Small Craft Advisory 714, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKDT Fri Sep 27 2019

.ANALYSIS AND UPPER LEVELS...
A ridge of high pressure currently extends from the Gulf through
eastern portions of the state. Further west, an area of low
pressure continues to spin over the western Bering, with its
associated surface front approaching the coast of southwest
Alaska.

&&

.MODEL DISCUSSION...
The models remain in good agreement through the weekend, before
diverging some for the start of next week. The main discrepancy,
as shortwave energy dives into the base of a Bering Sea trough
late weekend, is where does the new low form near Kodiak and what
track does it take (closer to the coast versus more of an
easterly component through the Gulf). The path this system
eventually takes affects who gets how much rain as we head into
the Monday night/Tuesday time frame. As is typically the case,
model uncertainty increases as we head through the remainder of
next week.

&&

.AVIATION...
PANC...Light winds with VFR conditions will prevail through the
forecast period, with some showers approaching the area Saturday
afternoon, ahead of an incoming warm front.

&&

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

A pattern change to wet, windy weather is on tap this weekend in
Southcentral. The ridge over Alaska is slowly weakening and
shifting eastward, and high clouds are moving in upstream of a
Bering trough. Later tonight, a surface low will form several
hundred miles south of the eastern Aleutians, under a 140kt jet.
The low will quickly track northeast toward Bristol Bay, and sweep
a warm front into the Gulf.

Expect precipitation on Kodiak Island by late Saturday morning...
then reaching the Kenai Peninsula and north Gulf coast Saturday
night, and inland early Sunday. Increased east winds will
accompany the front, with gales likely over the Barren Islands
into Kamishak Bay by Saturday afternoon. Southeast winds will
impact the usual gaps as well, including Turnagain Arm, Passage
Canal and the Copper River Valley late Saturday night into Sunday.
While strong winds are likely along the Anchorage Hillside,
models continue to indicate a surface low developing in the
southern Cook Inlet. If this comes to fruition, strong gusty winds
should be diverted away from lower elevations in Anchorage.
However, the latest runs still struggled with this feature and a
slight change in progression could cause stronger winds to mix
into town.

East winds will probably cause some initial downsloping along the
western Kenai Peninsula and Anchorage Bowl, however deep
southwest flow aloft should overcome downsloping. With the
moisture fetch behind the warm front reaching into the subtropics,
snow levels over Southcentral will generally be above 4500 ft.
There could be periods of heavier rain in Seward with this system.
The other area that will see heavier precipitation is the
northern Susitna Valley in upslope flow.

Precipitation will diminish Monday as the steering low exits to
the north, and a trailing cold front will approach the Gulf from
the west.

&&

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

A large upper level trough over the Bering will be a key player in
steering a North Pacific low over Southwest this weekend. In the
meantime, rain and gusty southerly flow along the Kuskokwim Delta
coast will diminish this evening as a front associated with a
broad low in the Bering moves inland. Behind this front, a North
Pacific low arrives over the Alaska Peninsula for Saturday
morning, bringing gusty southerly flow and moderate to heavy
rainfall. As the low lifts northward, a ridge of high pressure to
the east will aid to strengthen winds over the greater Bristol Bay
area Saturday night into Sunday. Widespread moderate to heavy
rainfall is expected across Southwest for most of the weekend. As
the low continues to move toward the northeast, precipitation
will taper to scattered showers by Monday.

&&

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

Cloudy skies and scattered showers will persist over much of the
western and central Aleutians through Sunday thanks to a broad
area of low pressure centered over the northwestern Bering. A
second low arrives over the eastern Bering from the North Pacific
for Saturday. Expect heavy rainfall east of Unalaska Island with
gusty southerly winds. As the low moves inland, wind and rain are
expected to taper by the end of the weekend with generally
westerly flow expected across the Bering by Sunday night. The next
inbound front arrives over the western Aleutians Monday afternoon.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...
All signs point toward a strengthening low in the Gulf on Monday
through Tuesday, though it's hard to say where in the Gulf it
will be located. Based on the expected strength and size of the
storm, widespread gale force winds appear likely, with some
potential for areas of storm force winds. Winds will come down
on Wednesday as the storm exits to the eastern Gulf. The Gulf
storm could also produce gale force winds along the Alaska
Peninsula. Otherwise, don't see any impactful storms across the
Bering Sea/Aleutians through the period.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Thursday)...
A deep low will be positioned in the Gulf of Alaska Tuesday,
though confidence in it's location is low. There is pretty high
probability of strong winds and moderate to heavy rain along the
Gulf coast and Kodiak Island. The low position will affect whether
the rest of Southcentral Alaska see anything out of this. In any
case, the low will exit eastward on Wednesday, with drier
conditions moving in.

Upper level flow will become quite progressive behind the initial
Gulf low, with a trough driving eastward out of Russia Tuesday
and Wednesday and reaching the Gulf Thursday and Friday. Models
disagree on timing and amplitude of this trough and display poor
run-to-run consistency so overall forecast confidence is low. In
general, expect a "weakish" low and frontal system to cross the
Bering Sea Tuesday and Wednesday, with rain spreading into
Southwest Alaska late Wednesday. All of this will then shift to
the Gulf and Southcentral Thursday or Friday. There is agreement
that the trough will amplify in the Gulf as a ridge builds
upstream in the Bering Sea ahead of recurving tropical systems
over the west Pacific. Thus, would expect good potential for
another round of heavy rain and strong winds along the Gulf coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 120 130 131.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...MM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...SEB