Small Craft Advisory 658, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
359 AM AKDT Mon Sep 4 2017

.ANALYSIS AND UPPER LEVELS...

The big surface low is now starting to fill south of Chignik.
North to northeasterly winds around the back side of the low have
been bringing gusty conditions to the eastern Aleutians and
western Alaska Peninsula especially where channeled through gaps
and passes. The frontal system associated with the large low
currently stretches from Southwest Alaska, across Cook Inlet and
the Kenai Peninsula to the northern Gulf Coast. To the west, upper
level ridging extends over the western and central Aleutians and
Bering, while a front associated with a deep 960 mb ex-tropical
low off of the Kamchatka Peninsula rides up the west side of the
ridge and pushes into the western Aleutians.

&&

.MODEL DISCUSSION...

Synoptic agreement on the large scale features remains very good.
A blend of the NAM and GFS was used for the western domain as all
models were handling the incoming ex-tropical system well. For the
eastern domain the details of high speed frontal waves rippling
up the strong southerly jet in the Gulf present more of challenge
to the models with small differences in timing and position of the
resulting compact surface low making a big difference to the wind
forecast Tuesday and Tuesday night. For the current forecast
package the 00Z GFS was preferred and the slower timing of the
00Z NAM deemed the outlier.

&&

.AVIATION...

PANC...VFR conditions are expected to persist through the period
with some isolated to scattered showers moving through this
morning. A northerly down Cook Inlet gradient will keep the
Turnagain Arm winds bent away from PANC through much of the day
with southeasterlies possibly pushing in the late afternoon or
early evening as ridging strengthens in the northern Gulf and the
down inlet gradient slackens.

&&

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

A strong weather front makes landfall over Southcentral this
morning then weakens. Winds are expected to diminish across much
of the area through this afternoon, though gales will linger on
through the morning west of the Barren Islands, as well as brisk
gap flows across the Matanuska Valley and Turnagain Arm. Rain and
showers will diminish in most inland areas, but should hold fairly
steady across the North Gulf Coast Today.

On Tuesday, a developing surface low under a strong southerly
jet, moves rapidly north across the Gulf making landfall over
western Prince William Sound/Kenai Peninsula Tuesday night. The
deep southerly moisture plume(jet) shifts west across the Gulf as
the upper ridge builds over British Columbia/Yukon territory. This
in combination with the surface low moving north will bring
moderate to heavy rains to the North Gulf Coast Tuesday. Rainfall
estimates of 2 to 4 inches are possible over the North Gulf Coast
Tuesday morning through late Tuesday night.

&&

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

An area of low pressure south of the Alaska Peninsula will
continue to weaken today as it remains nearly stationary. Rainfall
will continue to upslope along the Alaska and Aleutian Ranges
while cross barrier flow continues to limit rainfall across inland
portions of the Bristol Bay Area. As the front continues to
weaken through Tuesday, rain chances overall will continue to
diminish. Winds will also continue trending downwards although
some gusty winds will be observed through channeled terrain along
the Alaska Peninsula and through the Alaska and Aleutian Ranges.

&&

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

The remnants of Sanvu are currently located near the tip of the
Kamchatka Peninsula with the frontal boundary just beginning to
enter the western Bering Sea. As was the thinking yesterday, the
lack of cold air over Russia will prevent the remnant low from re-
strengthening and will weaken as it moves into the Bering Sea
Tuesday morning. Still, a small swath of gale force winds are
expected across the Bering Sea today and Tuesday. The biggest
concern with the low will be the amount of rainfall dumped over
the Aleutian Islands due to it's tropical origins.

&&

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

The long range forecast beginning Tuesday evening remains in an
active pattern through next weekend. A North Pacific low south of
the Alaska Peninsula lifts a strengthening secondary low across
the northern Gulf coast and into the Southcentral interior Tuesday
night. This system is expected to bring rain to much of the
southern mainland as it crosses to the interior Tuesday night
into Thursday, with the focus remaining on higher rainfall amounts
surrounding the Prince William Sound area. Gusty winds are also
expected to ramp up along the northern Gulf and Cook Inlet region
as this system tracks through the Gulf and into the southern
interior Tuesday into Wednesday.

Further West our focus shifts to the remnants of Typhoon Sanvu
quickly tracking along the Aleutian chain through Thursday. This
system will bring rain and gusty winds to much of the area through
the later half of the week, as it moves along the southern Bering
and eventually back into the North Pacific by Thursday. At this
time, a ridge begins to build behind this system from west to
east. However, models continue to struggle with the amplitude of
this feature as yet another North Pacific low is anticipated to
quickly move toward the western Aleutians for the weekend. The
forecast was primarily influenced with the ECMWF through Friday,
then trends through the weekend were nudged using blended ensemble
means.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 135.
MARINE...Gale Warning 119 125 130 172 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...KH

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1116 AM AKDT Mon Sep 4 2017

AKZ101-125-050730-
Anchorage-Western Prince William Sound-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Whittier, Seward, Girdwood, and Moose Pass
1116 AM AKDT Mon Sep 4 2017

...Rain and wind return to the forecast with developing fall
storm system...

An early season fall storm system will bring strong winds and
rain from Portage Valley through Turnagain Arm and the Anchorage
Bowl Tuesday into Tuesday night. Uncertainty still exists with the
exact track and strength with this system, but recent trends
suggest a stronger system with a track over the Kenai Mountains
into the western Susitna Valley. This favors strong winds from the
Hillside into the Anchorage Bowl.

What does this mean for the forecast? Confidence is increasing in
rain overspreading most of Southcentral Alaska Tuesday afternoon,
continuing into Tuesday night. The wind forecast is a bit more
tricky, where the most forecast uncertainty exists due to the
potential differences in storm track/strength. There is higher
confidence in getting gusty winds along the north Gulf Coast,
through Passage Canal/Portage Valley and Turnagain
Arm/Hillside/Anchorage Bowl, however the upper magnitude of these
winds is still uncertain. Stay tuned to the forecast for updated
information.