Winter Storm Watch 3, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 PM AKST Thu Nov 16 2017

.ANALYSIS AND UPPER LEVELS...
There is an eastward moving upper level trough over Southcentral
Alaska and another over southwest Alaska. Snow with the
Southcentral trough is generally located east of a line from
Seward to Anchorage to Talkeetna. Some snow/rain showers are
associated with the southwest trough. There is a large ridge over
the Bering Sea, centered just east of 180 degrees longitude.
Strong northwest flow at all levels is developing from the western
Gulf through the eastern/central Bering/Aleutians. There is an
upper level low over northeast Russia, with a trough extending
southward into the northwest Pacific. An associated front is
moving into the western Aleutians and southwest Bering.

&&

.MODEL DISCUSSION...
The numerical models are in reasonable agreement through the short
term portion of the forecast (Saturday afternoon). Some timing
differences begin to develop on Saturday with a couple of fronts.
One of these is a warm front along the Alaskan west coast, and
another is a cold/occluded front over the central Bering and
western/central Aleutians. Forecast confidence is thus near
normal.

&&

.AVIATION...
PANC...MVFR and IFR conditions are possible through this evening
in the wake of the snow from earlier today, then VFR conditions
and light winds are expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Widespread snowfall will continue dissipating from west-east this
evening, providing the highest amounts from Valdez/Cordova
northward into the Copper River basin where a few inches of new
accumulation is possible. There is an outside snow shower
redevelopment throughout the Cook Inlet, and in particular the
southern portions as a strong shortwave quickly descends towards
Kodiak Island. This should be a brief round of snow showers that
isn't expected to last more than a few hours, but it may provide
for a quick shot of light accumulations. A second concern tonight
over Northern Cook Inlet/Susitna Valley will be determining how
long the low stratus that quickly developed behind the snow today
will linger. There is a chance that the incoming shortwave could
help mix this out, however confidence is higher that this will be
a persistent layer that would hold off until Friday to burn off.

The bigger concern from the incoming shortwave will be with
strong northwesterly winds from the Barren Islands southward into
Kodiak Island, the highest of which should occur late this
evening/early Friday as the shortwave moves over Kodiak. This wave
has the potential to produce a gust or two approaching warning
level, but there is reasonable confidence that winds should
remain safely below warning level throughout the event. The
strong winds will then spread to the north Gulf Coast on Friday,
especially to the Seward, Whittier, and Valdez areas where another
round of strong and gusty (but sub-warning level) outflow winds
are expected to develop and continue into early Saturday. These
winds will be the primary forecast concern throughout the area for
Friday, as what has become a common weather pattern of sunny
skies and offshore flow continues.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Sunday evening)

A fairly decent coverage of rain and snow showers is currently
ongoing across much of the region, and this should continue
through the evening as another disturbance aloft crosses the area.
Brisk northwest winds will continue along the coast through
Friday morning. Further inland, snow will persist longer along
the western upslope regions of the Kuskokwim Mountains and the
Alaska Range, with breezy conditions continuing here as well,
especially for any passes.

Our attention then turns to a much more potent system which will
rapidly deepen as it moves from the northern Bering Sea into
southwest Alaska. The models show a 980mb low in the Hooper Bay
vicinity by late Saturday evening, with a warm front extending to
the southeast as it straddles the coast. The low will dive
southeast towards King Salmon through Sunday morning, with the
warm front pushing inland somewhat. Meanwhile, very strong dry
advection will develop following an attendant cold frontal
passage, with strong winds developing. Similar to yesterday's
storm, another strong tropospheric lowering will help compress the
air below it, helping increase winds via mixing and downward
momentum transfer processes.

As it stands right now, most of the interior (roughly east of a
Bethel to Dillingham line) will see predominately snow, with
several inches possible east of there, and a wintry mix closer to
the coast. Very strong wind gusts will develop Saturday night into
Sunday near the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Sunday evening)

Strong wind gusts will persist through tonight along the AKPEN
and to its south, especially for the mariners out of bays and
passes. Gusts over land look to continue up towards 70 mph
overnight, with locally higher gusts possible over the waters to
its south. A brief reduction in winds will commence for Friday,
before the next strong storm moves into the region for later
Saturday into Sunday. As a strong storm drops southeast across
southwest Alaska, an attendant cold front will sweep across the
area, with widespread precipitation developing.

Winds will rapidly increase following frontal passage for reasons
described above. Overall, look for sustain winds to potentially
reach hurricane force over both land and water if the models
don't change their forecast solutions.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Expect near to below normal temperatures across southern Alaska,
with periodic snow across the Southwest. Fronts will dissipate by
the time they reach Southcentral Alaska. Expect continued gap
winds along the north Gulf coast.

The long range forecast begins Saturday evening with a transition
in the upper level pattern taking place. A cold outflow pattern
with gap flow along the north Gulf coast will briefly give way to
a front and low pressure system sliding into the western Gulf
Sunday morning. The front will bring rain to Kodiak, and snow to
the Kenai Peninsula, potentially as far north as a line from
Skwentna through Anchorage/Palmer. The most likely place to see
accumulating snow would be from Kenai through Homer. Going into
the extended forecast, we return to a more familiar outflow
pattern. However there are differences between what we've been
seeing to the upcoming pattern.

The upper jet looks to be more zonal in the upcoming pattern while
the low-levels remain conducive for weaker outflow and gap winds.
The jet stream looks to be very active and progressive with
multiple embedded with a storm track from the northwest Pacific
through the southern mainland/Gulf of Alaska. Expect a few west
to east moving fronts to approach the mainland and weaken through
next week. Each of these systems will bring cold air from the
Arctic on the backside, which will prevent the progressive pattern
from moving into the Interior.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 127 136 137 138 155 160 165 180 181 185.
		 Storm 130 131 132 150.
		 Heavy Freezing Spray 130.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...MTL