Small Craft Advisory 661, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
355 AM AKDT Tue Sep 5 2017

.ANALYSIS AND UPPER LEVELS...
As the low south of the Alaska Peninsula weakens and tracks off
to the south, a new low is developing along a north/south
oriented baroclinic zone that stretches across the Gulf. This low
is moving north towards the Gulf as it strengthens. Out west,
weak ridging separates the low south of the Alaska Peninsula from
another low near the Western Aleutians. This low is continuing its
easterly track towards the islands.

&&

.MODEL DISCUSSION...
The models are now in decent agreement with the two main lows
moving through the area. The previous runs had a low tracking
through the Gulf towards the Barren Islands on Tuesday faster
than the current runs. However, they are all keying in on this
slower track now. The difference of the new runs compared to the
old ones does not change the impacts much, but does slow down
the timing of those impacts slightly.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
morning before northerly winds begin to pick as a low over the
Gulf tracks northwest towards the Barren Islands sometime around
noon. Wind shear is expected by late afternoon as winds out of
Turnagain Arm pick up out of a southeasterly direction while
surface flow stays out of the north. Overnight, surface winds will
shift to a south/southeasterly direction and be more in line with
the southeast winds overhead. This will limit the wind shear but
surface wind speeds will increase overnight until gradually
tapering off Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A rapidly developing surface low, embedded in a broad baroclinic
zone and under favorable upper jet support, heads northwest across
the Gulf. This system will intensify as it moves to the Barren
Islands late this evening, then weaken as it heads into Southwest
Alaska early Tuesday morning. The primary impacts will be wind and
rain. Gales to local storm force winds will develop across the
northern and western Gulf this afternoon in advance of the low and
associated front. Marine winds should weaken early Wednesday
morning as the low moves into Southwest ALaska. In addition to
marine winds, strong gap and local winds will develop through/near
channeled terrain (Turnagain Arm/Portage Valley/Matanuska
Valley/Knik Valley)this afternoon and should peak late tonight and
slowly diminish through Wednesday morning. However, brisk south
winds which develop across the Copper River tonight will continue
into the early evening hours Wednesday.

Moderate to locally heavy rainfall will also accompany this
system with the brunt of the rain along the North Gulf Coast and
southern and eastern Kenai through tonight. This will be a fast
moving system which should limit the duration of heavier rainfall,
so there are no flooding concerns at this time.

&&

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

Winds and rainfall will continue to diminish across Southwest
Alaska today as the low pressure system that brought the wind and
rain dives south into the North Pacific. A separate area of low
pressure will move near Kodiak Island late this afternoon into the
evening and quickly push across Southwest Alaska on Wednesday.
This will bring a period of moderate to heavy rainfall to a good
portion of Southwest Alaska as the low pushes through the region
but should taper off quickly on Thursday as the low continues to
move north. Some gusty winds will be observed along the Alaska
Range on Wednesday but gusty winds don't really seem to pose a
threat for Southwest Alaska with this system as antecedent low
pressure across the eastern Bering Sea will prevent a compact
pressure gradient from developing.

&&

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

The remnants of Sanvu will continue to drift eastward along the
Aleutian Islands through the forecast period. The low is expected
to continue to gradually weaken which will allow for winds
associated with the remnant low to weaken over time. Some gale
force winds will be observed near Shemya through this evening but
other than that winds will likely remain less than 30 kts. Some
fog has developed near the Pribilof Islands underneath the ridge
axis but this should dissipate during the morning hours as
cyclonic flow aloft returns to the region and the subsidence
inversion erodes.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast beginning Wednesday evening remains in an
active pattern through the weekend. Lingering rain showers along
the Gulf coast and southern interior will persist during this
onshore flow pattern through Thursday. Out west, the remnants of
Typhoon Sanvu tracks across the central Aleutians and into the
southwest Gulf by Thursday night, spreading gusty winds and rain
along the Aleutians and Alaska Peninsula Wednesday into Thursday.
This system will then traverse the Gulf and bring another round of
showers to Kodiak island and the Gulf coast Thursday into the
weekend. High pressure will build over the western Aleutians and
Bering on Thursday and spread eastward over much of the area by
Saturday. Guidance is struggling with the track and speed of the
next system tracking into the western Aleutians Saturday night,
and progressing further eastward into the Bering Sea by the early
part of next week. Even though there is decent agreement with
regards to the Aleutians, Bering, and southwest mainland seeing
some gusty winds and rain from this system, confidence remains low
on the exact strength, track, and timing of this system.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms...119.
		 Gales...120 121 125 129 130 131 132
				 138 139 140 177 178 351 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...TP

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
954 AM AKDT Tue Sep 5 2017

AKZ111-125-061330-
Matanuska Valley-Western Prince William Sound-
Including the cities of Palmer, Wasilla, Sutton, Chickaloon,
Whittier, Seward, Girdwood, and Moose Pass
954 AM AKDT Tue Sep 5 2017

...Very Windy and Wet Conditions on the Way...

A developing storm system over the southern Gulf of Alaska will
intensify as it skirts south of the Kenai Peninsula tonight.
Easterly winds with gusts of 60 to 70 mph will develop across
Portage Valley and the Eastern Turnagain Arm. Winds are expected
to peak in these areas around midnight before diminishing
Wednesday morning. Strong and gusty winds across the Matanuska
Valley will develop by late this afternoon and should peak
overnight.

Moderate to locally heavy rainfall will also accompany this
system with the brunt of the rain along the North Gulf Coast and
southern and eastern Kenai Peninsula through tonight. This will be
a fast moving system which should limit the duration of heavier
rainfall, so there are no flooding concerns at this time.

In Seward specifically, rain and winds will work in tandem to
cause potential impacts. 2 to 3 inches of rainfall are expected in
about an 18 hour period from this afternoon through Wednesday
morning. South winds will develop tonight ranging from 15 to 30
mph with gusts as high as 45 mph. Wet ground combined with the
presence of leaves on the trees this time of year makes them more
susceptible to being blown over as well.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1017 AM AKDT Tue Sep 5 2017

AKZ101-051930-
Anchorage-
Including the cities of Anchorage, Eagle River, Indian,
and Eklutna
1017 AM AKDT Tue Sep 5 2017

The Special Weather Statement has been updated. Please see the
High Wind Warning for details.