Small Craft Advisory 740, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKDT Tue Sep 29 2020

.ANALYSIS AND UPPER LEVELS...

A high amplitude trough extends from Shemya, south to 35N, and
back north towards Southeast Alaska. Over western Canada the
North American ridge continues to build to the northwest, which
will give the aforementioned trough a negative tilt over the next
few days. Over the southern Mainland, a stretched out upper level
low continues to weaken, bringing a few showers to the
Southcentral coastline and Bristol Bay. High pressure exists over
the western Bering as a front clips the western Aleutians.

The main feature for today's forecast package can be seen on
water vapor imagery as a well defined extratropical cyclone at 46N
155W. Multiple low centers can be seen within the broad comma
shaped storm system. The storms front is well north of the low
center and just south of the Gulf this morning.

&&

.MODEL DISCUSSION...

Models are in good agreement in regards to the first storm force
low moving over the AKPEN on Wednesday. The NAM, which had the low
stalling over and to the south of the AKPEN through Thursday, has
matched up with the other global deterministic models in bringing
the low into Bristol Bay by Thursday morning. Models are in
moderate agreement for the next system moving into the north
Pacific midday on Thursday. The GFS becomes the outlier in this
scenario with a slightly stronger storm (968 mb) that is well
south of the EC, Canadian, and NAM solutions which have a more
elongated low center with central pressures ranging from 970-975
mb. The main difference between the solutions will be the timing
of the front and gale force winds to Kodiak Island and the western
Gulf coast.

&&

.AVIATION...
PANC...Turnagain Arm winds should taper off late this morning as a
storm system moves into the Gulf. Expecting VFR conditions with
rain returning Wednesday morning as a front moves in. There are no
ceiling or visibility restrictions expected with the rain at this
time.

&&

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

An upper trough lifts north across Southcentral today. This will
bring breaks in the clouds and diminishing chances of showers. Locally
gusty winds with pressure rises on the backside of the trough will
diminish through this morning.

A storm force low complex over the North Pacific moves to over
the Alaska Peninsula Wednesday afternoon. Its associated front
moves north today bringing moderate to heavy rain to Kodiak Island
later this afternoon and to the northern Gulf Coast tonight. Rain
over Kodiak Island should transition toward showers as the front
moves through tonight, while steady rain holds along the Gulf
Coast until the front lifts inland Wednesday. Some rain will
likely make it west of the Coastal mountains, but the brunt of the
precipitation will be along the coasts. Gale to storm force winds
will accompany the front as it heads north today with winds
reaching hurricane strength over the western Gulf/Kodiak Island
late this afternoon/evening. Winds will slowly diminish over the
Gulf waters as the front moves inland Wednesday. Strong easterly
gap winds along Turnagain Arm and through Portage Valley are
expected tonight through Wednesday night. As the front moves
inland Wednesday, pressure rises on the backside will produce
strong southerly winds across the Copper River, Knik River Valley,
and will bring southerly winds across South and West Anchorage
Wednesday afternoon/night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Thursday Night)...

A pair of strong lows following nearly identical paths (one
following the other) will be impacting Southwest Alaska through
the short term. Both lows are expected to be of similar strength,
as the centers of both track northwestward out of the North
Pacific, cross the Alaska Peninsula near Chignik, then track up
the coast of Southwest Alaska.

The first low is already well developed over the North Pacific
south of the Gulf. Its leading front is already approaching the
coast. The leading front will be the more impactful aspect of the
storm, causing winds to hurricane force in the waters along the
Pacific side of the Alaska Peninsula this afternoon through the
first half of tonight. As the front lifts northwestward across
Southwest Alaska, the predominately northeasterly flow should
reduce (but not entirely eliminate) the downsloping to the lee of
the Aleutian Range. Thus, most of Southwest Alaska can expect
wetting rains from the front. The strongest winds will be in the
typical areas, primarily through Kamishak Gap and into Lake
Iliamna. The front will rapidly weaken with time Wednesday morning
as it pivots to track more towards the west, so little rain is
expected to survive the downsloping from the Kuskokwim Mountains
to reach the Kuskokwim Delta midday Wednesday.

