Winter Weather Advisory 9, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 AM AKST Sat Jan 28 2017

.ANALYSIS AND UPPER LEVELS...
A surface low in the Gulf of Alaska south of Yakutat continues
to track north this morning while an associated occluded front
stretches from the northern Gulf Coast south through the central
Gulf. A cold upper level trough stretches from an upper low over
the northern Bering, across southwest Alaska, over Kodiak Island
and into the southwestern Gulf. A deformation band being stretched
between the occluded front and the upper trough swinging in from
the southwest, extends across the northern half of the Kenai
Peninsula, Anchorage area and into the Susitna Valley. This has
been bringing banded snowfall to the area during the overnight
hours.

Out west a frontal system is pushing onshore across the Kuskokwim
Delta bringing some snow and blowing snow to that area. An upper
level trough in the central Bering is rotating east and swinging
a broad area of enhanced snow showers containing some isolated
thunderstorms from the central Aleutians, across the eastern
Aleutians and over the Alaska Peninsula.

&&

.MODEL DISCUSSION...
Models are in reasonable synoptic agreement through Monday.
Forecast challenges include the handling of finer scale details
including the mesoscale snow band over Southcentral Alaska
through Saturday evening, an upper trough lifting through the
western Gulf Sunday and the timing of an Aleutian frontal system
as it pushes across the Alaska Peninsula and Southwest Alaska and
into the western Gulf early next week.

&&

.AVIATION...
PANC...Snow bands will continue to move over the area bringing
IFR and occasionally lower conditions. The general trend this
morning will be for the more intense snow bands to drift north and
east of PANC for a gradual lightening of the snow fall through
midday but then briefly re-intensify mid-day as cold air
advection ensues. Snow will quickly end this evening with the
possibility of fog and stratus settling in tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The deep low over the eastern Gulf will head inland near Yakutat
this afternoon, with the bulk of impacts remaining east of the
Anchorage forecast area, primarily over Southeast Alaska. Some
light snow will spread across the Eastern Copper River Basin.
Showers which formed along a leading trough ahead of this low
advected across Western Prince William Sound, Anchorage, and
portions of the Mat-Su overnight. As a high amplitude negatively
tilted trough over the Western Gulf lifts northward and arctic
air begins to filter across the Gulf and into Southcentral this
morning, these showers will transition to one large band of steady
snow. Snowfall thus far has been quite wet, but as low level
temperatures cool expect it to become less dense (and fluffier).
Expect the heaviest snow to fall from Anchorage to the western
Mat-Valley to the southeastern Susitna Valley. Snowfall will
generally be light, though as the colder air arrives there could
be a brief period of heavier snow. Snow will taper off from south
to north tonight as the upper trough exits.

Aside from snow, the main impact will be wind focused along the
area of strongest cold advection, from Southern Cook Inlet and
Kachemak Bay across the northern Gulf. Winds will quickly diminish
tonight as a weaker short-wave moves in from the west. This is
characterized by deep instability so expect showers and gusty
winds with it. However, there is quite a bit of uncertainty in
the track of this trough, so have used a broad-brush approach to
the forecast for now. In any case, it will traverse the region
quickly, so no one place will see much snow out of it. Look for
the weather pattern to remain active as we head into early next
week with a front approaching from the Bering Sea.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There is a 983 mb low located northwest of Nunivak Island with an
associated trough swinging through the Kuskokwim Delta region this
morning. This feature brings more snow to the Kuskokwim Delta with
a brief period of blowing snow along the Kuskokwim Delta coastline
this morning. Meanwhile, the trough moves northward, and exits
the region by midday today. While, the Southwest Alaska region
continues with onshore flow through the weekend. By Sunday
morning an upper level shortwave moves into the Bristol Bay, and
the Alaska Peninsula region. This brings another round of snow
showers to the aforementioned area before the shortwave exits the
region by Sunday afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The gale force low positioned over the northern Bering this
morning tracks into the Chukchi Sea by Sunday morning. The next
system is a gale force low dropping down off the Kamchatka
Peninsula Saturday afternoon. The associated weather front moves
through the Western Aleutians during the same time-frame before
tracking into the Central Aleutians/Bering by Sunday morning. This
brings warmer temperatures resulting in a wintry mix in the lower
Bering. The upper levels have a weak shortwave moving eastward
along the chain near Unalaska. Expect a slight chance of
thunderstorms through Sunday coinciding with the shortwave in an
unstable airmass. This shortwave moves east of Kodiak Island into
the Gulf by Sunday afternoon.

Meanwhile, by Sunday midday a secondary low develops just west of
the Pribilofs with gale force southerly winds through the Central
Aleutians/Bering Sea region. By Sunday evening this low tracks
near Saint Matthew Island with strong southerly gale force winds
through the Eastern Aleutians/Bering extending northward into the
Northern Bering Sea. This brings warm air advection which spreads
farther north resulting in a wintry mix for the northern Bering,
and all rain for the Eastern Aleutians/Bering Sea as we head into
the Sunday evening time-frame.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A pair of troughs will affect the entire forecast area Sun night
into Mon. In the eastern domain, an eastward moving trough will
bring snow to eastern Prince William Sound before exiting the
region early Mon. In the west, a front associated with a Bering
trough will make a windy and snowy landfall over Southwest AK
beginning Sun night. Between these features will lie the western
flank of an upper ridge. A disturbance moving along this western
flank will bring another round of snow to Southcentral AK Mon
afternoon.

Beginning Mon night a second trough begins to consolidate over the
central Bering. The associated front will bring more snow as it
makes landfall over Southwest AK Tue. High pressure will then
build over much of the Bering, effectively cutting off most
precipitation through the end of the workweek. A front approaching
the western Bering on Wed will become essentially stationary and
bring strong wind to the western Aleutians Wed into Thu.

Over the eastern domain south to southwest flow consolidates over
the Gulf and a stream of moisture continues to aim at the Kenai
and Prince William Sound through Tue. This will bring snow to
those areas, with lesser amounts over the rest of the region.
Precipitation will taper off on Wed as high pressure builds across
mainland AK. Thu into Fri a strong gradient sets up along the
north Gulf coast, with significant outflow winds developing into
Fri.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 180 185.
		 Gale Warning 119 120 130 131 139 141 155 165 170
					  171 172 173 174 175 176 178 179.

&&

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...DS

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