Small Craft Advisory 117, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 AM AKST Tue Feb 7 2017

.ANALYSIS AND UPPER LEVELS...
The weather pattern remains stagnant with the polar and
subtropical jet streams oriented in a split flow pattern with the
ridge over the Chukchi Sea. The closed low, that cut through the
mid levels on Monday, is currently spinning over the eastern
Bering. To the west, a closed North Pacific low is gaining
northward momentum and pushing the next front into Shemya. At the
surface, relatively benign conditions continue across much of the
southern mainland with the exception of dense fog across the northern
Cook Inlet expanding inland during the early morning hours.

&&

.MODEL DISCUSSION...

Models are in excellent agreement through mid week as the slowly
evolving pattern shifts into motion. Only minor changes were
made overnight using the NAM for Southcentral and the Gulf with a
combination of both the GFS and NAM for the Southwest and Bering.
The biggest challenge in the short range forecast through
Wednesday is defining snow development along the Bering low that
sets into southerly motion later today. This system lacks in a
dynamic set up for producing snow without a front or low at the
surface. On Wednesday, all models are showing better organization
of a surface feature as the system begins to track east with a
deepening surface trough just north of the Alaska Peninsula.

&&

.AVIATION...
Light surface flow continues through the next TAF package keeping
reduced visibilities due to dense fog from the Inlet. A period of
improvement is expected during the afternoon due to diurnal
mixing, however diminishing conditions are expected to quickly
ensue after sunset.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The extent of dense fog across northern Cook Inlet and the
Anchorage Bowl will diminish through Wednesday as the upper ridge
retrogrades westward and temperature inversions weaken. That said,
there will likely still be patchy fog lingering across northern
Cook inlet with some locally dense conditions near west Anchorage
and along Knik arm into Wednesday morning. An upper trough dropping
south and surface low moving into the northern Gulf on
Wednesday/Wednesday night will bring increased chances of light
snow to Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A closed upper level low moving into the Eastern Bering will
increase cloud cover and a chance of flurries/light snow to much
of the region today. With very limited deep layer moisture
available, significant accumulations are not expected as flurries
spread throughout the area by Wednesday. Beginning late Wednesday,
the upper level low will strengthen as it dives to the southeast
towards the AK pen, tapping into increased moisture as it moves
over the Bering Sea. This will bring much more widespread and
heavier snow to the Bering coast of the AK pen from King Salmon
southward, though accumulations should remain under a few inches.
The system will then move into the Gulf by Thursday morning,
bringing clearing skies and colder temperatures back to the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
After the remnants of a gale force front weaken over the Western
Bering today, a slightly stronger storm force front will quickly
replace it. This system will bring strong winds and rain to the
Western Aleutians beginning Wednesday morning. The front will
then spread into the Central Aleutians as it weakens to gale force
on Thursday. To the east, shortwave ridging stretching from the
Pribilof Islands into the Eastern Aleutians will prevent most of
the impacts from this front from moving into the area, with
continued benign weather outside of a chance of rain showers.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Snow shower activity will be on the rise on Wednesday ahead of a
cut off upper level low meandering through Southwest Alaska, and
into Southcentral Alaska by Thursday morning. On Wednesday
afternoon, this cut off low will merge with a trough dropping out
of the arctic (actually has some polar origin) and help bring down
some colder air to much of the state. The trough will bring quite
a punch of upper level lift while cold air advection behind the
trough looks quite promising to bring snowfall to portions of
Southcentral Alaska on Thursday. Following this first push of cold
air on Wednesday and Thursday, a reinforcing shot of cold air will
drop out of the arctic once again on Friday to help bring back
some well below normal temperatures to much of the state.

At the same time, a pretty significant pattern change will start
to take place over the Bering Sea and northern Pacific Ocean. A
series of shortwaves will propagate eastward along the upper
level trough situated between Kamchatka and Attu on Wednesday
which will in turn amplify the upper level pattern. This will help
set up a moisture tap well into the subtropics that will start
pushing toward the gulf coast of Alaska late Saturday into Sunday.
The forecast gets very tricky this weekend as the upper level
become quite a mess with multiple shortwaves embedded in the
amplified flow which impacts the location and propagation of
surface features. One can say with high confidence that several
areas of low pressures will move into the Gulf of Alaska this
weekend through early next week, but it's nearly impossible to
even imagine where these features may track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE..STORMS 178.
        GALES 176 177.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...PEPE