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FXAK68 PAFC 201159

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
359 AM AKDT Sun Aug 20 2017


An upper level low embedded within the arctic trough over the
Alaska mainland continues to drop south along west coast of
Alaska. Water vapor imagery this morning depicts a weak shortwave
rotating along the western periphery of the upper level trough but
looks as if it will not phase with the upper level trough and will
be ejected into the North Pacific. Rain showers have developed
over Southwest Alaska in response to the upper level low but an
infusion of colder and drier air is suppressing the overall
coverage and intensity of the showers. Shower activity across
Southcentral Alaska has been widely scattered with showers across
both coastal and inland locations. Mesoscale precipitation banding
developed over Anchorage during the evening on Saturday as convergence
associated with southerly up-inlet flow and southeasterly Turnagain
Arm winds aided in creating some light to moderate rain bands.

A surface low pressure system has been rapidly developing this
morning southwest of Kodiak Island. While infrared imagery this
morning looks unimpressive, water vapor imagery shows just how
impressive the upper levels presently look. A very impressive
atmospheric river has begun to set up but it looks like it will
remain south of our forecast area with the Alaska Panhandle likely
to feel the full force of the atmospheric river in the coming



Models are in overall pretty good agreement this morning. There
are still some timing and placement issues with the developing low
pressure southwest of Kodiak Island. This will have an impact on
when rainfall will start across Southcentral Alaska today and
Monday and just how much rain will fall across interior locations.
At this time, coastal locations will likely feel the brunt of the
front associated with this low with upslope flow along the 
coastal mountains maximizing precipitation processes. Interior 
locations remain a little bit more of a question mark as to how 
far inland rain makes it and how much rainfall Anchorage, Palmer 
and other locations will receive. Models had been trending drier 
on Friday but now they are showing higher rainfall amounts across 
the interior. Elevated instability will be another component that 
will be hard to parameterize as it will easily enhance rainfall 
over some locations leading to decent mesoscale banding. 
Otherwise, models are in good agreement with high pressure 
building in the wake of this surface low across the Bering Sea. 


PANC...Gusty southeasterly winds will diminish shortly after
sunrise this morning. VFR conditions will prevail through the
morning with MVFR ceilings and vsby likely by noon as rain begins
to fall. Conditions will begin to improve Monday morning.


The upper level low over southwest Alaska will continue to move 
south, entering the southwest Gulf tonight. The associated surface
low, which is currently south of Kodiak Island, will move
northeast and stall just south of the Kenai peninsula this
evening. There is plenty of moisture with this low that is moving
into Southcentral this morning. Rain will develop over much of the
area today, and continue tonight as the surface low remains
stalled. On Monday the surface low will retreat southward,
as the upper low moves eastward just south of Gulf. Conditions
will thus begin to dry out from west to east. By Tuesday morning
the low complex will have exited the area, with just some stray
showers remaining.


An upper low will track southward over the southwest mainland 
today, keeping some showery conditions over the area through early
Monday morning. At this time, a tight pressure gradient will 
materialize as the low departs for the Gulf of Alaska and a ridge 
builds in from the west, resulting in gusty winds over Bristol 
Bay through Monday night.


High pressure will continue to build over the Bering Sea,
resulting in widespread stable conditions over much of the area.
By early this afternoon as the ridge pushes further east, gusty
north to northwesterly flow will build over the eastern
Bering/Aleutians and Alaska Peninsula. Expect gap winds south of 
the Alaska Peninsula to ramp up tonight and reaching Gale force 
by Monday morning.


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

Beginning the extended period forecast Tuesday night, there will
be a closed upper low over the far southeastern Gulf of Alaska
with a mobile shortwave trough over the Central Bering Sea. In
between these systems there will be a shortwave ridge axis nosing
into Southern Alaska with an anchoring trough centered over the
North Slope of Alaska. This pattern will be the end of a short
quiescent break in the weather. By Wednesday afternoon, the trough
over the Bering Sea will merge with the anchoring trough over the
North Slope with increasing southwest upper level flow ushering in
more clouds and rain to Southern Alaska. This cloudier and wetter
pattern looks to persist through Friday, although uncertainty
grows by this period as the GFS and ECMWF differ with respect to
the position of the trough and the degree of interior downslope
flow. Coastal locations, especially near the Gulf of Alaska, will
be wet, but by Thursday and Friday it looks like there is a
possibility that downslope drying dramatically limits interior
precipitation, especially in the ECMWF solution. For now a middle
ground solution will be favored, depicting coastal rain into the
weekend and more showery conditions inland by the end of the week.


MARINE...Gale...119 130 131 150



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