Small Craft Advisory 638, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
700 PM AKDT Mon Aug 28 2017

ANALYSIS AND UPPER LEVELS...
A sprawling and loosely defined longwave trough continues to
encompass mainland Alaska this morning as it extends from the
Arctic into the eastern Bering Sea. Weak cyclonic flow associated
with this feature is continuing to support benign shower activity
across much of the region. A weak low pressure system off the
Kamchatka coast is bringing light rain to the western extent of
the Aleutian chain, but the remainder of the Bering is largely
quiet owing to the influence of a large ridge over eastern Russia.
Meanwhile, the next system poised to influence the region in the
coming week is beginning to take shape approximately 400 miles
south of Kodiak Island as a North Pacific low begins to rapidly
deepen, with an associated gale force front entering the southern
Gulf.

MODEL DISCUSSION...
Models remain in good agreement regarding the strength and timing
of the front moving across the Gulf today. Mainly utilized the
NAM to refine the details of the forecast through tonight. Model
spread then increases considerably on Tuesday as the low tracks
north towards Kodiak Island, with guidance generally split amongst
two different camps: the EC/GEM which keeps the low farther east
in the vicinity of Kodiak Island, and the GFS/NAM which depicts a
westward jog towards the Chignik area. These differences in track
will have rather large implications on the forecast wind field,
yielding lower confidence in some of the details beyond the first
24 hours of the forecast. The morning package sticks closer to the
inherited forecast which more closely resembles the eastern end
of the guidance envelope, but will continue to closely watch the
incoming model runs in the next 24 hours as they hopefully begin
to converge on a solution.

AVIATION...
PANC...Rain will move into the terminal around midnight with the
arrival of a front which will also be accompanied by northerly
winds as the down inlet pressure gradient continues to tighten.
Rain should become more showery in nature by Tuesday morning as
cross barrier flow begins to increase across the Chugach Range.
VFR conditions are expected through Tuesday morning.

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

A powerful cyclone is in the process of rapidly deepening south of
Kodiak Island this evening. The upper level baroclinic leaf on IR
and WV satellite imagery is moving northward ahead of the
advancing upper level trough, with the Middleton Island radar
picking up the large area of warm frontal rain moving toward the
coast. ASCAT satellite data and local buoys are already reporting
strong gale force winds in excess of 45 knots. Eventually this low
will bottom out around 975 hpa and near 55 knots as the barrier
jet advances toward the coast late this evening. Warm frontal rain
is expected to move inland late this evening and overnight as
difluent flow aloft aids in vertical forcing. Deep layer moisture
will also help counteract the downslope forcing, so while not
heavy, light rain will continue into Tuesday morning before the
upper level dryslot and downslope flow exert their influence.
Meanwhile, however, with a subtropical moisture fetch and strong
orographic ascent, rain will pile up over the next few days along
the coast, especially Prince William Sound where 4-8" of rainfall
is likely.

The upper low will eventually weaken and drift north through
Thursday, keeping rain along the coast and rain/shower threats
inland. Lobes of energetic waves will wrap around the main low,
so precipitation threats inland will be more dependent and
uncertain given the uncertain nature and track of these waves.

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

A strong low approaching Kodiak Island will be the dominant
weather feature affecting Southwest Alaska over the next couple
days. Rain will begin to cross the Aleutian Range south of King
Salmon this evening, and overspread much of the Bristol Bay area
east of Dillingham tonight. During the day Tuesday, the
precipitation will gradually creep westward, reaching the
Kuskokwim Mountains during the afternoon. Strong, dry, northerly
winds will temper the westward advance of the rain, with very
little moisture expected to encroach into the Kuskokwim Delta.
Thus, Bethel and points west are expected to stay dry, though none
of the area will escape the gusty winds.

Late Tuesday night into Wednesday morning, a strong push of colder
air will move southward out of the Bering Strait along the coast.
Temperatures through the Kuskokwim Delta will drop by Wednesday
and especially Thursday as a result. Lows dipping into the 30s
will be more common by Thursday morning. The cold air will
eventually reach into Bristol Bay as well, with upper 30s and
lower 40s possible Thursday morning. There remains some
disagreement as to how far east the cold air gets to, which would
result in a somewhat unusual temperature pattern for this time of
year with warmer temperatures persisting inland across the Lower
Kuskokwim Valley as the colder air remains near the coast.

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

Persistent northerly flow continuing across the Bering will keep
conditions relatively dry for much of the area for the next few
days. There are a few exceptions: Near the Alaska Peninsula, where
a strong low near Kodiak Island will spread rain throughout the
region, especially south of the Peninsula, also, a front stretched
out across the western and central Aleutians will dive
southeastward through the morning Tuesday, and allow for that area
to join in on the drier north to northeast winds by Tuesday
afternoon. A small, weak disturbance marking the leading edge of
some colder air moving across the eastern Bering may touch off a
few showers over the central Bering and potentially the Pribilofs
late in the day Wednesday.

.LONG TERM FORECAST (Days 3 through 7)...
At the onset of the forecast period there is a vortex over
western Alaska with a secondary low over Kodiak and a negatively
tilted trough extending from SE AK to Prince William Sound.
As time elapses, expect the plume of Pacific moisture to continue
into south central Alaska with precipitation lingering into the
weekend over the Anchorage Metro area with Whittier getting
consistently pummeled into the weekend. As the next low descends
southward diffluent southwest flow will set up over the Cook
Inlet which also indicates a wet weekend.

Looking west, there is a ridge axis in the vicinity of the Western
Aleutians. The next feature to watch will be a low spinning up
and tracking in from the west along the Aleutians in the Friday or
Saturday time period.

&&

AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM DISCUSSION...JW

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...PZ