Small Craft Advisory 884, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
531 AM AKST Sat Nov 18 2017

.ANALYSIS AND UPPER LEVELS...

A Gulf low departing into the northeast Pacific this morning is
helping to dig a longwave trough centered over the Alcan border
region while a ridge builds over the central Bering Sea. This has
allowed the bulk of the southern mainland to return to a more
amplified offshore flow pattern, leaving gusty outflow winds along
the Gulf coast as the primary sensible weather impact. Skies have
cleared out over Southcentral AK once again with the exception of
some patchy fog and stratus lingering over parts of Cook Inlet
and the Matanuska Valley. Skies are mostly overcast across
Southwest AK as mid level cloud cover streams in over the top of
the Bering ridge. A warm front associated with a low over far
eastern Russia extends southeast along the Kuskokwim Delta
coastline and is poised to bring another round of mixed
precipitation to the Delta as it moves inland today. A trailing
cold front is making slow eastward progress through the Western
Aleutians.

&&

.MODEL DISCUSSION...
Models are in generally good short term agreement as the pattern
remains very progressive over the coming days. The Bering Sea
ridge will quickly break down this weekend as a strong upper level
wave drops out of the Bering Strait region Saturday night and
moves into the western Gulf on Sunday, spreading significant
winter weather and strong winds over a wide swath of the eastern
Bering and Southwest AK. Some differences remain regarding the
placement of a surface low developing east of Kodiak Sunday
morning, which will have large implications on precipitation type
and intensity for Kodiak Island. Preferred the GFS solution as
somewhat of a middle ground for the morning forecast package.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist
through late Saturday night. Snow will spread back into the
terminal from the west early Sunday morning, dropping conditions
to MVFR/IFR after 12z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Strong gap/outflow conditions and gales over the western Gulf
will weaken through today as surface pressure gradients and upper
wind support wanes. The focus then turns to the frontal trough and
associated triple point center moving into the western Gulf
tonight. Models are initially in agreement with the occluded
portion of the front as it spreads overrunning snow to
Southcentral beginning Sunday morning. The triple point is another
matter. The preferred solution is to take the triple point which
develops near Kodiak Island Sunday morning and carry it just to
the east and then retrograde the system to the west/southwest some
Sunday night. This will be a fairly strong low that will bring
gales and storm force winds to the Gulf waters Sunday and Sunday
night. The result of this solution for the Southcentral Mainland
would be to spread snow to along the north Gulf coast through
Sunday afternoon with the heaviest over the Kenai Peninsula. Snow
amounts may approach advisory levels over the western Kenai by
Sunday afternoon. Snow will tapper off throughout the evening and
overnight hours over the Mainland and be followed outflow winds
along the north Gulf coast.

Kodiak island will likely see warm advection with a mix of rain
and snow with the front and developing triple point Sunday. The
preferred solution of keeping the low just to the east would
likely hold out any significant cold advection and tend to keep
the precipitation mixed or a very wet snow. So with the strong
winds moving in we are not expecting significant reductions in
visibilities in blowing snow with the strong northerly winds
develop during Sunday and Sunday night. A slight change in track
however would result in potential heavier snowfall and significant
blowing snow. Stay tuned for updates.

&&

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

The storm expected to impact Southwest Alaska remains on track.
The storm's center is still over eastern Russia, however the
storm's warm and cold fronts extend well south of the center of
the low across the Bering. The warm front is over the eastern
Bering, and is responsible for the snow and mixed precipitation
already being reported along the coast this morning. The cold
front and arguably the bigger weather maker is the cold front now
racing across the western Bering. As the warm front slowly pushes
into mainland Southwest Alaska, the snow will change over to rain
first at the coast and gradually through much of the Kuskokwim
Delta west of the Kuskokwim Mountains. During that transition from
snow to rain this afternoon, a brief period of freezing rain is
possible, but the warm air is expected to scour out the
subfreezing surface air quickly, with a little help from the weak
November sun.

Further south through Bristol Bay, snow will begin around midday
and push eastward to the Alaska Range by the evening. At the
coast, the warm air will transition the snow to rain during the
afternoon, but with slightly weaker onshore winds, it will have
more difficulty scouring out all of the cold air. Thus a longer
duration freezing rain is expected north of King Salmon through
Koliganek, New Stuyahok, and Igiugig, though ice accumulations
should remain less than a tenth of an inch with a wide variety of
precipitation types expected. Inland Bristol Bay and the entire
Kuskokwim Valley are still expected to pick up close to a foot of
loose, powdery snow. In the windier areas, that will cause blowing
snow concerns both as the snow is falling and well after the storm
ends through Monday morning.

The cold front will race across the Bering today and reach the
coast tonight. It is the leading edge of very cold air, which
coupled with the rapidly intensifying low over the Bering Strait
will cause a large area of storm force winds with gusts over
hurricane force. This is most likely in Southwest Alaska along the
coast of the Kuskokwim Delta and especially Nunivak Island.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A storm over Russia with distinct warm and cold fronts over the
Bering are the primary weather features this morning. The warm
front will move into Southwest Alaska this morning. The cold front
and the arrival of a much colder air mass will be the catalysts
for strong winds across much of the Bering. The first land to be
impacted will be the Pribilofs. The cold front will race through
this afternoon, but the highest winds will really be associated
with the strengthening low over the Bering Strait late Saturday
night that will dramatically increase the winds out of the
northwest late tonight through Sunday morning. A few hours later,
those same winds will reach the Alaska Peninsula and Eastern
Aleutians, where gusts as high as 90 mph through bays, passes, and
channeled terrain will be possible.

The strong winds associated with the low will diminish late Sunday
night through Monday morning as another area of warm high pressure
builds over the central and western Bering. Yet another low
tracking along the Russian Coast will cause yet another cold front
with another blast of cold air on strong winds to traverse the
Bering Monday for the western Bering and Monday night for the
remainder of the Bering, though these strong winds will be more
out of the west.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Expect near to below normal temperatures with periodic snow across
the southwest, and periodic cloudiness for Southcentral Alaska.
Gap winds will pulse up with every exiting disturbance.

Monday evening begins day 3 with an exiting system in the Gulf of
Alaska. Dry conditions will take hold over much of southern Alaska
within an outflow pattern, mainly along the north Gulf coast.
Overall, the upper level pattern remains very progressive with
strong, discrete disturbances propagating from the northwest
Pacific into the Bering Sea. Each of these systems will send a
surface low pressure east of Kamchatka while sending a front from
west to east across the Bering. However, troughing from northwest
Canada southward will turn the storm track from the Bering into
the Gulf of Alaska. As far as sensible weather is concerned, the
periodic fronts will push snow onto the mainland while the front
falls apart as it moves west to east.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning: 155 181 185 195
Winter Weather Advisory: 121 152 161
MARINE...Storms: 131 132 155 165 170 172 179 180 181 185 412 414
Gales: 119 120 138 150 160 171 173 174 178 351 352 411 413

&&

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