Small Craft Advisory 127, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 AM AKST Fri Feb 10 2017

.ANALYSIS AND UPPER LEVELS...
Water vapor satellite imagery shows a broad trough over the state
this morning with a closed upper low digging south over the
Kuskokwim Valley. Another lobe of energy is rounding the base of
this trough and approaching the Southeast. Two other closed lows
are located to the west, with one south of Atka and the other
southwest of Shemya. At the surface, snow showers are developing
around a low in the northern gulf with light snow activity across
a weak convergent zone which stretches along a broad line from
Palmer to the southern Cook Inlet. Offshore winds increased
overnight as the gulf low tightened it's core, with the gustiest
conditions along the western coastal areas of the gulf.

Cold air is advecting across the Southwest on the backside of the
low. Light snow showers are also noted along the east Bering and
surrounding coast as a several weak shortwaves are moving south
along the western side of the trough. A mix of rain and snow
showers are occurring along a weak frontal boundary across the
Aleutian chain.

&&

.MODEL DISCUSSION...
Models are relatively good agreement through the weekend, however
they are all struggling with snow amounts and timing. Models have
continued with the trend that moisture would develop and spread
west through today and then taper off by Saturday as the strong
polar wave shifts into the gulf. The associated surface low in
the gulf is expected to diminish as it moves south on Saturday.
Two fast moving waves approach the gulf late Saturday and will
need to be monitored as they have the potential to bring
additional precipitation; one dives from the Northwest and the
other races in from the North Pacific.

&&

.AVIATION...
PANC...MVFR conditions are expected to improve to VFR late this
afternoon as a dry and colder air mass advects from the interior.

&&

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

The main forecast challenge and also the greatest degree of
uncertainty will be with the areal coverage and snowfall amounts
inland through tonight.

General synoptic setup: A strong surface low complex over the
northern Gulf retrogrades westward this afternoon as a strong
upper low drops southeast across the southern Kenai Pen. The
surface low then moves south overnight as the upper center moves
north and weakens across the Copper River Basin.

Snow should become more widespread across much of Southcentral
through the day with the heavier snowfall along the coasts and
over western Prince William Sound/Eastern Kenai. Snowfall west of
the Chugach, particularly across the Anchorage Bowl/MAT Valley,
should increase in intensity through the day. QPF (liquid Water
Content) will be fairly low, but cooler air moving in should boost
snow amounts as snow ratios increase. Current thinking is that
snowfall will be measurable, but should remain below advisory
levels. With the exception of the eastern Copper River Basin, snow
should taper of over most areas overnight tonight/Saturday
morning.

Cold advection with strong pressure gradients will continue
westerly gales to local storms (Barren Islands) with strong gap
flows over the eastern Kenai and Thompson Pass. Winds should
diminish across the western Gulf tonight, but will linger into Sat
elsewhere.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frigid arctic air mass and offshore flow will remain entrenched
over the southwest mainland through Sunday morning as the main
upper low shifts east of the Alaska Range while the longwave
trough axis remains overhead. This will keep temperatures well
below zero across most of the area, with the exception of
locations along the coast where temperatures may climb to around
zero during the day. The combination of these cold temperatures
and persistent northerly winds will continue to produce advisory
level wind chills across the Kuskokwim Delta through Saturday
morning before winds begin to diminish in response to a weakening
pressure gradient.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A compact and relatively deep North Pacific low passing south of
Atka will bring gale force winds to the Central and Eastern
Aleutians through tonight as an associated front brushes the
Pacific side of the chain. Otherwise, cold northeast flow will
encompass much of the Bering through the weekend. This will yield
widespread Small Craft Advisory level winds and freezing spray
along with scattered convective snow showers across much of the
region.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Significant pattern changes begin this weekend as a more active
southerly storm track pushes into the western Gulf. The first in a
series of deep lows bringing abundant warm and moist air, will
enter the western Gulf south of the Alaska Peninsula Sunday. While
the surface low tracks to just south of Kodiak Island Sunday
night, the accompanying front will lift north to stretch across
the entire Gulf Coast bringing windy conditions and snow quickly
changing over to rain along the immediate coast. Further inland
over Southcentral Alaska, the snow will last longer with the location
of the rain/snow line a rather tricky forecast. The potential for
some areas to receive significant snow fall followed by warming
temperatures and abundant rain has the potential to make for a
rather messy weather pattern.

This general flow pattern will continue through midweek with at
least two additional lows tracking through the western or central
Gulf. Each of these systems will bring the stream of warm, moist
air further inland across Southcentral Alaska. To the west, a
colder air mass will persist over the Bering with Southwest
Alaska on along the boundary between the two air masses. As
frontal systems swing from Southcentral Alaska to the west over
the Alaska and Aleutian Ranges, periodic snow will be likely in
Southwest Alaska with heavy snow possible at times where favorable
deformation bands set up and persist.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blowing snow advisory 125.
MARINE...Storm 130.
         Heavy Freezing Spray 130 137 138 139 141 150 160 165 170
         179 180 185.
         Gales 131 132 138 150 170 172 173 174 175 176 177 .

&&

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