Small Craft Advisory 713, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Sun Sep 24 2017

.ANALYSIS AND UPPER LEVELS...
A low pressure system is currently located over Southwest Alaska
and contains two distinct low centers. The first low is located
just east of Kodiak Island at 993 mb while the other low is
located over Bethel at 994 mb. An occluded front has formed on the
north side of the low and is draped along the North Gulf Coast
down to the Eastern Kenai Peninsula, bringing heavy rain to
locations along the coast. Southeasterly cross-barrier flow is
leading to downsloping across the Anchorage Bowl and the
Northwestern Kenai Peninsula, limiting precipitation in those
areas. Precipitation is also wrapping around the low over
Southwest Alaska bringing rain and showers from the West Coast
east into the lower Kuskokwim River Valley. Further west, a ridge
of high pressure is located to the southwest of the Western
Aleutians and nudging east into the Bering.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement through the first 48 hours
showing the progression and dissipation of the two surface lows
in the Gulf and Bristol Bay. By Monday night, all models depict a
low moving up into the Gulf from the North Pacific. Numerical
guidance begins to differ at this point with the EC being the
outlier as it moves the low center up towards Kodiak by Tuesday
morning while the NAM, GFS, and Canadian models were a bit slower
and further south by Wednesday. Therefore, forecasts leaned
heavily on the GFS/NAM guidance over the EC.

&&

.AVIATION...
PANC...Patchy low stratus and fog will be present in the Anchorage
Bowl through the morning hours due to weakening southeasterly flow
over the Chugach and abundant moisture in the boundary layer from
the previous day's rain. Fog was present as far south as Merill
Field overnight before retreating back north and slowly
diminishing, keeping the Anchorage terminal VFR. VFR countdowns
will prevail through the day as weak southeasterly flow
continues to keep most of the precipitation over the mountains.
Fog and low stratus may develop again tomorrow night as the flow
switches to up inlet.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The weakening low over the western Gulf will continue to bring
rainfall to much of the Gulf Coast, but with a weakening barrier
jet no significant winds are expected. Rainfall over Southcentral is
expected to redevelop one last time from this morning as the upper
level shortwave passes overhead with weakening downsloping. One
concern ahead of this shortwave will be with patchy fog that
developed throughout the Anchorage Bowl late Saturday night. This
fog will likely be the most dense and widespread along Knik
Arm/Glenn Highway, with patchy coverage expected elsewhere, but with
some mixing from incoming rainfall widespread dense fog is
however not expected. Rain and fog is then expected to break up by
this afternoon as the upper level trough lifts to the north,
leaving cloudy skies and the chance of a passing shower by this
evening. Weak cyclonic flow aloft will keep generally cloudy skies
throughout the area into Monday, however most of the shower
activity should be confined to the Gulf Coast. There is the good
possibility that fog/stratus will redevelop throughout Upper Cook
Inlet tonight, especially as flow shifts to light southwesterly.
The good news is that persistent upper level cloudcover should
limit any significant or widespread development, though patchy
areas should be expected again tonight into the Monday morning
commute.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A wet pattern persists through the beginning of the next work
week as a low over the Southwest lifts to the north and another
low slides south across the eastern Bering. Patchy morning fog
develops during the mornings across inland areas as weak
temperature inversions set up at night. Winds across the Kuskokwim
Delta are expected to prevail out of the north with winds along
the Valley and Bristol Bay shifting from the south becoming east
to northeast Monday night as a North Pacific low moves closer to
Kodiak Island.

&&

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

A wetter regime ensues across the eastern Bering and Alaska
Peninsula as a low pressure system moves south along the eastern
waters. This slow moving system is expected to cross over the
Peninsula by Tuesday morning on its track to the North Pacific. A
ridge holds across the western Bering through early week
advecting a cooler air mass along northerly flow. The central
Bering and eastern Aleutians will see influences from both
systems as low pressure across the eastern Bering takes a slight
westward shift...primarily the pattern stays on the drier side for
this area with increasing chances for rain as the low nears the
Peninsula.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Confidence continues to increase in regards to an area of low
pressure quickly moving into the Gulf of Alaska from the North
Pacific on Tuesday. The ECMWF is the fast outlier currently but it
now looks like gale force gusts could be expected near Kodiak
Island and the northern gulf Tuesday afternoon through Wednesday
afternoon as a front associated with the low pressure quickly
moves through the region. Rainfall with this system will upslope
along the coastal mountains but there is bigger question for
interior locations and it really depends on the low track.
Currently it looks like inland areas like Anchorage and Palmer may
miss out on much of the precip on Wednesday.

Looking at the big picture from late next week through the
weekend, it appears the wet pattern across Southwest and
Southcentral Alaska will remain the norm. A jet streak on the
backside of an upper level low over Southwest Alaska on Tuesday
will help amplify the longwave trough across the state of Alaska.
This will keep cyclonic flow across the region with colder air
being advected south which should keep increase instability and
keep rain showers in the forecast for much of the state. There
could be a few breaks with some shortwave ridging across Southwest
Alaska over the weekend but zonal flow will return to the state by
late Sunday and allow for another unstable pattern to develop
across the state. Afternoon high temperatures look to remain below
normal with low temperatures across the region remaining near
normal. The majority of the Bering Sea will be under the
influence of high pressure come Tuesday with low stratus and fog.
By late Friday, a strong low will push off of Russia and into the
Bering Sea which will force the high south into the North Pacific
and a rather wet and windy period will be observed across the
Bering through the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121 125.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...MO