Small Craft Advisory 379, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
813 PM AKDT Wed May 9 2018

Updated Long Term

ANALYSIS AND UPPER LEVELS...
The main features on the weather map remained unchanged from
previous days, with a large low spinning in place over the central
Bering Sea and high pressure over western North America trying to
nose into the eastern interior portions of mainland Alaska. This
is perpetuating the cloudy and wet (except for the usual downslope
areas) southeast flow over most of our forecast area. The front
associated with the Bering Sea storm is slowly moving through the
area today, with the back edge of the steady rain just past Kodiak
Island this afternoon. The winds are not particularly strong with
this front, so orographic enhancement of precipitation along the
coast is not in play as much as it often is, so rainfall rates
have been fairly moderate at most locations.

A vigorous shortwave south of the Alaska Peninsula near 45N
latitude will be the next big storm to cycle into position in the
stagnant longwave trough over the Bering. This shortwave will
support a strengthening low pressure system that will impact all
of southern Alaska beginning Thursday.

MODEL DISCUSSION...
The models are in very good overall agreement. There is a bit of
a discrepancy as relates to the timing and placement of a new low
(the one that spins up underneath the shortwave currently near
45N) as it moves into the eastern Bering Thursday evening. The NAM
is the furthest north with the storm center, which would help
adjust the pressure gradient to bring a bit stronger winds into
parts of Anchorage. Because all of the global models are in better
agreement on a further south solution, and because the makings of
this storm are very near the boundary of the NAM model at the
moment, we're generally discounting this solution for the time
being. And, late update, the 18z NAM is trending toward the global
consensus solutions with a bit further southward solution.

AVIATION...
PANC...Cloudy but VFR conditions should continue throughout the
TAF period. A strong southeast wind regime should move in during
the early afternoon on Thursday, and last through the evening
period.

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The frontal system currently stretched from Kodiak Island, north
across the Kenai Peninsula and extending southeast into the Gulf
will continue to gradually weaken as it lifts north tonight, to
stall over Prince William Sound and the northern Gulf Coast. A
stronger storm force front spinning up south of the Alaska
Peninsula will lift north to reach Kodiak Island Thursday morning
and the Barren Islands midday Thursday. The front will slow as it
continues north up along the Kenai Peninsula Thursday afternoon
and evening in response to a triple point low forming along the
front in the southwestern Gulf Thursday evening. Precipitation
rates along the coast look impressive, especially Thursday evening
through Thursday night along the eastern Kenai Peninsula and
western Prince William Sound. Additionally, as this frontal system
approaches, east to southeasterly gap winds as well a the
northern Gulf coast barrier jet will increase rapidly. Once the
the front moves inland on Friday breezy southerly winds push
inland behind it with the strongest south to southeasterly
channeling through the Copper River Basin.

SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There is an upper level low over the Bering and a shortwave has
been kicked out of it this afternoon and is pushing across
southwest Alaska. The net result has been scattered convection
which has been detected at both the Bethel (PABC) and King Salmon
(PAKC) radars. As this shortwave lifts, expect the convection to
move with it. Drier conditions are expected tomorrow except for
the AKPEN. There is a new low south of the Aleutian Chain that is
rapidly closing in on the AKPEN and the surface low will make
landfall somewhere between Port Heiden and King Cove. This storm
will have a lot of gusto as it moves into the AKPEN as a result of
its tight pressure gradient, the orientation of the boundary
layer winds and their alignment with the gaps and complex terrain.
Additionally, there will be a lot of warm air advection with this
new disturbance. By Friday, the moisture plume will be transported
deeper into southwest Alaska.

SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
There is a vortex of low pressure wobbling over the Bering.
Tonight it slowly starts to migrate southward. There is developing
storm south of the Aleutian chain. The shortwave energy
associated with this new storm will track northeastward and
become entrained within the broad cyclonic flow. The new shortwave
rapidly becomes negatively tilted south of the AKPEN which
correlates to the rapid deepening/intensification of the surface
low. This low will become a well-organized system and make
landfall somewhere between Port Heiden and King Cove. The latest
ASCAT pass has not detected any gale force winds yet but highly
confident that another round of gale force winds will materialize
with this system. Gusty winds anticipated in passes along the
Eastern Aleutians as this low closes in Thursday and into Friday.
This storm originated from a warmer region of the Pacific and has
retained subtropical characteristics. The precipitation associated
with this disturbance will be an all rain event.

.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...
The extended portion of the forecast begins Saturday with the
weakening low lingering just offshore of the Y-K Delta coast,
promoting continued showery conditions for much of the southern
mainland and localized gusty winds through favorable gap flow
areas including Turnagain Arm. By Sunday, a potent shortwave
trough rounding the base of the Bering trough will track rapidly
to the northeast Gulf coast and help to spin up a compact surface
low. It appears at this time that the primary impacts from this
feature will be largely confined to the panhandle, but a period of
heavier rain and gusty winds cannot be ruled out along the north
Gulf coast. Forecast details become a little more muddled by the
start of next week, but the general pattern generally looks to
maintain some semblance of longwave troughing over the eastern
Bering Sea and keep southern Alaska under the influence of moist
southerly flow. This will keep Southwest and Southcentral AK in a
generally cloudy and showery regime well into next week.
Meanwhile, a broad North Pacific low will be tracking south of the
Aleutians, keeping the threat for rainy and windy conditions over
the chain for much of the long term while the remainder of the
Bering experiences more benign showery conditions.

AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 130 138
  		 Gale Warning 120 131 132 136 137 150 155 160

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...CB