Small Craft Advisory 393, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 AM AKDT Wed May 22 2019

.ANALYSIS AND UPPER LEVELS...

In the upper levels this morning, there are two upper level lows,
one near the Pribilofs, and a second much stronger and nearly
vertically stacked low tracking north towards the Alaska
Peninsula. A secondary wave is over the northern Gulf. The wave
over the northern Gulf is dissipating and is no longer having any
significant impact on the weather. The low approaching the Alaska
Peninsula will be the primary weather maker across Southcentral
and Southwest Alaska through at least Thursday. Ahead of the low,
much of Southcentral west of the Talkeetnas are enjoying nearly
clear skies, while remnant cloud cover from earlier convection and
some cloud cover from the upper wave over the northern Gulf are
keeping conditions mostly cloudy for the Copper River Basin. Cloud
cover ahead of the low continues to spread northward over
Southwest Alaska.

&&

.MODEL DISCUSSION...

The models remain in good agreement through Thursday night. They
all show the low south of the Alaska Peninsula tracking to near
southern Kodiak Island Wednesday night. After that there is some
disagreement on whether the low center stays on the Gulf side,
reforms over Bristol Bay, or both. Ultimately it should have
limited difference in the forecast other than the strength of the
winds around where the low center tracks. All of the models show
strong high pressure building over the central Gulf behind this
low, as a new North Pacific low approaches the eastern Aleutians
Friday morning. As the leading front associated with the low moves
into the north Gulf Coast Wednesday night, there is some
difference in the models regarding the resulting gap winds through
Turnagain Arm and the Copper River Basin. Forecast confidence
remains high.

&&

.AVIATION...
PANC...VFR conditions will persist. The Turnagain Arm wind is
expected to return this afternoon. With the strongest heating in
the Susitna Valley today, it will help the wind move into
Anchorage. Gusty winds to 35 mph are possible this evening. The
wind will turn down-Inlet tonight, returning the winds to light
and variable. However, mountain showers will be possible in the
vicinity as well.

&&

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

A Gale force low, currently well south of the Alaska Peninsula,
weakens as it moves to over Kodiak Island early Thursday morning
and the North Gulf Thursday night. The associated occluded front
approaching Kodiak makes landfall over the Kenai Peninsula/North
Gulf Coast late tonight then weakens. Winds will increase to gale
force across many marine areas today/tonight with the approach of
the front and will linger on into Thursday over the western Gulf
with the trailing low center. Expect increasing gap winds to
develop this afternoon as the coastal ridge builds in advance of
approaching front and the thermal trough lowers pressures inland.
The result will be strong winds across Turnagain Arm, out of the
Knik River Valley, and along the Copper River this afternoon. Also
expect gusty southeast winds to spread across south and west
Anchorage this afternoon. Local/gap flows will diminish overnight
tonight, but will likely redevelop Thursday afternoon.

Rain, associated with the occluded front, will spread to Kodiak
Island this morning then to the north Gulf Coast/Kenai this
evening/tonight. The brunt of the rain will be along the coasts
as downslope drying to the lee of the Chugach Mountain Range
should minimize precipitation. Most areas will trend towards showers
Thursday as a cool upper low and associated short wave spread
across the area. Expect another warm today today, cooler on
Thursday with increased cloud cover and cooling temps aloft.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The main forecast challenge surrounds a storm system currently
developing south of the Alaska Peninsula this morning. This
occluded system tracks slowly north spreading gusty winds and rain
across Bristol Bay this afternoon. The strongest gusts are
expected to impact locations along and near the Aleutian Range
this afternoon through the evening. Northeasterly winds across
the Kuskokwim Delta shift late this afternoon as a weak mesoscale
low center develops along the Kilbuck Mountains...winds will vary
from a southerly wind near Bethel to northerly flow along the
coast.

Precipitation and winds taper off tonight as the surface low
weakens on a northward track. There is still some uncertainty
regarding rain chances on Thursday...all models are in good
agreement on trends; however, the upper low center tracks directly over
over the Southwest which usually keeps periods of light rain
across the region until it diminishes or the next synoptic
pattern moves in. A brief respite from rain is anticipated on
Friday as a transient ridge gives a pause before the next North
Pacific low pushes a front to Bristol Bay Friday evening.

&&

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

Active weather is on tap across the eastern Bering today as a
North Pacific low south of the Alaska Peninsula pushes a front
across to the southeastern waters this morning. Gusty gale winds
and rain can be expected with this front as it moves
through...then tapers off tonight as the low fills quickly while
pushing inland across the Southwest. Meanwhile, a dirty ridge
builds across the western Bering keeping low to mid level clouds
streaming overhead. The ridge builds across the eastern waters on
Thursday...diminishing rain chances until the next North Pacific
low pushes a front to the eastern Aleutians and Alaska Peninsula
early Friday. The front will likely take a northeasterly track as
the ridge holds across the western waters.

&&

.MARINE (Days 3 through 5)...
A low tracking north towards the eastern Aleutians late Thursday
night through Friday morning and its associated front will bring
the potential for wind in the 25 to 30 kt range with localized
areas up to 35 kt to the waters from the eastern Aleutians east
into the western Gulf. Following that, a more active southerly
storm track will set up in the vicinity over the western Gulf,
increasing the potential for periodic Small Craft Advisory to Gale
strength wind through the weekend as a series of frontal waves
push north over the western Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By Thursday evening the upper level pattern will have progressed
to having an upper level trough extending from a North Pacific
upper low across the central and eastern Aleutians to a second
upper low over southwest Alaska with ridging pushing slowly east
over the western Aleutians. To the east, a strong upper level
ridge centered over northwest Canada will extend a spur ridge
west across northern interior Alaska. The upper low over
southwest Alaska with troughing extending into south central
Alaska will lift gradually north into interior Alaska Thursday
night through Friday as a broad upper level ridge over the eastern
Pacific builds north into the Gulf of Alaska. Although conditions
will be cool for fire weather concerns, this pattern shift will
lead to rather breezy conditions in the Copper River Basin
Thursday afternoon/evening through Friday.

By Friday, another North Pacific low will have tracked north to be
located south of the eastern Aleutians and western Alaska
Peninsula with its associated front lifting north into the western
Gulf of Alaska. The ridge in the Gulf of Alaska will sharpen in
response on Friday with its axis shifting east over the eastern
Gulf of Alaska and southeast Alaska Friday night as the front
pushes north over Kodiak Island and southwest Alaska. The ridge
will continue to amplify over the eastern Gulf of Alaska and
build into western Canada as well on Saturday, while the front
pushes north to stretch across both Southwest and Southcentral
Alaska as its supporting surface low tracks into Bristol Bay.

With a very long southerly fetch and temperatures warming at all
levels under the ridge, concerns are growing for an atmospheric
river event developing for southern Alaska. While details regarding
the location of the heaviest coastal precipitation and whether or
not it will stall over one spot or oscillate back and forth
across the western and northern Gulf coast remain uncertain,
confidence is increasing in the pattern shifting to a significantly
wetter one starting Friday night and persisting through the
weekend.

By around Monday, the upper level ridge shifts east and inland to
be centered over British Columbia and Alberta with its axis
negatively tilted and extending northwestward across the Yukon
Territory and into far northeastern Alaska. With an elongated
upper low setting up over western Alaska and the eastern Bering,
south to southeasterly flow will continue to bring frontal systems
across the Gulf of Alaska and into southern Alaska through the
first half of next week. This will keep the pattern on the damp
and active side.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 120 130-138 150 155.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...JR