Area Forecast Discussion
Issued by NWS Anchorage, AK

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868
FXAK68 PAFC 210034
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKDT Wed Sep 20 2017

.ANALYSIS AND UPPER LEVELS...
There are two main features impacting the area. The first is a
shortwave moving west to east through the Susitna Valley area
headed towards the Copper River Basin. Most of the rain with this
feature is staying north of the greater Anchorage area. The other
feature is a low south of the Central Aleutians and its
associated front that stretches across the southern Bering and
over the Alaska Peninsula. The front is continuing its track to
the northeast towards the Bristol Bay and Kodiak Island areas.

&&

.MODEL DISCUSSION...
There are several areas that the models are struggling to lock in
on with the low near the Aleutians and its associated front. One
of which is a low forming on the frontal boundary south of the
Kenai Peninsula Thursday. Models are struggling with the timing
and positioning of this low. Then on Friday there are differences
with how much rain to bring to the Kenai Peninsula. Out west there
are issues with the main low late Thursday and into Friday as it
approaches the Alaska Peninsula and eventually moves into the
Bristol Bay area. Timing is the biggest concern for this change.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Clouds will diminish this evening across the Anchorage and Mat-Su
areas as a short wave trough pushes out of the area and a short-
lived ridge builds in behind it. Temperatures will quickly cool off
tonight before stabilizing early in the morning as cloud cover
will rapidly increase from the south. Lows should not be as cold
as they were last night, with the exception of the northern
Susitna Valley as cloud cover will not reach there until the
early morning hours.

A low pressure system currently centered south of the Aleutians
will send a warm front north into Southcentral overnight. Warm
moist air behind the front supported by an upper level southerly
jet will lead to steady precipitation spreading across the region.
Rain will begin in Kodiak later this evening and move up into the
Anchorage and Mat-Su areas by the morning hours, while reaching
the Northern Susitna Valley by noon. Rain will be heaviest along
the Gulf Coast and Prince William sound before shifting east as
the upper level jet support takes a more southwesterly jive.
Precipitation will continue across most of the region through
Friday as several upper level waves move down into southcentral,
reinforcing the upper level trough over the state. The forecast
becomes more tricky by Friday evening as an upper level wave digs
south into the Southern Gulf changing to low level flow to
southeasterly. This will lead to heavier precipitation on the east
side of the Kenai Coast but also looks to dry out the leeward
side some, including the Western Kenai Peninsula and the Anchorage
Bowl, though the extent of downsloping is still uncertain at this
time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Steady stratiform rain will move over coastal portions of Bristol
Bay this evening and then spread inland overnight as the Bering
front moves into Southwest AK, pushing into the Lower Kuskokwim
Valley by Thursday morning. The rainy pattern will then continue
through Friday as the trough over the northern half of the
mainland phases with the upper low south of Adak and the front
pivots from E-W to more of a NE-SW orientation while broad
southwest flow is established over Bristol Bay and the Kuskokwim
Valley. This evolution will support drier conditions across the
Kuskokwim Delta in northeast flow on the dry side of the front,
with areas from Bethel west likely seeing little or no
precipitation through Friday evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The low south of Adak will continue to be the primary feature of
interest through Friday evening, producing rain and gusty winds
across the southeast Bering as it moves slowly eastward and
evolves into a longwave trough spanning the western mainland and
the Akpen. The western half of the Bering will remain in a drier
pattern in northeast flow as the low moves to the east, with high
pressure building into the Western Aleutians by Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Friday night, there will be
an arctic trough centered over eastern Bristol Bay with waves of
North Pacific moisture rotating northward toward the Gulf of
Alaska. This pattern will support wet weather across the region
with the heaviest rain along the North Gulf Coast. Saturday night
into Sunday morning, a stronger front will move toward the coast,
but this feature now looks much weaker than it potentially was
looking the past few days. The main impact will be increasing
winds along the coast, along with more rain across Southcentral.

By Monday and Tuesday, the upper low will begin to dig south into
the North Pacific. This change will cut off the main moisture
supply and allow for gradual drying heading through the middle of
the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 121 125.
MARINE...Gales 177 412 413.


&&

$$

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



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