Small Craft Advisory 203, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKDT Tue Mar 12 2019

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low, positioned over the central Aleutians,
sits near the base of a broad trough that encompasses the western
and central Bering. This low continues to advect colder air over
the western Aleutians and Bering, with light snow and
northwesterly wind gusts of 30 knots reported at Adak and Atka.
Ahead of the low, a weak ridge over the AKPEN is tracking east
over the western Gulf of Alaska. Farther downstream, a negatively
tilted shortwave stretches from Prince William Sound northwest
overtop the ridge to the lower Kuskokwim Valley. At the surface,
a front along the leading edge of the Aleutian low is pushing
eastward over the eastern Bering, AK-Pen and the southwest
mainland coastline. In IR satellite imagery, this front is seen
as the band of cloud cover over the eastern Aleutians to Bristol
Bay. These clouds wrap back along the occlusion to the surface
low now north of Dutch Harbor. Gusty winds and precipitation are
present along the eastern Aleutians as well as coastal locations
along the southwestern mainland. In Southcentral, conditions are
generally calm and dry. The shortwave is bringing a mix of rain
and snow showers to areas of Prince William Sound and along the
northern Gulf Coast, as seen on the Middleton Island radar.
Southerly winds are still prevalent through much of Southcentral,
with gusts still persisting along Cook Inlet up through the
Anchorage Bowl. Clearing behind the trough has also allowed
temperatures across much of Southcentral to drop below or near
freezing.

&&

.MODEL DISCUSSION...

Numerical guidance remains in fairly good agreement with the
overall synoptic picture through the end of the week. This is
especially true with the broad upper-level trough stretching from
the Bering to the western Gulf and the evolution and track of the
next significant surface feature set to impact Southcentral.
Models have been locked onto this feature for a few days now,
with the low tracking north into the Gulf for late Tuesday and
Wednesday then steering northwest and tracking over the Barren
Islands. The recent 00z and 06z NAM runs have produced a slightly
weaker low center along with a more eastward track. The GFS and
EC, however, have been holding steady with the aforementioned
track and a slightly deeper low. Given this, and the similar
solution of the NAM and GFS concerning the position of the trough
along the coast of Southwest Alaska, the GFS was the preferred
model for the morning forecast package. There are still some
forecasting challenges on the smaller scale (e.g. local- effect
winds and precipitation type for some areas), but forecast
confidence on the whole remains above average given the good run
to run consistency. Models do diverge more moving into day three,
but remain within tolerance with respect to the general pattern.

&&

.AVIATION...
PANC...General VFR conditions will persist over the airport
complex as any lingering cloud cover today and increasing clouds
tonight remain above 5,000ft AGL. Gusty southerly winds early
this morning will diminish and becoming northerly by late morning.
LLWS is expected by early tomorrow morning as northerly surface
winds continue and winds aloft turn southeasterly and increase in
advance of the next storm system lifting north from the Gulf.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Tue and
Wed)...

One front will move away and dissipate today while a new storm
system gathers strength in the Southern Gulf of AK tonight. First,
some lingering showers will hold on this morning across parts of
the Copper River Basin and the Northern Susitna Valley. This front
is also making for a very well mixed atmosphere this morning
allowing the breezy conditions to continue around Cook Inlet. A
shortwave ridge building over the Northern Gulf of AK will bring
an end to just about all shower activity and allow the winds to
relax as it moves through today. An upper level low near the
Eastern Aleutians will pivot towards Southcentral tonight. This
feature will help to usher in more energy under southeasterly flow
from the Gulf. It will allow precipitation to return to the Gulf
Coast by this evening and work its way into Prince William Sound
and eventually Cordova overnight. This system will not have the
same warm air surge that the previous system did. That said, right
now we are expecting snow (or at least snow/rain mix) for all
inland locations with only the immediate coast seeing strictly
rain.

