Small Craft Advisory 885, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...
A longwave trough is moving east from the Gulf of Alaska as a
developing shortwave sets its sights on the Eastern Bering Sea and
the Gulf of Alaska in the next 24 hours.

At the surface, a warm front is moving into southwest Alaska with
mixed precipitation falling along the coast and snow falling
inland ahead of the boundary. As a result, a Winter Weather
Advisory for freezing rain, rain, and snow is in effect for the
Kuskokwim Delta until early Sunday morning. Behind the warm front,
rain and fog are found over the Pribilofs and Central Aleutians.
The cold front is already past Shemya, where northwest winds are
gusting. High Wind Warnings and marine Storm Warnings are hoisted
from Southwest to the Eastern Aleutians and for much of the Bering
Sea.

A stationary front lingers over Prince William Sound and the
Anchorage area. The region of low pressure is pulling gusty winds
off the mainland and keeping patchy areas of fog and stratus over
northern parts of Cook Inlet and the Matanuska Valley.

&&

.MODEL DISCUSSION...
A 160-knot jet streak descending along the southwest coast will
encourage rapid deepening of the low moving into the Gulf of
Alaska. A triple-point will form in the vicinity of Kodiak Island.
As the low deepens, a temperature whiplash of warmth over
Southwest will end with significant cold air advection. In the
lower Kuskokwim Valley and Delta, 850mb temperatures will drop to
around -12 degrees C Monday morning.

Wind and snow will combine to lower visibility over the Western
Kenai Peninsula beginning early Sunday morning and lasting through
most of the day. Wind is not expected to create similar issues
further north for Anchorage or Valley locations.

Wind will continue to be a forecast issue in the coming days for
both aviation and marine interests. Gap wind and outflow issues
should be expected in bays and passes. Aviators should be mindful
of wind shear, especially in southwest Alaska.

Forecasters preferred the GFS for timing and track of shortwave
and surface features. A blend with the Nam-Nest was used after
Day 2 for weather effects over Southcentral, Prince William Sound
and the Gulf of Alaska.

&&

.AVIATION...
PANC...Snow will spread back into the aerodrome from the west
early Sunday morning, dropping conditions to MVFR/IFR after 12z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Once the few remaining gap winds along the Gulf coast diminish
early this evening the focus will shift to the incoming front and
developing triple point low in the Gulf overnight. Confidence is
good that the occluded front will move into the Gulf late
tonight, bringing snow from Kodiak Island northward into
Southcentral, with the heaviest snow occurring along the Western
Kenai Peninsula where a winter weather advisory remains in effect
for snow and blowing snow concerns. Widespread accumulating snow
of 2-4 inches is also expected over the Anchorage Bowl/Matanuska
Valley during the day Sunday. With much of the energy remaining
well west and south of the area, confidence is good that these
areas will remain safely under advisory level (6 inches).

The forecast becomes much more dynamic and less certain as the
triple point low develops and rapidly intensifies off of Kodiak
Island and moves into the central Gulf. This development will
largely bring an end to the snowfall over Southcentral and even
most of the Gulf coast, and outflow winds from Cook Inlet/Barren
Islands eastward into the gulf coast redevelop. Throughout Kodiak
Island, this will also bring widespread strong northwest winds
along with heavy snow, creating the potential for blizzard
conditions on the Island. It does appear that temperatures from
this system will remain too warm (just above freezing) for
widespread blizzard conditions to develop, however strong winds
and significant accumulations of wet snow are still expected. As a
result a winter weather advisory was also issued for Kodiak for
reduced visibilities in falling/blowing snow with strong winds.
Snow and winds are then expected diminish on Monday morning as the
low moves out of the Gulf and a ridge of high pressure moves in.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A strong winter storm will continue to impact Southwestern Alaska
and is expected to continue through the day tomorrow. A warm front
will push east into the interior this evening bringing warm air
advection in the mid-levels overrunning cold air at the surface.
This will lead to a mixed bag of precipitation for areas near the
coast as well as far inland as Bethel. Precipitation will
transition briefly to rain this evening as cold air is finally
scoured away at the surface, especially near the coast, before a
strong cold front moves in tonight transitioning the precip to all
snow. Further inland, precipitation is expected to remain all
snow and will be heavy at times, with snow accumulations expected
to be around a foot for locations Iliamna and north, with even
higher amounts in the mountains.

Snow will taper off tomorrow afternoon and evening as cold dry
air is pulled south on the backside of the low as it departs,
bringing strong winds along the coast and near sub zero
temperatures inland by Monday night. Blowing snow will be an issue
tomorrow and Monday in the Upper Kuskokwim River Valley as gusty
NW'ly winds will kick up the new snow causing areas of decreased
visibility.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A storm with a low centered over northeastern Russia and a strong
cold front extending over the Bering will be the primary weather
makers over the next 24 hours. Cold and dry air coming off of
Siberia will be drawn down behind a cold front screaming SW across
the Bering Sea and the Aleutians. A rapidly building ridge behind
the low will increase the pressure gradient and lead to very
strong northwesterly winds behind the front tomorrow. The core of
the strongest winds will first hit the Pribilofs tomorrow morning
through afternoon then down through the Bering straight and the
Eastern Aleutians and AKPEN by late morning through tomorrow
night. A high wind warning has been issued for these locations
with gusts expected to reach 75-90 mph, especially through
channeled terrain.

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 a cold front with
another blast of cold air and strong winds to traverse the Bering
Monday and Monday night, though winds are not expected to be as
strong as the current storm.

&&

.LONG TERM FORECAST (Days 3 through 7)...

After Tuesday's system moves out of Alaska, a shortwave ridge will
once again move in over most of the state. This will bring a
period of clear skies and seasonably cold to below normal
temperatures in some valleys through Wednesday evening. Yet
another system will approach Alaska from the west beginning
Wednesday night, traversing the Bering Sea on a very similar path
to the previous two systems. This will bring a repeat of strong
westerly winds followed by strong northwesterly winds for the
Bering Coast and the AK Pen. Precipitation type looks like it will
be less of a challenge with this system, as it has colder air
aloft than the current systems. Current thinking is that it will
remain all snow for the West Coast on Thursday, but that may
change as the system gets closer.

This system is currently expected to bottom out at 970 mb as it
approaches the West Coast. As it crosses the AK Pen, it develops a
secondary low in the Gulf of Alaska which should help to draw
some colder air down from the Interior. As a result, outflow winds
are expected to pick up on the southern side of the AK Pen and in
the usual channeled areas throughout Southcentral. If the system
remains as strong as forecast, the weather challenge for the end
of next week will be strong northerly winds for both east and
west.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181 185 195.
		 Winter Weather Advisory 121 171 152 155 161.

MARINE...Gale 119 130 137-139 150 160 173 174 178.
		 Storm 120 131-136 155 165-172 179-185.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...LF