Small Craft Advisory 720, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKDT Sat Sep 22 2018

.ANALYSIS AND UPPER LEVELS...

A 984 mb surface low west and north of Nunavak Island is moving
northeast with gales over the Eastern Bering Sea this afternoon.
This feature is supported by a stacked upper-level low with the
main long-wave trough still west of 180-degrees. An occluded
front was passing over Kodiak Island and moving into the southern
end of the Kenai Peninsula with a warm front preceding it. The
triple-point was working up the west side of Cook Inlet.

A stiff southwest flow aloft is moving along the AK PEN, over
Kodiak Island with the expectation that this flow will continue
into Cook Inlet over the next period.

Despite recent rains in the region from two-tenths to as much as
1.4 inches at Independence Mine at Hatcher Pass, no significant
runoff issues have developed likely due to an record-setting dry
streak which ended yesterday.

Unrelated to local rainfall, the Skilak Lake Glacial Dam release
has crested and conditions will continue to improve along the
river.

&&

.MODEL DISCUSSION...
The models indicate that the next upper-level wave will intensify
around Sand Point and track northeast. Isolated lightning strikes
identify the region of instability nearing 180-degrees West.

Models were in good agreement yesterday. Even better, they have
improved more today with the NAM coming into better alignment
with other models' timing of the vort max and surface features.
This is most notable with the development of the feature along the
Pacific Coast of the AK PEN toward Kodiak Island.

Local pressure difference forecasts along Cook Inlet indicate
Saturday evening gusts should wane after 03z Sunday. But with the
next wave moving in, wind concerns will quickly become the next
challenge for Sunday night into Monday midday for regions west of
the Alaska Range, over Kodiak Island, and up Cook Inlet.

&&

.AVIATION...
PANC...Generally, VFR conditions will persist through tonight and
into Sunday. There are some isolated showers in the vicinity this
afternoon and some of these showers have contained some small
hail. It is unlikely that any hail will fall at the airport, but
there is some in the vicinity. Winds will hold the potential to
be gusty through the period, with better confidence on stronger
winds and gusts developing through the day Sunday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3,
Saturday night through Tuesday night)...
A frontal system that had brought rain to most of the region
overnight has pushed east this afternoon allowing skies to clear
some, especially from Anchorage south. Cold air moving in aloft
behind the front combined with some modest surface heating will
lead to the development of isolated showers this afternoon and
evening, especially over higher terrain and the Susitna Valley.
Some of these showers may also contain small hail or graupel due
to lowered freezing levels between 4,000 and 5,000 ft. In fact, an
observer in South Anchorage reported small hail this afternoon,
with radar imagery confirming the report.

The front will continue to push north and east overnight as a
weak shortwave ridge builds in over the area ahead of the next
system. This will lead to clearing skies over the southern half of
the area tomorrow, though a good push of southerly winds up the
Cook Inlet will continue through the morning before gradually
diminishing in the afternoon.

Attention shifts to the next system coming into the region as a
surface low rapidly develops over the Northern Pacific and moves
north over the AKPEN and Kuskokwim River Valley through Monday.
The rapidly deepening system will swing a warm front north across
the region Monday, which will be accompanied by heavy rain and
strong winds. Winds will be favored through the typical terrain
gaps, including the Turnagain Arm and Upper Hillside, where gusts
up to 65 mph can be expected Monday morning. The lee (western)
side of the mountain ranges will be initially downsloped with
strong cross barrier flow but rain should fill in quickly as the
front pushes north Monday afternoon. As the front progresses
north, a secondary surface low will form in the northern Cook
Inlet area and move north, this will lead to rapid pressure rises
and subsequent strong southerly winds up the Cook Inlet Monday
afternoon through Tuesday morning. Winds could gust as high as
40-50 mph along the Western Kenai Coast Monday evening. For
Tuesday, dry air will again sweep in behind the departing system
leading to decreasing cloud cover across the region and slowly
diminishing winds.

&&

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

Wet and breezy conditions will continue through the evening,
before the pressure gradient weakens overnight as the Bering low
moves ashore south of Norton Sound while weakening. A moist
onshore flow and disturbances aloft rotating the low will keep
showers going through the period. A separate area of
strengthening low pressure will race up from the north Pacific,
reaching the AKPEN by Sunday evening, then ridge the Alaska Range
on Monday, before stalling as it weakens near McGrath Monday
night. Easterly winds will increase ahead of this system with
sustained winds of 20 to 40 mph possible. This system will bring a
steady and widespread rainfall to much of the eastern half of the
interior, some of which may be moderate to locally heavy at
times.

&&

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

A persistent showery pattern looks to continue across most of the
Bering Sea and Aleutians over the next few days, as disturbances
dive southeast across the region on the back side of a slowly
weakening low along the Yukon Delta/coastal area. Winds will
decrease from low-end gales across the eastern Bering, with small
craft winds along coastal waters of the AKPEN this evening, to
below small craft conditions by morning. The remainder of the area
will see light winds though most of the period.

The exception to this will be for the Bristol Bay region and
coastal waters along either side of the peninsula Sunday night
into Monday, as a rapidly strengthening storm reaches the
peninsula from the North Pacific Sunday evening, riding the Alaska
Range on Monday. This storm will bring gale force winds, seas of
9 to 13 feet, and widespread rainfall (some of which may be
moderate to heavy intensity) to the Bristol Bay and coastal
waters of the AKPEN. Winds and rainfall will then diminish by
Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday Night through
Saturday)...

The models have come into much better agreement for the first few
days of the long-term forecast as compared to yesterday. An area
of high pressure, centered southwest of Adak, will extend
northeastward into Southern Alaska to start out the forecast
Tuesday night. The center of the high will quickly shift across
Southwest Alaska by Wednesday morning. Thus, nearly ideal
conditions for radiational cooling will be in place (clear skies,
light winds), with the exception of any high cloudiness
associated with a building 110 kt jet in the upper levels moving
over southern mainland Alaska. As such, many inland areas have
their first real chance this fall of seeing their first freeze
Wednesday morning. As the average date of the first freeze in
Anchorage is tomorrow, September 23rd, the growing season is
already on borrowed time. The high pressure system will slowly
lift to the north on Wednesday, but most of the area should
continue to enjoy another beautiful fall day. Despite the expected
continuation of mainly clear skies, warming temperatures in the
mid-levels should reduce the frost threat Wednesday night, though
by no means can it be ruled out.

An area of low pressure in the models has been struggling in
recent days as to what it's going to do may be finally showing its
hand this afternoon. For fans of sunny, dry, fall weather, you'll
be happy about the trend. Previously, the models had brought the
associated front all the way to the Gulf Coast and inland, with
rain associated with the low itself overspreading Southwest Alaska
by Wednesday afternoon. Now, the models have shifted south and
west, strengthening the high pressure over the interior. As a
result, the front now looks to struggle even making it to Kodiak
Wednesday afternoon, and the center of the low looks to track west
of the Pribilofs on Thursday, with doubt even the easternmost
fringe of the precipitation affects the Bering coast. Therefore,
high pressure and increasingly amplified upper level ridging
suggest yet another run of gorgeous, dry weather with above
average afternoon highs and warming overnight lows going right
into the weekend for Southcentral. The models try to bring periods
of rain to the coast of Southwest Alaska Friday and Saturday, but
there is a lot of uncertainty in those details. September will
certainly go down in the record books as an unusually warm and dry
month for all of Southern Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 132.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JW