of snow from Portage Valley through
Anchorage and into the Matanuska Valley. However, if the low stays
east or the trough drives too far south, dry air will win out and
snow will be confined to mostly the Gulf coast and the Copper
River Basin. We have bumped up PoPs across South Central. But at
this time, we held accumulations in check due to the number of
variables at play. The more certain aspect of this system is the
cold air behind it. This cold air will move in late Fri and should
help to create a fairly potent offshore gradient, especially for
places like Whittier, Seward, and Thompson Pass.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The low continues to track south along the west coast and will
make it over the Alaska Peninsula tonight. Before it passes, weak
onshore flow will keep low stratus and patchy fog over much of
southwest Alaska. However, after it passes, flow will become
northerly, dry, and cold. This will clear out much of the cloud
cover which will further aid in colder temperatures through the
end of the week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main impact for the Bering and Aleutians is the front that is
stalling over the Western and Central Aleutians. It is bringing
gale force winds to the area that will slowly diminish through
Thursday. This system is also bringing up enough warm air for
precipitation to primarily fall as rain. Then on Friday a new low
will approach the Central Aleutians from the south bringing more
gale force winds to the area.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A cold longwave trough over interior Alaska will begin to drop
down over Southern Alaska Friday, bringing colder temperatures to
much of the region through Sunday. As this colder airmass settles
in over the area Friday, a surface low over the Gulf of Alaska
begins to strengthen and move over Prince William Sound. There is
good agreement in the models and higher confidence in precip
developing along the Northern Gulf coast areas and over parts of
the Copper River Basin Friday and into Saturday. However, models
are still struggling with the positioning of the surface low,
which has lead to lower confidence in snow potential/amounts over
the Anchorage, Mat-Su Valleys, and Western Kenai areas. One can
say with higher confidence that several areas of low pressure
will move into the Gulf of Alaska this weekend through the middle
of next week. Southerly flow associated with each of these
systems will help transport warm moist air from the Pacific
northward over the Southern Mainland. Even though confidence
remains low on the exact track of these systems, confidence is now
higher in warming temperatures and an overall wetter pattern for
the Southern Mainland next week.

Out west, a pattern change will also be underway over the Bering
Sea and northern Pacific, as the receding high pressure over the
areas gets pushes westward by the above mentioned cold longwave
trough tracking through the mainland. This will make way for the
upper level trough to set up over the Bering Sea and Northern
Pacific by early next week, with multiple systems at the surface
mainly staying along or south of the Aleutian chain through the
middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale...130 131 132 150 174 175 176 177 178 Heavy
Freezing Spray...121 127 129 130 150 165 179 180 185

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...PEPE