Small Craft Advisory 199, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Mon Mar 11 2019

.ANALYSIS AND UPPER LEVELS...

A broad long wave trough extends from a vertically stacked low
that is anchored over the Kuskokwim Delta. Cyclonic flow around
the low is advecting cold air over all of the Bering Sea and the
southwestern Gulf of Alaska. A second, negatively tilted,
vertically stacked low spins over the western Aleutians. A third
low sits over the Kenai Peninsula. Southerly flow associated with
this low is bringing warm air into the Gulf and across Southcentral
from the Pacific. This feature is also promoting southeasterly
surface winds which is creating gusty conditions through Prince
William Sound and along Turnagain Arm. Temperatures remain above
normal for many locations throughout Southcentral, with overnight
temperatures remaining in the lower 30s to near 40 degrees, as
warm air continues to migrate northward. Shower activity over the
western and northern Gulf overnight is also tracking north, moving
over Prince William Sound and along Cook Inlet and portions of
the Anchorage Bowl. For the southwest, the King Salmon and Bethel
radars are indicating widespread rain and snow shower activity
stretching across the Kuskokwim Delta south across Bristol Bay and
over the AKPEN. Strong northwesterly flow wrapping around the
back side of the low positioned over the Y-K Delta is also
continuing to produce showers and winds gusts of 30 to 40 kts
across portions of the eastern Aleutians, Pribilofs, and AKPEN.

&&

.MODEL DISCUSSION...

Numerical models remain in good, consistent synoptic agreement
through Wednesday morning before diverging in their respective
solutions. Guidance continues to have a good handle on the
eastward progression and strength of the low over the western
Aleutians, as well as the developing low in the northern Gulf.
Through the first 24 hours, the biggest forecast challenge (and
differences between models) is the development and timing of
precipitation over Southcentral, especially over Anchorage. The
NAM is the wetter of the solutions, and is slightly faster with
both the onset and ending of the precipitation. Farther west, the
first hint of model disagreement begins Tuesday evening as the
low over the western Aleutians moves over the eastern Aleutians.
Here, the Canadian is the main outlier while the EC, NAM and GFS
remain in good agreement with track and timing. The next system
is a Pacific low that is advancing towards the southern Gulf on
Wednesday morning. Here, models differ greatly with location and
strength of the system.

&&

.AVIATION...

PANC...Ceilings and visibilities will drop to MVFR this morning
as a low tracks to the northern Gulf and spreads a mix of rain and
and snow showers over the airport complex. Conditions are
expected to rebound back to VFR by mid afternoon as with ceilings
lifting back above 5,000ft AGL and showers diminishing in
coverage. VFR conditions are then expected to remain through the
evening and overnight hours. Gusty south to southeasterly winds
will continue throughout the day, with a slight decrease in the
gustiness coinciding with the shower activity.

&&

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

Southerly flow aloft continues across all of southcentral this
morning bringing areas of rain and snow showers. The Kenai radar
has shown widespread showers on it most of the night, but very few
reports of precipitation have been noted except for areas along
the immediate coast. This is likely due to the rain and snow
evaporating or sublimating in the dry layer below 5000 ft. This is
the layer below mountaintop level for the majority of the Kenai
and Chugach mountains which is remaining well mixed with the
southerly surface winds. Colder air aloft is also filtering in
from the west and southwest today which may transition some of
these rain showers back to snow at the surface. However,
temperatures will once again be in the upper 30s to lower 40s for
much of the region which will probably keep the majority of the
precipitation a mix of rain and snow despite the cooling
temperatures aloft. Tuesday should be another mild day across the
region with fewer showers around as we will be between weather
systems. The next low will move into the southern Gulf of Alaska
Wednesday and to the northwestern part of the Gulf by Wednesday
evening. This will bring a strong front to the north Gulf coast
and Prince William Sound which will bring gales and some storm-
force winds to parts of these regions. Precipitation type will be
tricky again with this next system. It looks to be mostly rain
along the coast but the rain/snow level in the mountains is still
a bit uncertain at this time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
There is a surface low near Norton Sound and a broad area of
troughing at 500 mb spanning from Norton Sound to west of St.
Matthew Island. There are numerous weak embedded shortwaves aloft
that will impact the northern Bering through Southwest Alaska.
In general, expect more unsettled weather today with intermittent
showers. A ridge axis will nose its way over the AKPEN and into
Southwest Alaska Tuesday resulting in a brief period of drying.
Then another low will move towards the region with the surface low
crossing the chain just outside Bristol Bay by 12z Tuesday. The
surface low actually tracks almost due north on Tuesday which
would make this more of a coastal precipitation event for
Southwest Alaska. However, a secondary slug of energy will enable
a secondary cyclogenic event which will bring a deep surface low
near Kodiak Island with moisture wrapping around the back side of
the low into the Alaska Range.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
At the onset of the forecast period there are three lows of
concern. The first is anchored over Norton Sound producing broad
cyclonic flow over the Northern and Eastern Bering. The second low
is positioned slightly west of Adak. The third low is farther
upstream, near mainland Japan. The first of these lows will bring
light to heavy freezing spray from the Pribilof Island to north of
St Matthew Island along with gale force winds. The low near Adak
will intensify as it moves eastward. The third low will deepen as
it moves toward the chain later this week.

&&

.MARINE (Days 3 through 5)...
Confidence remains high that winds will remain out of the
northwest across the Bering Sea and below Gale force through the
weekend as high pressure builds in. The main focus is across the
east and with a low moving into the Gulf in the Tues/Weds
timeframe. Current forecasts are for low end storm force easterly
winds as a front drives into the coast. Behind the front, high end
gale to low end storm winds out of the south look likely across
the eastern offshore zones and into the Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast remains relatively good with respect to the
large scale flow across Alaska and the north Pacific. Persistent
upper level troughing near the Aleutians will remain, with a ridge
axis building across western Canada and portions of southeast AK.
Relatively benign weather will persist across southwestern Alaska
and the Aleutian chain, with general showery conditions through
the weekend. The bulk of weather will be across the oceans south
and near the Gulf of Alaska. Southerly winds will usher in warm
and moist air to the region, with much of the precip falling along
the coastal mountains. Much of the Anchorage/MatSu area should
remain rain-shadowed and dry. Temperatures for the most part will
continue to raise through the day, keeping the outlook above
normal.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 132 150 155 165 175 176 177 180.
		Heavy Freezing Spray 185.
FIRE WEATHER...NONE.

&&

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