Small Craft Advisory 391, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKDT Tue May 21 2019

.ANALYSIS AND UPPER LEVELS...
The synoptic pattern is made up a complex series of lows aloft.
The first is a decaying vertically stacked low over the eastern
Bering. This low is driving much of the cyclonic flow that
encompasses the Bering Sea and Gulf of Alaska. The second is a
weakening low aloft in the southern Gulf of Alaska. This low sits
at the base of a shortwave that is moving through the flow around
the low in the Bering. At the surface, conditions are fairly mild
with some showers occurring in the Kuskokwim Delta ahead of the
surface low in the eastern Bering. An inverted trough has
positioned itself over the Gulf of Alaska, with high pressure
inland, is bringing calm, dry and generally sunny conditions to
much of south central Alaska. This area of general low pressure
can be observed in visual satellite imagery as the slight curl of
clouds in the central Gulf of Alaska. The next active system, a
strong 985 mb low in the North Pacific, is moving north towards
the eastern Aleutians.

&&

.MODEL DISCUSSION...
Models have come into good agreement through Thursday and there
is very little variation between them at this point. All models
depict a similar track of the North Pacific low as it moves
through the western Gulf of Alaska on Wednesday. The main,
although slight, difference is the overall strength of this system
as it moves through the western Gulf.

&&

.AVIATION...
PANC...VFR conditions will persist. Light wind are expected into
Wednesday morning. Expect gusty south wind to develop Wednesday
afternoon, persisting well into Wednesday evening, as a weather
front approaches.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Tuesday
afternoon through Thursday afternoon)...

A surface ridge in place along the northern Gulf coast has
brought generally clear skies to much of south central Alaska today.
However, a pattern change is underway for Wednesday as a North
Pacific low lifts northward and tracks over the Alaska Peninsula.
A front associated with this low will extend over the Gulf of
Alaska, bringing rain to Kodiak Island beginning Wednesday
morning. As the front approaches, the usual southeasterly gap
winds (including Turnagain Arm into Anchorage and Knik River
Valley into Palmer) will begin during the morning hours on
Wednesday, increasing by Wednesday afternoon. The front will
progress northward and lift over south central Alaska by
Wednesday evening, by which time most of the northern Gulf coast
will see rain. Given the southeasterly flow, the Anchorage Bowl
will likely not see very much precipitation due to downsloping.
The heaviest precipitation will be along the northern Gulf coast
and will eventually taper off by Thursday afternoon as the front
weakens.

&&

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

Showers and possible thunderstorms will continue tracking south
to north along the Kuskokwim Delta today under a weak front. Low
pressure will approach Kodiak Island Wednesday morning and
continue to move northward through the day, bringing gusty
easterly winds to southwest Alaska (with gusts potentially
exceeding 50 mph in the Kamishak Gap). Downsloped areas including
King Salmon should see very little rain. Widespread showers will
remain well north of the low center over the Kuskokwim Delta. The
low will weaken Thursday and winds will subsequently diminish,
although scattered showers will persist over much of the region.
After a brief break Friday morning, another low will lead to rain
developing over southwest Alaska, north of its associated warm
front.

&&

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

Generally benign conditions are expected for the majority of the
region. The lone exception will be a low that will approach the
Alaska Peninsula and eastern Aleutians Wednesday morning bringing
gales and moderate rain to areas generally east of Unalaska
through the day. Conditions will improve Thursday as the low
weakens. As a ridge amplifies over the western Bering, another low
will approach the same aforementioned area Friday morning
bringing more rain to the central and eastern Aleutians north of
its associated warm front.

&&

.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 around 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 south central
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 even if not with as much continuous moderate to
heavy rain as the weekend has the potential to bring to coastal
areas.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 130 131 132 137 138 351 352 150 155.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...DK/KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...JR