Gale Warning 416, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 PM AKDT Thu Sep 19 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level trough over the eastern Bering Sea, with a
ridge over the Yukon Territory. This is bringing moist southwesterly
flow aloft to all of southern Alaska, and some showers to much of
the area. There is a strong surface low rapidly forming south of
Kodiak Island, and moving toward the northeast into the Gulf of
Alaska. There is a flat upper level ridge over the central and
eastern Bering. There is a trough and a developing surface low
over the far western Bering moving eastward.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement. There are some timing
issues with a low that moves from the northern Bering Friday night
into southwest Alaska on Saturday, but this low is not real strong
and the forecast implications are minor. The forecast confidence
is higher than normal with the exception of this Bering/southwest
low.

&&

.AVIATION...
PANC...Steady rain with the next storm system will develop this
evening. Cross barrier flow will initially maintain VFR conditions.
However, this will quickly weaken overnight as the deepening low
over the Gulf lifts northward toward the eastern Kenai Peninsula.
While the low doesn't quite move to the "sweet spot" for heavy
precipitation in Anchorage, still expect to get under a portion of
the deformation band ahead of the low. Thus, will likely drop to
MVFR ceiling and visibility as heavier rain moves in early Friday
morning. Intensity will then diminish by 00Z Saturday, though
continued cyclonic flow aloft will keep some light rain in the
area. This could also lead to IFR clouds in the vicinity, though
would expect most of these clouds to form near the mountains.

&&

.FIRE WEATHER...
Widespread wetting rains are once again expected across
Southcentral with this next storm. The big difference from
previous storms will be the storm track, with a low tracking
into Prince William Sound. This event will be much more favorable
for significant rainfall across the Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Saturday)...

Widespread wetting rains from an incoming low are making their way
into Southcentral this afternoon. The shower activity over
Anchorage has moved west of town for now, but is expected to
refill in on radar over the next few hours. The radar is already
showing rain moving into Cook Inlet and the Gulf Coast around
Homer and Seward. Steadier rain will overspread all of Southcentral
tonight. The low causing all of this rain is centered well south
of Kodiak right now. It will intensify as it moves into the
northern Gulf south of Seward Friday morning. This will cause
strong winds associated with the barrier jet to increase overnight
along the coast mainly from Hinchinbrook Entrance eastward, where
a Storm Warning is in effect. The center of the low will move
into Prince William Sound Friday evening, which will keep light
rain ongoing through Anchorage, the Matanuska Valley, the Gulf
Coast, and the Copper River Basin. This will be a very good track
for rainfall in the Copper River Basin, so much more rain is
expected there with this low than the previous rainfall event.

On Saturday, the low will circulate back into the northern Gulf,
as the associated moisture only very gradually shifts eastward
with time. This will lead to strong westerly winds ongoing through
the Barren Islands and southern Cook Inlet from Friday right into
Saturday. Gradual clearing is expected from west to east during
the day Saturday, mainly over the Susitna Valley and Cook Inlet,
taking until late in the day at the earliest to reach Anchorage
and the Matanuska Valley. With the clearing skies, colder air will
move in as well behind the low, with a real threat for freezing
temperatures in interior areas of the Kenai Peninsula, east
Anchorage, and the Mat-Su Valleys Saturday night. Where
precipitation persists longer in the Copper River Basin, snow
showers are possible Saturday night.

&&

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

Rain will lift northeast from Bristol Bay to the Middle and Upper
Kuskokwim Valley through the overnight hours as a weak trough
tracks inland along the Alaska Range. For the Lower Kuskokwim
Valley and Y-K Delta, clearing this evening will once again allow
for patchy fog to develop as overnight temperatures drop to near
freezing. A cool, northwesterly flow along the backside of the
trough will persist through late Friday as scattered showers
linger in the higher elevations. Showers and cloud cover diminish
overnight Friday into the first half of Saturday as a ridge tracks
over the region. Overnight lows will again drop to a few degrees
either side of the freezing mark as patchy fog settles along the
Kuskokwim Valley. The ridge slides quick to the east on Saturday
as the next frontal system approaches the coast. This system will
bring gusty southerly winds and rain to the Kuskokwim Delta before
lifting north and east in response to a stronger low moving from
the central Aleutians to the AKPEN. This second system looks to
bring another round of widespread rain across the AKPEN and
Bristol Bay up to Kuskokwim Bay as it sweeps a front along the
coast on Sunday before tracking southeast away from the region.

&&

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

Northwesterly small-craft winds will develop from the eastern
Bering south across the eastern Aleutians and AKPEN by early
tomorrow in response to a tightening pressure gradient between a
deepening trough in the Gulf and a ridge building and moving east
over the Aleutians. By late Friday these winds will increase to
gales over the coastal waters south of the AKPEN. A low over the
northern Bering will move to Saint Matthew Island by Friday night
with rain and small-craft winds stretching from the Pribilofs to
the Y-K Delta coast. Rain and winds from this system then diminish
on Saturday as the low moves to Norton Sound. For the Aleutians,
the eastward moving ridge will reach the AKPEN by Saturday as a low
moves south of the western Aleutians in its wake. The center of
the low in expected to be near Nikolski by late Saturday with its
associated occluded front draped from Dutch Harbor to Cold Bay.
Widespread rain and small-craft winds will accompany the front
along with an area of northerly gales wrapping around the
backside of the low. The low makes moves to near Cold Bay by
Sunday as its front advances to the southwest coast. Behind the
low, the northwesterly winds strengthen to strong gales with
the possibility of storm-force winds developing south of Dutch
Harbor.

&&

.MARINE (Days 3 through 5)...

Starting Sunday, winds in the Gulf will diminish as a low exits to
the east. Small craft advisory level seas driven by fresh swell
will persist Sunday morning, but likely diminish by the afternoon.
Attention turns to a low currently developing south of the
central Aleutians. Gales are expected over the central and eastern
Aleutians by Sunday, then models continue to project a triple
point low developing late Sunday into Monday that would bring
gales into the Gulf. There's uncertainty with the path/strength
of the secondary low, but at this point confidence is growing that
there will be gales in the Gulf Monday, possibly persisting into
Tuesday.

&&

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

A relatively progressive pattern will continue as a low traverses
the Aleutians, bringing widespread precipitation and gusty winds
to the Aleutians Sunday. The Southwest mainland will probably see
some precipitation from this system, especially over the AKPEN...
but the primary impacts will be felt over the Aleutians. The low
is expected to track into the Gulf. Thus, as the low progresses
eastward Sunday into Monday, precipitation is expected over Kodiak
Island and the North Gulf Coast. Farther north, east/northeast
flow will prevail and relatively dry weather is expected. An
Arctic trough over the Bering Strait will strengthen by midweek,
maintaining relatively cold conditions and unsettled weather over
southern Alaska. Another low will develop in the central Bering
Thursday into Friday, but it's too early for confidence in
details.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119. Gale Warning 120 130 131 132 138 150.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...MM