Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 250310

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
710 PM AKDT Mon Jul 24 2017

.PREV DISCUSSION... /issued 450 PM AKDT Mon Jul 24 2017/

A strong blocking ridge is centered south of the Western Aleutians
with low amplitude (nearly zonal) flow over top of this ridge
across the Bering Sea and Mainland Alaska. The main feature of
interest today is an old elongated frontal boundary which extends
from the Kuskokwim delta southwestward across the eastern Bering
Sea and down to the Eastern Aleutians. This is producing widespread
clouds and a narrow zone of rain just ahead of it. However, an
upper level wave crossing the Bering is helping to reinvigorate
the southern end of the front, resulting in increased rainfall.
Fog and stratus continue to dominate the Aleutians and much of
the Bering Sea.

A separate short-wave crossing the Alaska Interior is just
clipping the northern tier of Southcentral, with light rain over
the northern Susitna Valley and Talkeetna Mountains along with
portions of the Alaska Range. A marine layer is firmly entrenched
along much of the Gulf coast and Kodiak Island, with just hints
of some lower clouds in Cook Inlet. In between, variable clouds
extend across the rest of Southcentral

Model guidance is in good agreement through Tuesday night.
Differences then evolve with track of closed upper low as it dives
southeastward across Southwest Alaska and into the western Gulf
Tuesday night through Wednesday. This will affect timing of rain
over the Susitna Valley-Cook Inlet-Kodiak Island corridor. For
now, will utilize a mean position of the 12z models.

PANC...Ceiling will gradually lower over the next 12 to 24 hours
as weak upper wave and surface front approach from the west.
Expect to bottom out at marginal ceilings, though some patchy
IFR clouds could make there way up Cook Inlet to the terminal.

A weak front that is pushing through southwest Alaska towards
Southcentral Alaska will be reinforced by a shortwave tonight
helping to bring rain to Southcentral. The majority of the rain
will stay over the mountains with some light rain at lower
elevations. Then on Tuesday night a low will drop down from the
north and skirt the western edge of Southcentral Alaska. As the
low passes it will bring more widespread rain to Southcentral
Alaska Tuesday night into Wednesday. Then it will track into the
Gulf Wednesday evening.

A frontal boundary continues to weaken and track into the
Southwest Mainland this evening, keeping some light showery
conditions over the area this evening. Dominant onshore flow along
with a short-wave disturbance moving in aloft overnight tonight will
keep ample moisture and renewed dynamic lift over the area. This
will act to re-invigorating the precipitation over the area,
keeping it in the forecast through wednesday afternoon. Look for
heavier precipitation in upslope regions aided by orographic lift.

Areas of fog are also expected to develop and move into areas
along the southwest coast, under the persistent onshore flow, a
weak surface low developing along the Western Capes Tuesday
morning, and ample amounts of moisture at the surface. Therefore,
look for the marine layer stratus to advect into the AOR, and
areas of dense fog to form over areas surrounding the Kuskokwim
Delta and Bristol Bay coasts.

The western Bering and the Aleutian Chain remains under the
influence of a high pressure ridge, which is accompanied by
marine layer stratus/fog. A front is tracking across the eastern
Aleutians/Bering and Alaska Peninsula this evening through
Tuesday morning. The biggest challenge remains with how widespread
the marine layer stratus/fog will be and how it will impact the
communities along the Aleutian Chain and Bering Sea. The Western
Aleutians will likely have dense fog issues in the morning time-
frame as the high pressure builds into the Bering by Tuesday
morning. Persistent zonal/westerly flow continues to support the
marine stratus/fog to continue advecting into the central Bering
region. Another wave of precipitation will round the top of the
Bering Sea ridge over the Northwest Bering and track over the
central/eastern Bering and Pribilof Islands by Wednesday morning.

LONG TERM FORECAST (Days 3 through 7).../updated at 700 PM AKDT
Mon Jul 24 2017/

The forecast for Thursday and into the weekend continues to be a
challenging one. The upper levels across Southcentral will be
dominated by a low that will practically anchor itself in the
southern Gulf of Alaska. At the surface, a weak ridge of high
pressure will try to nose into Prince William Sound this weekend,
increasing the chances for Turnagain winds. The overall pattern is
a bit of a sloppy one. With an influx of moisture and persistent
onshore flow, coastal areas should see showers leading into the
weekend. The Anchorage area may be spared with the usual
rainshadow/downsloping - but it is still too early to hone in on
those specifics. Long range guidance is beginning to hint at
another ridge of high pressure expanding across Canada and into AK
beyond the forecast period (e.g. next work week) meaning dry and
warm weather could be on the rebound. Out west across the Bering
and Western Alaska, continued summertime stratus dominating the
forecast. The biggest weather player of note is a well agreed upon
system that will dive towards NW Alaska from Siberia Thursday,
bringing a large area of rain with minimal thunder chances. All
guidance members are in excellent agreement with this feature and
forecast confidence into the weekend is high.




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