The aforementioned stronger system will develop and deepen over
the offshore marine waters tonight into Wed. It should bring a
fairly wide swatch of storm force winds towards the coastal areas,
especially along the barrier jet region (Cordova to Seward).
While there are some discrepancies on the exact track (and
strength) of the low center, it looks to split between Northern
Kodiak Island and the Southern Kenai Peninsula. Regardless, the
effects will be much the same; Moderate precipitation along the
coast, strong winds moving inland, and a hefty amount of
downsloping across the Chugach and Kenai Mountains. This forecast
package leaned towards the GFS as it has been superior in run to
run consistency. This brings the low track a bit further west than
the NAM. The system reaches max intensity and is actually in the
occluding phase before much of the QPF reaches the Southern
Mainland, so we don't expect as warm of an airmass to move through
as we did over the weekend. However, surface temperatures will
still feel quite warm in places where the winds will blow (Mat
Valley, Anchorage, Turnagain Arm, etc). Most of these locations
will be in the "rain shadow" of the mountains regardless. This
system is slow to move as it fills and drifts over the Alaska
Range to end the work week.

&&

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

A decaying low over western Alaska will wobble around the region
and slowly get absorbed into the longwave pattern. Aloft there are
multiple weak shortwaves which have kept unsettled conditions over
the northern Bering and lingering showers over Southwest Alaska.
The models bring a minor ridge axis into the AKPEN and Southwest
Alaska today and overnight which will serve as a respite between
storms. For the next storm, it almost goes without saying that
the windward side of the AKPEN will get another round of
precipitation. By 12z Tuesday, the leading edge of the next front
will move over Goodnews Bay and the Walrus Islands. The remnants
of the occluded front will then push northward. Meanwhile,
another round of cyclogenesis south of the chain will result in a
deep low that will track into the Gulf of Alaska on Wednesday. It
is very possible that wrap around moisture will work its way into
pockets of Southwest Alaska, specifically the western Alaska Range
and portions of the Aleutian Range.

&&

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

A gale force low is progressing along the Aleutians. Expect
another batch of unsettled weather complete with gusty winds,
rough seas and the possibility of snow or a rain/snow mix that
will transition over to rain. In the wake of the cold front, there
is a healthy amount of cold air advection and it looks fairly
convective/showery in the post frontal environment. Looking at
500 mb and 300 mb there is a dynamic forcing mechanism that will
steer arctic air from Russia into the Bering.

&&

.MARINE (Days 3 through 5)...

We are still expecting storm force winds with a low pressure
system in the Gulf of Alaska into Thursday morning. Models differ
slightly in the placement of the low, but due to the setup of the
system we have high confidence storm force winds will be present.
Another low enters the Gulf on Saturday and we have low confidence
in its track at this time due to poor model agreement. However,
its looks like small craft to gale force winds will be present.
Meanwhile in the Bering, it will be quiet with moderate northerly
flow. This does mean the ice pack could grow in zone 185 and along
the coast near Nunivak Island.

&&

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

Models are agreeable to start the longterm, but quickly fall
apart going into the weekend. General low pressure will be present
over the eastern Bering Sea. Two rounds of low pressure will be
impacting the Gulf of Alaska, with one of them already present to
start the long term Thursday morning. The next will move in from
the south on Saturday. However, there is considerable
disagreement in the placement and timing with the second low
pressure system. What this means for the extended forecast is a
broad brush approach using an ensemble of models to get very
general timing and and placement of weather. This also means low
forecast confidence in the long term. So, we will remain in an
active weather pattern for the next week or so, but we will have
to work on the details over the coming days.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 125 130 131 132 138 139 155 174 .
		Storm Warning 119 120 351 352.
		Heavy Freezing Spray Warning 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/TM
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...BB

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
254 PM AKDT Tue Mar 12 2019

AKZ125-140100-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
254 PM AKDT Tue Mar 12 2019

...HEAVY SNOW FOR EASTERN KENAI PENINSULA WEDNESDAY THROUGH FRIDAY...

Heavy snow is expected starting Wednesday and continuing into
Friday for the Eastern Turnagain Arm, Turnagain Pass, and south
to around Moose Pass along the Seward Highway. A low pressure
system moving in from the Gulf of Alaska is expected to drop 2 to
3 feet of snow in mountains, with up to 2 feet of snow along the
Seward Highway corridor from Turnagain Pass south to Moose Pass.
Temperatures will warm enough at sea level for snow to transition
to rain. Avalanche danger will be increasing in the mountains
during this time. Please check the current avalanche forecast at
www.cnfaic.org

Travel will be difficult in mountainous areas due to the heavy
snow and will be difficult at sea level due to slick roads from
rain falling on top of snow. Please use caution if you need to
drive, especially along the Seward Highway. A continued surge of
warmer and wetter systems are expected into the weekend bringing
continued heavy precipitation.