Area Forecast Discussion
Issued by NWS Anchorage, AK

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504
FXAK68 PAFC 220114 CCA
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...
A cut off upper level low has developed in the Gulf of Alaska as
it breaks away from the upper level trough that remains over the
mainland part of the state. The upper level ridge is over the
central Aleutians and protruding into the central Bering Sea. A
jet streak between the ridge and low is bringing increased
northwesterly winds aloft over the eastern Bering Sea, Southwest
mainland Alaska and the Alaska Peninsula.
The Gulf coast and much of inland areas of Southcentral Alaska
remain under persistent shower activity today which should taper
off this evening to overnight as the upper level low moves east-
southeastward and the ridge moves eastward as well.

&&

.MODEL DISCUSSION...
Models remain in fairly good agreement through the middle of the
week. The only real change to note is that they are slowing down
the arrival of the next batch of rain moving into the southern
mainland on Wednesday by six to twelve hours. Models also keep the
train of Arctic troughs digging over the state as the continuing
pattern looks like the classic transition from late Summer into
Autumn.

&&

.AVIATION...
PANC...VFR conditions should prevail through the night as the
northeast winds today keep cigs even higher during the periods of
rainfall. While winds may switch to more of a Southeast direction
overnight, cigs should remain above 5000 ft. A short drying trend
tomorrow will also mean VFR conditions and winds under 10 kt.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Thursday)...

Rain will gradually break up across Southcentral Alaska this evening
through the overnight hours as the upper low marches eastward
across the southern Gulf, pulling the surface low currently east
of Kodiak island with it. Strong and gusty northwesterly winds
will linger through Tuesday along the Alaska Peninsula, Shelikof
Strait, and western Gulf due to persistently tight pressure
gradients between the exiting low and the ridge building in.

Tuesday does not look quite as nice as previous forecasts indicated.
All guidance now indicates a short-wave digging down from the
northwest and crossing inland portions of Southcentral Tuesday
afternoon through Tuesday night, with a weaker wave to follow
through Wednesday morning. This will hold in clouds over interior
areas of the Susitna Valley and Copper River Basin and produce
showers, especially across the northern to eastern Copper River
Basin. The best chance of seeing sunshine on Tuesday is the
western Kenai Peninsula up to Anchorage where westerly downslope
flow will dominate south of the approaching short-wave.

The leading edge of a broad positively-tilted trough will approach
Southcentral from the Bering Sea on Wednesday, leading to increasing
clouds and showers out ahead of it. This trough will quickly head
eastward for the Gulf while another trough drops southward across
the Bering Sea, forcing the leading trough to become negatively
tilted and curve northward toward Southcentral. All of this will
lead to another prolonged soaking rain across Southcentral Wednesday
night through Thursday night - and likely beyond for at least a
portion of the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The bulk of the Southwest mainland looks to remain dry through
Wednesday morning as weak ridging replaces the upper low
departing into the Gulf. The exception will be across the northern
extent of the area as a trough moving across the northern half of
the state clips the Delta region tonight and then the Lower
Kuskokwim Valley on Tuesday, bringing a chance for some light
rain to these areas. There is also some fog potential across the
Delta tonight as onshore flow helps to advect the marine layer
inland. The arrival of a front from the Bering on Wednesday
afternoon will bring the next round of widespread rain to the
area, with rain then continuing into Thursday as yet another
Arctic trough digs into the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Gusty northwest winds will begin to subside overnight along the
Alaska Peninsula as the upper low moves farther away from the
area. This will usher in a period of benign cloudy conditions
through Tuesday across the eastern half of the Bering as high
pressure builds overhead. Farther west, a front will move over the
western half of the Aleutian chain this evening and then slowly
push to the north and east through Wednesday, bringing rain and
Small Craft Advisory level winds to much of the Bering by
Wednesday evening. The parent low will push into the western
Bering by Wednesday, keeping the rainy pattern going across the
Western and Central Aleutians.

&&

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

Beginning the extended period forecast Thursday night, the weather
pattern across the region will be dominated by a large trough over
the state, stretching from the Arctic Circle to the North Pacific.
A highly amplified upper ridge will be in place over the Western
Bering, so this will be a rather slow moving pattern. As one may
expect, such amplified troughing will result in a continuation of
typical Alaska early fall weather with clouds and rain impacting
most of Southern Alaska. On Friday and Saturday, the surface low
will move from eastern Bristol Bay to the Gulf of Alaska.
The forecast gets a little more tricky here as weak pieces of
shortwave energy will meander around the larger upper trough, so
it seems that there may be waves of rain instead of continuous
rainfall, but it will still be wetter than drier, especially
across Southcentral and the Gulf Coast as Southwest AK slowly
dries out Sunday.

By Monday, the pattern will become more dominated by a zonally
driven North Pacific jet stream which will cross the southern
Bering Sea and eventually amplify as it moves south of the Alaska
Peninsula. This should result in a brief break in the weather
(note, rainfall) across Southern Alaska sometime early next week
before troughing over the North Pacific moves back in sometime
Tuesday and Tuesday night. In other words, it seems that summer is
effectively over...or...it is the beginning of the new season.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JA



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