Small Craft Advisory 673, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
600 PM AKDT Mon Sep 16 2019

...Updated Extended Discussion...

.ANALYSIS AND UPPER LEVELS...

An active progressive upper level wave pattern is in place across
Alaska as the low which brought widespread rain to Southern
Alaska the last two days shifts east through the Gulf of Alaska.
This is promoting weak off offshore flow along the North Gulf
Coast which is helping to clear out some of the lower clouds.
Farther to the north, an Arctic trough is digging southward across
the Brooks Range and is beginning to impinge upon the Alaska
Range. A deep trough is stationed over the western Bering Sea,
and this trough is tapping an impressive region of low level
subtropical moisture. This low will be the beginning of a period
of rainy and stormy weather for the Southern Alaska and North
Pacific regions heading through the weekend.

&&

.MODEL DISCUSSION...

The numerical weather models are in surprisingly good overall
agreement with a continuation of the overall "progressive" upper
level flow and general storm track. With the next low moving into
Southern Alaska Tuesday and Wednesday, models are in excellent
agreement with widespread rain and the overall low track
(regarding details of gap winds in Southcentral). Forecast
confidence is very high.

&&

.AVIATION...

PANC...VFR conditions are expected to prevail, along with light
winds, through the TAF period before rain returns to the terminal
Tue evening when conditions will likely drop to MVFR at times.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Mon
night through Thursday night)...

Low pressure over the Gulf exits southeast tonight with a weak
ridge building across the western waters overnight. An Arctic low
diving across the northwest mainland this evening will bring
increased chances for rain overnight across the Susitna &
Matanuska Valleys as well as the Copper River Basin with isolated
amounts farther south. This frontal push will increase offshore
flow with gusty conditions expected along the Gulf coastal
locations. In areas outside of these winds, patchy fog is possible
as temperatures cool close to dewpoints with the recent abundant
rainfall.

Attention then quickly turns to the next front as it
moves across the Western Gulf and Kodiak Island Tuesday morning,
making progress northward over the Kenai Peninsula during the
afternoon. Rain quickly spreads north to the rest of Southcentral
Tuesday evening through Wednesday. Deep subtropical moisture
with this low pressure system will bring appreciable rainfall
amounts area-wide. Gusty south to east winds will be strongest
late Tuesday night through Wednesday morning...with gale force
strength along the Northeastern Gulf and Prince William Sound.
Turnagain Arm and the Upper Hillside will see strong winds gusting
between 50 to 60 mph...and may possibly be stronger, so stay
tuned for updates.

On Wednesday, the low driving the front swiftly moves east from
Southwest AK to the Copper River Basin...diminishing rapidly as
it moves across the terrain. Rain will become less organized
during the afternoon and winds taper down; although, another
system will should make for light rain again through Thursday as
weak disturbances move up from a storm system tracking eastward
along the Eastern Aleutians and Alaska Peninsula.

&&

.FIRE WEATHER...Fire weather concerns will continue to diminish as
we remain in a very typically Fall pattern. Tonight, a cooler
upper level trough will dive into the Susitna Valley and the
Copper River Basin. This will bring some steady showers. Then a
much stronger front will start to move into the area from west to
east late Tue into Wed. This front will bring gusty southeast
winds through the typical gaps (Turnagain Arm and Knik Arms, the
Copper River Valley, etc.). It will also bring healthy amounts of
wetting rains to just about all areas with QPF amounts averaging
near 1 inch.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Mon Night
through Thu)...

A ridge cresting over Southwest Alaska today is bringing a quick
period of diminishing cloud cover along with a drier,
northwesterly flow. The exception to this is along the upslope
side of the Kilbuck and Aleutian Ranges where some low stratus
will linger through the evening hours. Radiational cooling under
the ridge will lead areas of fog overnight, especially for
locations that saw the majority of clearing this afternoon such as
interior Bristol Bay and the Kuskokwim Valley.

Rain and gusty southerly winds return Tuesday morning along the
coast as a warm front moves onshore. The front lifts northeast
through the day as it continues to push widespread rain over the
interior Southwest. A trailing shortwave then treks across the
Southwest on Wednesday as the main surface low moves over the Y-K
Delta. These two features will help maintain a southwesterly
onshore wind and allow showers to continue across the region
through late Wednesday. A weak trough then drops over the northern
half of Southwest Alaska early Thursday, allowing for a colder,
drier northwesterly flow across the Kuskokwim Delta with
continued showers along the Alaska and Aleutian ranges.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Mon
night through Thu)...

A surface low over Amchitka is currently swinging a warm front
toward the eastern Aleutians and the AKPEN with a trailing cold
front tracking east towards Nikolski. Widespread areas of small-
craft winds and rain will continue along both fronts as the system
continues to jog east tomorrow. Small-craft winds will increase
to gales along the warm front as it occludes and lifts north of
the Pribilofs. Northwesterly gales will also develop over the
southern Bering and central Aleutians tomorrow as colder air wraps
around the back side of the surface low. Westerly gales persist
through the first half of Wednesday along the Pacific side of the
eastern Aleutians and AKPEN as the low moves onshore near Kipnuk.

Farther west, a weak ridge will build behind this first system,
moving from the western Aleutians Tuesday to the central and
eastern Aleutians Wednesday. Upstream, a deepening north Pacific
low will sweep a front with widespread rain and areas of gales
over the Western Aleutians Tuesday night into Wednesday. This
feature will then weaken as it slides east, with the center of
circulation staying well south of the chain through Thursday.

&&

.MARINE (Days 3 through 5)...

Beginning the extended period marine forecast on Thursday, there
will be a low moving northeast from the North pacific into the
Gulf of Alaska. This low will reach the North Gulf by early
morning Friday, likely bringing gale force winds to much of the
North Coast as the warm front/barrier jet increases in strength.
The low will then occlude and remain in the Gulf of Alaska through
Saturday with winds becoming westerly/offshore as the low moves
east toward the ALCAN. Yet another strong low moves into the Gulf
of Alaska on Sunday and Monday as the storm track remains directed
over the Gulf with much more quiescent weather over the Bering
Sea through the weekend.

&&

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

An active weather pattern continues through the extended period.
An upper level low remains in place over the northern part of the
state while several troughs rotate across the southern part of the
state. The first surface low travels south of the Aleutian Chain
and arrives over the Gulf of Alaska Friday. This low will bring
cloudy skies, rain, and gusty winds as it traverses into the Gulf
and eventually lifts inland over the southern half of the state
for the weekend. A second low arrives over the Gulf of Alaska via
a similar path as the first by late Sunday. This will continue the
trend for cloudy skies, rain, and gusty winds. Models have been
in fair agreement regarding an unsettled weather pattern through
early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 351 352 155 172-178 185 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...JA/KO