Small Craft Advisory 730, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKDT Sat Sep 26 2020

.ANALYSIS AND UPPER LEVELS...

Much of Alaska remains under a broad longwave trough again this
morning. Weak upper level shortwaves in the flow continue to
rotate around this longwave trough keeping Southcentral and
Southwest Alaska in the continued unsettled pattern with on and
off rain showers. Some patchy areas of fog were also observed
this morning in and around Anchorage. This is expected to be
short-lived, however, as latest satellite shows cloud cover moving
back into the region. The jet stream remains well south of Alaska
aimed into British Columbia which is the major driver in moving
weather systems around. However, the jet stream is beginning to
buckle this morning over the North Pacific. A strong piece of
energy is rotating around the bottom of the trough and is helping
to redirect the jet stream further north aiming towards Southeast
Alaska. This piece of energy rotating around the trough will be
the storm force/hurricane force low in the Gulf of Alaska over the
weekend.

Quiet conditions continue across the Bering Sea and the Aleutians
with just some cumulus clouds due to the cool air creating an
unstable air mass as it moves over the relatively warm waters in
the Bering.

&&

.MODEL DISCUSSION...

Models remain in really great agreement on the synoptic scale
features through the next 3 days. There remain the subtle
differences between models on timing, intensity, and exact track
but overall the models are in fantastic agreement on this system
for today through Sunday. The NAM and Canadian NH are trying to
develop another low closer to Kodiak Saturday while the other
global models (EC and GFS) keep one low center moving through the
central Gulf.

&&

.AVIATION...

PANC...VFR conditions and light winds are mainly expected this
morning, though, some patchy areas of fog were present around the
terminal that lowered visibility and ceiling for a time. With
cloud cover moving in now this has largely dissipated. LLWS (Low-
level wind shear) is possible for a six hour period beginning
tonight through early Sunday morning.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Monday Night)...

A large diffuse low over the Gulf this morning is slowly drifting
westward towards the southern Cook Inlet. This low is causing
shower activity, mostly over the Kenai Peninsula and the western
Susitna Valley. The low will dissipate this afternoon. Elsewhere,
patchy fog developed over portions of west Anchorage, but with
cloud cover moving in, the fog has mostly diminished. Easterly
flow will increase across the northern Gulf and Prince William
Sound today, which will increase the coverage and intensity of the
rain shower activity along the coast of the Kenai Peninsula. The
easterly flow will also increase the winds through Turnagain Arm
and higher elevations today, with breezy conditions developing
tonight out of the Matanuska Valley as well.

All of this is in response to a strong low moving north into the
Gulf this evening. The leading front will lead to rapidly
increasing rainfall rates across Kodiak Island late tonight, with
the north and east facing slopes expected to pick up the most
rain. The rain will be heaviest along Kodiak and Afognak Islands
Sunday morning, rapidly ending Sunday afternoon. Rain will also
increase in intensity along the north Gulf coast, particularly the
eastern Kenai Peninsula in the predawn hours Sunday morning. For
Seward and much of eastern Prince William Sound, as the flow turns
northeasterly, the drying offshore flow will dramatically cut
into rainfall totals. Thus, this is not expected to be a big
rainfall event for most areas, except Western Prince William Sound
and the north and east slopes of Kodiak and Afognak Islands.

The main hazard with this low will be the winds. Since the low
will be entering the Gulf at the low 960 mb strength, it will be
powerful enough to cause hurricane force winds in the waters
between Middleton Island and the Copper River Delta, as the
barrier jet ramps up early Sunday morning. Storm force winds will
also impact the rest of the north Gulf Coast waters. The low
turning the winds northeasterly fairly quickly should spare most
of the Prince William Sound from above gale force winds, though
gusts to storm force are likely, especially around Hinchinbrook
Entrance.

The low will begin weakening during the day Sunday as the center
moves into the northern Gulf. Strong southwesterly flow south of
the low center will itself cause storm force winds with hurricane
force gusts in the offshore waters well east of Kodiak Island
Sunday afternoon through the evening. The center will make
landfall near Cordova late Sunday night, though associated
precipitation will stay on the windward side of the mountains,
leaving Anchorage and the Mat-Su dry, but mostly cloudy for the
day. On Monday, as the surface low tracks up the Susitna Valley
and the upper trough pivots westward, broad southwesterly flow
will make for a wet day for nearly all of mainland Southcentral. A
cooler air mass will briefly follow behind the low Monday night,
which with upslope flow continuing to produce precipitation, could
lead to high elevation snow, including into Thompson Pass. The
air mass warms quickly during the day Tuesday, and for now appears
to be the last chance of any kind of seasonably cold air moving
into Southcentral for the foreseeable future.

&&

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

Northerly flow will continue into Monday. Saturday, a trough over
the northern Bering will be the feature responsible for this
pattern. Into Sunday, a low in the Gulf of Alaska will continue
this flow. Precipitation will be scattered across the Southwest
on Saturday with most precipitation type being rain. Snow is
possible in the higher elevations of the Alaska Range. Sunday into
Monday, warmer and drier conditions will move into the area, on
the backside of the low moving inland from the Gulf of Alaska.

&&

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

Unsettled weather will continue for the forecast period as
shortwaves bring showers and small craft criteria winds to the
area. Monday afternoon, expect an increase in precipitation in the
western Bering and Aleutians as an area of low pressure
approaches the area. With this, expect gale force winds as the
front associated with this feature moves across the western
Aleutians.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...

A Gulf low weakens and moves into Prince William Sound then
inland Mon. Its front extends across the Gulf with southwesterly
gales dissipating late Mon. A new North Pacific front pushes
widespread easterly gales across the Gulf from south to north for
Tue and extends around Kodiak Island and the Alaska Peninsula
coast by Wed.

For the Bering Sea and Aleutians, a polar shortwave drives
southward across the eastern Bering into the North Pacific, with
gusty small craft northerlies dissipates late Mon. High pressure
settles over the Bering through Wed. A strong North Pacific low
and front moves into the western Aleutians and Bering. The low
tracks south of the Aleutians through Wed with widespread east to
southeast gales over the western Aleutians late Mon and Tue, and
dissipating into the North Pacific by Wed.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Friday)...
A fairly active and amplified pattern will continue next week. A
short wave rounding the base of the upper trough brings a strong
surface low north toward the Alaska Peninsula Tuesday. Model
spread is fairly broad with this feature with the Canadian NH and
ECMWF the fastest and the GFS the slowest. While confidence is
low on the timing and positioning of this system, all solutions
indicate a large storm system affecting the southern Mainland of
Alaska Tuesday through Wednesday night. Widespread strong winds
and heavy rain are likely. This system weakens Wednesday
night/Thursday morning. Yet another developing low then heads
north toward the Alaska/western Gulf Thursday afternoon and
Friday. Continuing chances of strong winds and widespread rain to
the Alaska Peninsula, Gulf, and southern Mainland.

Out west, the remnants of Tropical Storm Dolphin will transition
to an extratropical low and pass just south of the western
Aleutians Monday through Tuesday. This will bring a quick shot
of rain and wind. Otherwise, with the primary storm track much
farther east, conditions across the western Bering and Aleutian
Chain will remain relatively quiet.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Force Wind Warning...119.
 Storm Warnings...351 352 120 131 132
 Gale Warnings...150 125 130 136 137 138 139
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...MK/BG/SEB