Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 211323 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
408 AM AKDT Mon Aug 21 2017

.ANALYSIS AND UPPER LEVELS...

The upper level pattern is beginning to show signs of becoming
more progressive as longwave ridging builds across the Bering
Sea. Impressive isallobaric flow coupled with weak cold air
advection on the back side of the upper level low and building
high pressure has allowed for some 40 to 45 kt winds to be
observed across portions of the Alaska Peninsula. The gradient has
begun to relax early this morning allowing for the stronger gusts
to have diminished but gusts in the 30 to 35 kt range continue
across the Alaska Peninsula. The upper level low that had been
diving south along the west coast of Alaska has rapidly pushed
into the northern Pacific Ocean this morning but the arctic
trough remains thoroughly entrenched across much of the Alaska
mainland. The aforementioned high pressure has begun to nose in
across coastal locations of Southwest Alaska which has helped
suppressed shower activity. Luckily, enough of an offshore
gradient (northerly winds ushering in drier air from the
interior) to keep fog at bay. Elsewhere, widespread showers
continue with a frontal boundary slamming across the northern gulf
coast with copious amounts of moisture being advected inland.

&&

.MODEL DISCUSSION...

Models are once again in pretty good agreement through the short
term with the overall propagation of the synoptic pattern. Models
are struggling the most with how much rainfall will continue to
fall across Southcentral Alaska, especially the interior, and when
this rainfall will begin to move out of the area. Otherwise,
models continue to show surface high pressure moving into
Southwest Alaska throughout the day as a low pressure system
begins to move into the western Bering Sea. Some discrepancies are
evident in exact low placement but with the departing ridge and
building surface low, a gale force front is now progged to develop
by Tuesday morning. High-res models were utilized for the forecast
package today with models being in pretty good agreement with a
shift toward the GFS on day 3.

&&

.AVIATION...
PANC...Ceilings will fluctuate between VFR and MVFR throughout the
day with shower activity lingering through much of the day. A
brief break in showers may be observed during the late morning and
early afternoon hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low over the southwest Gulf will track east
across the southern Gulf today. The surface low that is centered
east of Kodiak Island will be pulled southward toward the upper
low leading to a gradual tapering off of rain over Southcentral
today and tonight, lingering longest along the Gulf coast. Tuesday
will be a warmer and drier day across Southcentral as the Gulf
storm exits and weak ridging slides in from the west. Expect
increasing sunshine from the western Gulf to Cook Inlet (including
Anchorage and the Matanuska Valley) with favorable north to west
flow aloft. Clouds and even some showers are expected farther
north over the Copper River and Susitna River basins as trough
crosses interior Alaska. Increasing southwest flow aloft late
Tuesday night and Wednesday morning will bring an increase in
moisture and some showers to the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2).
With the upper low tracking further south into the north Pacific and
the ridge axis to the west continuing to close in on the western
coast, drier conditions over the southwest Mainland are expected to
continue through this afternoon. As the high pressure tracks further
east and acts to tighten the gradient over the Alaska Peninsula and
Bristol Bay area, expect gusty winds to ramp up this morning and
diminish during the overnight hours.

Stable conditions over the Bering will be conducive for fog to
develop, and with increasing onshore flow today, we are expecting
fog to slowly advect into the Kuskokwim Delta and parts of the
Bristol Bay area tonight. The potential for fog will persist over
these areas through Tuesday night, as the onshore flow continues
to push low level moisture inland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure will continue to build over the Bering Sea,
resulting in widespread stable conditions and fog development
over much of the area. The caveat being the eastern Bering and
Alaska Peninsula, where a tight pressure gradient will set up and
produce gusty gap winds to the marine areas south of the Alaska
Peninsula. These Gale force winds will peak during the day today
and begin to diminish during the overnight hours into Tuesday
morning.

Further west, a front associated with a low over the north Pacific
will begin to push into the Western Aleutians from the southwest
this afternoon. This system will bring more rainfall and gusty
winds to the western and central Aleutians by Tuesday morning,
then spread these conditions north and east into the Bering and
eastern Aleutians by Tuesday afternoon.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended period forecast Wednesday night, there will
be a deepening upper level trough over the state of Alaska with a
subtropical warm front at the base of this trough over the North
Pacific. Some of this moisture will be moving north toward the
Gulf, so this system will have some enhanced moisture aiding it.
Rather widespread rain in southwesterly flow aloft will be
impacting most of Southern Alaska through Thursday night. However,
by Friday, the main upper low and associated trough position will
dig into the Gulf of Alaska, so the steadiest rainfall will begin
to shift towards the coastal areas and Bristol Bay while more
showery conditions (instead of steady rain) develop over interior
locations. All the global numerical models depict a similar fate
to the upper trough as it weakens in place over the North Gulf
Coast through the weekend, likely keeping some clouds and shower
threats through the weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale...150 155 175
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA



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