Small Craft Advisory 691, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
312 PM AKDT Sat Sep 16 2017

.ANALYSIS AND UPPER LEVELS...
A deep vertically stacked low is weakening as it tracks eastward
across Bristol Bay and the Alaska Peninsula. A strong low level
jet persists on the back side of the storm from the eastern Bering
Sea southward across the eastern Aleutians and southern Alaska
Peninsula. Winds have peaked across this region and are steadily
diminishing. As flow shifts from west to west-northwest this
afternoon there may be a bit of an uptick in winds through north-
south oriented bays and passes along the Alaska Peninsula.

Out ahead of the low an elongated occluded front has dissipated
over Southwest Alaska, with just some patchy light rain remaining.
A triple point low has formed over the northwestern Gulf with a
robust occluded front tracking northward across the Gulf. This is
leading to strengthening winds along the north Gulf coast, Prince
William Sound and Cook Inlet. Rain has overspread Prince William
Sound and much of the Kenai Peninsula, with just some spotty light
rain as you head north across the remainder of Southcentral.

Meanwhile, a moderate strength low over the northwest Pacific is
passing south of the Aleutian Chain. At this point, it is too far
south to produce any impacts, so conditions are very quiet along
the central and western Aleutians.

&&

.MODEL DISCUSSION...
Model guidance is in excellent agreement over the next couple days
and forecast confidence is generally high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The triple point low has moved just off the Eastern Kenai
Peninsula with the associated weather front positioned over the
northern Gulf accompanied by a barrier jet over the aforementioned
location this evening. This is bringing widespread rain with
gusty southeasterly to easterly winds to the northern coastal
communities. While the Anchorage Bowl remains under the influence
of somewhat drier conditions due to the cross-barrier flow.
Meanwhile, the frontal boundary will weaken before dissipating by
late evening tonight. By Sunday the triple point low will have
phased in with the main low pressure system that tracks over
Kodiak Island. This system continues to track southward before
exiting into the North Pacific Ocean by Sunday afternoon.
Therefore, Sunday is shaping up for drier conditions with a
weaker gradient resulting in weak winds throughout the
Southcentral Alaska region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The low near Bristol Bay will continue to track east towards the
Gulf as it weakens. This will diminish showers in the area
slightly until a trough swings through from the northwest Sunday
afternoon brining more rain to the area that will last into
Monday, moving further inland over time. Overnight tonight, light
onshore flow and moist conditions near the surface will allow for
fog formation for locations near the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As the low near Bristol Bay pushes off to the east, the gradient
over the eastern Bering and Alaska Peninsula will weaken causing
the winds over the area to diminish as well. Even though the winds
are on the downward track, gale force winds are expected for the
marine zones south of the Alaska Peninsula into Sunday with higher
gusts out of bays and passes. Out west, high pressure is building
in over the western Bering while a new system is approaching the
Western and Central Aleutians from the south. The ridging will
stall the approaching system before making it to the islands with
only a small increase in winds expected.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Monday night, there will
be two mobile shortwave troughs moving in northwest flow, one
diving into the Gulf of Alaska and another moving off Eastern
Siberia. There will be a cutoff low in the North Pacific south of
the Western Aleutians with weak high pressure over the southern
Bering Sea. The low moving into the Gulf of Alaska will have
enough moisture and difluent upper level flow for areas of light
rain and showers over most of Southcentral and the Alaska Range.
Eventually, the Gulf low will quickly move southeast Tuesday, with
offshore flow drying out most of Southern Alaska.

By Wednesday, the aforementioned mobile shortwave moving east from
Eastern Siberia will begin moving onshore along the western Alaska
coast. Much uncertainty develops by this point, but it seems the
brief quiet weather pattern will briefly shift back to wetter and
more active as a North Pacific front moves north and merges with
the northern stream low. This should bring in a more southwest
flow regime late Wednesday night into Thursday with increasing
chances of rain as the trough digs and strengthens over Southern
Alaska. Eventually this progressive flow pattern is expected to
amplify by the end of the week as a ridge builds over the Western
Bering Sea. This will keep cooler temperatures and light shower
threats across Southern Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 132 150 155 165 170 171 172 179.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...JA