Small Craft Advisory 655, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
905 PM AKDT Tue Sep 10 2019

.ANALYSIS AND UPPER LEVELS...
The afternoon analysis map is dominated by a few key features.
The first is a significant ridge across NW Canada that has been
steering and stalling systems moving in from the west. Across
southcentral, is a weak trough of low pressure that is allowing
scattered showers this afternoon. As of the noon hour, this
trough can be noted on satellite as a SW to NE oriented band of
clouds draped along the Alaska range to our west. Farther west in
the central Bering resides an area of low pressure that continues
to deepen. This area of low pressure and associated front are
beginning to merge with an ex-tropical system with a deep fetch of
moisture.

&&

.MODEL DISCUSSION...
Synoptically speaking, models are in good agreement in the first
few days. In the upper levels, the story all guidance is
suggesting remains similar, but there are timing and strength
differences towards the end of the week in handling the previously
mentioned Bering low. At the surface, guidance is in great
agreement through Thursday morning. Models start to show
differences as the newly merged ex-tropical system phases with
the front that is attached to the Bering low. All guidance spins
up a triple point low that traverses Kodiak and lifts to the
northeast, but differences begin to appear with how the surface
features of the main low shift southeast. The Canadian, NAM and
GFS are all in lock step this afternoon moving it towards Bristol
Bay by Friday morning. The ECMWF is the fastest, by 12 hours and
is weaker by a few millibars.

&&

.AVIATION...
PANC...MVFR to VFR conditions are expected to continue through the
evening hours. Cloud bases will float between VFR and MVFR with
passing showers. Shower activity should wane by the evening hours
giving way to VFR conditions overnight. Winds will remain light
and variable through the forecast period.

&&

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

Areas of showers across interior Southcentral will diminish
overnight as an upper-level trough lifts through the region,
replaced by a weak, transient ridge. With the ridge cresting
overhead Wednesday, conditions should be mainly dry and rather
mild for many locations across the region, albeit with some patchy
early morning fog possible from Talkeetna to Kenai. The exception
to this will be Prince William Sound where an inverted trough
will spin up a compact area of low pressure that will lift toward
the coast from the northern Gulf. This system will bring areas of
rain from Whittier, to Valdez and Cordova through through the day.

The bigger weather concern, a strong frontal system, arrives in
the western Gulf overnight Wednesday and lifts into the northern
Gulf on Thursday. This system is attached to a deep Bering low
that is entraining tropical moisture from the remnants of typhoon
Faxai. Given the tropical moisture feed, this feature is expected
to bring some of the heaviest rains and strongest winds we have
seen in a couple of months. Rain will first overspread Kodiak
Island late Wednesday, lifting north by Thursday morning. Rain
will also spread across eastern Kenai Peninsula overnight
Wednesday into early Thursday, due in part to the westward track
of the compact low in Price William Sound as it phases with the
larger frontal system. The rain then makes it into Prince William
Sound by midday Thursday. Rainfall will be heavy for an extended
period of time along the coastal areas, especially for locations
such as Seward and Whittier where 2 to 4 inches of rain could fall
(with even higher amounts along the upslope side of the coastal
ranges). Conversely, a strong downsloping effect from the cross-
barrier flow will make for lower precipitation totals inland.

Gusty southeasterly gap winds and marine gales will also develop
in advance of the front, peaking during the day on Thursday across
Turnagain Arm, the Anchorage Hillside, the Knik River Valley, and
along the Matanuska Valley. With a secondary low expected to
develop near Kodiak Island, the southeasterly gap winds around
Anchorage should bend down inlet, keeping the strongest of winds
out of Anchorage, but ramping up winds across the Matanuska
Valley. Southerly winds will also be on the increase across the
Copper River Valley later on Thursday as the front pushes closer
to the coast.

Winds will diminish overnight Thursday into Friday with showers
lingering along the coast and higher elevations of the interior
as the front pushes onshore and weakens.

&&

.FIRE WEATHER...

The Copper River Basin will continue to be the primary fire
weather concern. Temperatures will remain in the 60s while minimum
RH values slowly climb higher each day into the 40-50% range.
Gusty southeast winds will develop in the river valley again Wed
afternoon. For other locations, showers will begin to diminish
overnight across the interior while areas of rain develop across
Prince William Sound for Wednesday. A strong frontal system then
moves over the Southcentral Mainland with rain and gusty
southeasterly winds for late Wednesday through Thursday. While
rain will be likely over inland areas, amounts will generally be
less than a tenth of an inch through Thu night.

&&

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

Weak ridging builds in this evening which will allow for clearing
skies and possibly some local patchy fog and cool temps.
Attention quickly turns to an impressive fall storm which will
impact the region Wed and Thu (and will also be reinvigorated by
former Typhoon Faxai), bringing a swath of strong south-
southeast winds and heavy rain, especially along the coast where
some gusts to 60 MPH are possible. Some very minor coastal erosion
is also possible across parts of the Kuskokwim Bay with both the
front, and then the back- end southwest winds. The front shifts
east Thu night with the low slowly weakening and moving inland on
Friday which will keep cooler and showery weather in place.
However, some coastal erosion may also impact parts of the
southern AKPEN Friday eve, but the track of the low is too
uncertain at this time, and impacts are very marginal at this
juncture in time. Stay tuned to future forecasts.

&&

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

A strong gale force low is wrapping up and strengthening over the
Bering this afternoon. The front will be reinvigorated by former
Typhoon Faxai Wednesday as the front moves east, reaching peak
gale strength (45 knots) by the time it reaches the coast. Cold
air advection on the backside of the low (from the Pribs south)
Wed through Thu night will promote favorable long fetch SW winds
with seas reaching into the 25+ foot range across parts of the
open Bering Sea.

The low moves inland Friday with brief high pressure followed by
another low which moves into the western Bering Sea Friday
afternoon and evening.

&&

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

The forecast is on track for the low in the Bering Sea on Thursday
with its frontal system extending into the Gulf of Alaska. We are
confident in the wide swath of gale force winds in the Bering and
Aleutians, as well and the gale force winds associated with the
front in the gulf. High seas are still expected with the strong
winds. The system will weaken as progresses east across the Alaska
Peninsula Friday night, with just some small craft advisory winds
in the gulf of alaska by Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7/Friday through Monday)...

The long term starts with the strong Bering Sea storm weakening
and exiting the Bering Sea over the Alaska Peninsula. It will
still be strong enough at this point to produce some gale force
winds, but will lose its strength as it crosses land Friday night
and moves into the western Gulf of Alaska. Another weaker low
pressure system is expected to move east through the Bering Sea
Saturday into Sunday, while weak low pressure continues to exist
in the gulf. This means generally cool and cloudy weather with
some rain showers for the weekend. Models do indicate a shortwave
ridge briefly in the gulf on Saturday, so there is a chance of
some clearing, but hedge your bets on mostly cloudy with showers.

Models have come into decent enough agreement over the past 24 hrs
to be confident in this trend, however smaller features like
placement of areas of precipitation still need to be resolved, so
there is moderate confidence in the forecast into the weekend.
To start next week, models are still loathe to agree with each
other so forecast confidence remains low. Right now it looks like
general low pressure for our forecast area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 132 155 160 165 170 172 173 175>177
 179>181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...BJB