Another concern of note will be potential for low water along
coastal Bristol Bay tonight. While fortunately the strongest
northeasterly winds of 55 to 65 mph will not correspond well with
the low tide (the tide will be coming in as the strongest winds
are occurring), it may make for water levels dropping several feet
below mean low tide this evening. In addition, the incoming tide
and strong offshore flow will make for very choppy seas in Bristol
Bay tonight.

The center of the first low will cross the Alaska Peninsula
Wednesday evening in a much weakened state, with gusty winds
through Kamishak Gap and into inland Bristol Bay and continuing
upslope rain along the Kuskokwim Mountains being the primary
lingering effects by Thursday. The second low will rapidly
approach the AKPEN Thursday evening with another round of strong
easterly winds, heavy rain along the coast, particularly along the
south side of the Alaska Peninsula, and favored east facing
slopes of the Kuskokwim Mountains. There will be more on this
second low in upcoming discussions. Temperature-wise both lows
will reinforce the warm air over the area, making for well above
average temperatures, especially with overnight lows.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Thursday Night)...

A strong low moving out of the North Pacific will track up the
coast over the eastern Bering through Thursday, weakening as it
goes. The low will work to bring seasonably cool air southward out
of Siberia and across the central and western Bering. A large, but
weak area of high pressure over the western Bering this morning
will slowly shift south into the North Pacific by Wednesday, with
the low eventually needing to do all the work of bringing cooler
air south across the central Bering by Thursday. This means a
continuation of the current pattern of widespread cumulus over the
area, some of them producing shower activity, but otherwise
remaining a broken cloud field, allowing many areas to get at
least a little sunshine between the clouds the next several days.
Gales to occasionally storm force northerly winds will impact the
southern Alaska Peninsula and eastern Aleutians starting tonight
and slowly diminishing through the day on Wednesday.

&&

.MARINE (Days 3 through 5/Thursday through Saturday)...

Bering/Aleutians

A weakening area of low pressure in the eastern Bering will bring
small craft winds to the area. A second and much stronger low will
move northwestward across the AKPEN and into the Bering Thursday
night into Friday morning. This low will bring widespread small
craft winds with small areas of gale force winds possible.
Confidence in the exact track of this system is low to moderate as
there are discrepancies on the placement of the low center between
models.

Gulf of Alaska

A strong area of low pressure will move across the western Gulf
Thursday afternoon through Friday evening. Widespread gale force
winds are likely, especially in the western Gulf. Confidence is
lower on the timing of these winds as there are discrepancies
between models on the placement of the low center. Another low
will move into the Gulf Saturday afternoon, though confidence
remains very low with regard to this low due to major
discrepancies in the placement and intensity of the low center.

&&

.LONG TERM FORECAST (Days 3 through 7/Thursday through Monday)...

The pattern will be highly amplified on Thursday with a deep
negatively-tilted upper level trough over the Bering/Aleutians and
a strong upper level ridge over the west coast of Canada. The
negative-tilt of this upper trough will be favorable for
cyclogenesis as a strong area of low pressure moves into the
western Gulf on Thursday and crosses the AKPEN into the Bering Sea
on Friday. Confidence is low in the exact placement of this low
due to discrepancies in guidance as this low crosses into the
Bering. Once in the Bering, this low will gradually weaken through
Sunday as the center of this low moves farther away from the core
of the jetstream and loses synoptic support. Another strong low
will move northeastward into the Gulf of Alaska on Saturday
afternoon then will gradually weaken while remaining in the
northern Gulf through Monday. Confidence with respect to this
system is very low as there are significant discrepancies with the
placement and intensity of this low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Force Wind Warning 131 132 138 150.
Storm Warning 119 120 130 136 137 139 155 351 352.
Gale Warning 140 160 165 170 172.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
MARINE/LONG TERM...ED