Small Craft Advisory 713, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
458 PM AKDT THU SEP 8 2016

Update to Southcentral

.ANALYSIS AND UPPER LEVELS...
A closed upper level low lies over Southwest Alaska this morning,
connecting with a deep longwave trough that extends over the
Northern Pacific. This trough is providing the upper level support
for a developing surface low that is currently moving into the
Gulf of Alaska. The air mass currently over the mainland is fairly
moist in the low to mid levels, and combined with the upper level
instability showers continue to persist over southcentral this
morning. The showers include some moderate rain, but are falling
apart rather quickly so reductions to visibility across the area
are brief. Out west, some lingering stratus along the Alaska
range is also indicative of these higher moisture levels, but
daytime heating is creating some breaks in the stratus over the
Bering, resulting in some relatively benign weather over the
forecast area before the next pattern change.

&&

.MODEL DISCUSSION...
Some interesting minor differences in the short term with the
amount of moisture moving into the Southcentral area and how
closely the upper level lows phase together. This could make the
difference in whether rain falls at certain locations,
specifically Anchorage. GFS seems to be the outlier, phasing the
lows more closely and pulling in moisture from the system in the
Gulf, while GEM keeps the two upper levels completely separate and
EC/NAM lie somewhere in between. The EC/NAM intermediate solution
was chosen, but this bears watching with satellite data to see if
either EC or GFS verifies better.

&&

.AVIATION...
PANC...SHRA expected to remain around the area for the next TAF period.
Model resolution is poor as to when they might move over PANC.
Mod rain may fall but any reduction to vis will be very
short lived. Gusty northerly winds will begin at the end of the
TAF period but continue through the day tomorrow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A closed upper low centered over the Western Alaska Range and Cook
Inlet, along with a rather unstable environment over the North
Gulf, is aiding ascent and a widespread region of showers
stretching from the Susitna Valley southward. A North Pacific
shortwave trough and a very strong warm front (aided by the
remnants of post-tropical Lester) moving toward the Eastern Gulf
of Alaska will partially phase with the Cook Inlet low, setting in
a rather impressive early season offshore flow event. These
northerly gap winds will increase dramatically late tonight and
peak Friday from the Barren Islands eastward all along the North
Gulf coast. Rapid inland drying will develop Friday with warming
temperatures aided by downslope warming. Offshore winds will
weaken quickly Saturday as high pressure aloft builds in and the
surface pressure gradient weakens. Saturday looks to be dry and
bluebird region wide.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Showers associated with the departing upper level low will be mainly
east of a line from King Salmon to Aniak tonight. On Fri these
showers will be limited to the AK and Aleutian ranges. Dry high
pressure will continue to build over the area through Sat.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Deepening low pressure well south of Attu will bring mainly gale
force winds to the Aleutians as it moves east later tonight
through Sat. A small area of storm force winds will affect waters
near Amchitka Fri night. This system will also be accompanied by
significant rainfall over the Aleutians through the period.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The Southcentral Alaska region has an upper level closed low
located over the Southern Cook Inlet. This synoptic feature tracks
into the Southeast Panhandle by Saturday morning before exiting
the region by Saturday evening. Look for a scattered showery
regime, and the precipitation confined mainly to the northern
coastal communities. While, an upstream ridge builds into the
Southcentral Alaska by Saturday morning resulting in drier
conditions with sunny skies through Sunday morning. Meanwhile, the
models are depicting a low pressure system tracking into the
Bristol Bay by Sunday afternoon with the associated weather front
tracking into the eastern Gulf of Alaska. This produces strong pressure
gradients with gusty southeasterly winds through the Barren Islands
as this frontal system moves northward. Therefore, look for
abundant moisture streaming into the Southcentral Alaska region as
the frontal boundary moves through the Prince William Sound, and
the Anchorage Bowl by Sunday afternoon. This synoptic feature
coincides with gusty Turnagain Arm, and gap winds before tapering-
off heading into Monday. Meanwhile, the Bering Sea has high
pressure over the region, and a North Pacific Low south of the
Western Aleutians that tracks eastward along Aleutian Chain. This
system produces gale force easterly/northeasterly winds along the
Aleutian Chain, and the lower Bering before this systems elongates
into two low pressure systems by Sunday. Therefore, the central
Aleutians/Bering will transition to small craft advisory northerly
winds by Sunday accompanied by a showery weather regime pattern.
This synoptic feature transitions into a broad area of low
pressure over the eastern Bering before moving inland over the
Kuskokwim Delta region heading into Monday. By Monday evening the
next Bering low tracks into the West Aleutians, and moves along
the same pattern as previous system before being located near the
Eastern Aleutians by the midweek time-frame. This will keep the
temperatures in the upper 50s along the chain, and lower 60s in
the Southwest/Southeast Alaska coastal communities.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 177. Gales 130 131 172 173 174 175 176 178 351 352
                          413.

&&

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