Small Craft Advisory 675, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 AM AKDT Tue Sep 12 2017

.ANALYSIS AND UPPER LEVELS...
The current upper level pattern features a ridge prominently over
the 49th state...with the axis bisecting Prince William Sound and
pushing into the North Slope. Upstream there is a vortex over the
Bering Strait with an open wave pushing along the Aleutian
Archipelago with southwest flow from the Pribilof Islands to the
Kilbuck Mountains. The radar over the Kenai Peninsula and the
Anchorage Metro area is quiet and signifying a break between
systems for southern mainland Alaska. While the radar in Bethel is
picking up on shower activity on the western portion of the
state.

&&

.MODEL DISCUSSION...
Models are in sync with the major features of the long wave
pattern through 72 hrs. The run to run consistency streak
continues this model cycle. The higher resolution models ping into
the wind details through the variant terrain of southcentral AK.

&&

.AVIATION...
PANC...VFR conditions will persist through Wed morning, although
CIGS will drop below 5000 feet at times. Southeast winds will
quickly increase this afternoon then peak this evening and overnight
with gusts into to 30-35 kt range. They will remain gusty through
Wed morning.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A pattern change is in store for south central Alaska as a
frontal system approaches from the west. A large low over the
Bering Sea will only slowly move east through Thursday, leaving a
tight pressure gradient along the North Gulf Coast that will
bring strong winds through Turnagain Arm into Wednesday. There
will be some downsloping associated with these winds along the
Chugach, however with the deep and moist southwesterly flow
aloft, rain will still spread into most areas west of the Copper
River Basin by this evening. The heaviest rainfall from this low
is expected to occur from the Kenai Peninsula westward, as ridging
over the Eastern Gulf will limit any significant rainfall from
moving east of Middleton Island through Thursday afternoon. On
Thursday night the low will begin to move eastward, and
precipitation will likewise spread eastward as well.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Areas of rain associated with the frontal boundary are working
their was across the mainland this morning. The diffuse nature of
the front and weak upper level forcing will make rain spottier
over the Kuskokwim Valley and eastern portions of the Greater
Bristol Bay area. Rain will be more steady from Dillingham east as
an upslope component will aid in lift. Steady, more moderate rain
will ramp up late this morning from Bethel west as a jet streak
and disturbance aloft move over the area. These conditions will
move eastward toward the afternoon hours, continuing steady rain
through the overnight in most places. On Wednesday, areas of rain
will continue for most areas, less intense and more occasional as
forcing weakens.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Gusty gale-force southerly winds out of bays and passes will
affect the Alaska Peninsula today as the front continues to
squeeze the gradient. The current low pressure system will slowly
work eastward and weaken through Wednesday, with rain turning
more showery after today.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The main focus for the long term forecast will be on a strong low
pressure system developing in the central Bering Sea on day 4
(Friday). A surface low moving in from the subtropical waters of
the Northwestern Pacific will move over the Western Aleutians and
cross into the Bering Sea on Friday. This low looks to interact
with an upper level system dropping south from the Arctic at the
same time. There have been significant run-to-run and model-to-
model inconsistencies with the strength and track of this system,
leaving forecaster confidence low on the exact details of this
system through the weekend. The GFS solution phases the
subtropical and Arctic lows together leading to a much stronger
low in the Bering compared with other models, with the central
pressure up to 20 mb lower than the EC and Canadian. The overall
consensus right now seems to be a sub 980 low in the Bering with the
system taking a southeast track across the AKPEN and into the Gulf
of Alaska by Sunday. Regardless of the model inconsistencies, there
is high confidence that this system will bring heavy rain and high
winds to the Pribilofs, Central/Eastern Aleutians, and Southwest
Alaska Friday and Saturday with rainy and unsettled weather expected
in Southcentral Alaska Saturday and Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 160 177 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...KVP