Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 250041

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKDT WED AUG 24 2016

The pattern aloft is a very amplified one owing to a couple stout
upper level ridges, one over the northeast Pacific and another one
over the northwest Pacific. in between these two is a decaying
upper trough spinning south of the Eastern Aleutians. Such an
amplification leads to a large stream of moisture, an atmospheric
river, with subtropical connections. Satellite based observations
of precipitable water show around 1.5 inches in this moisture
plume and the PAFC 24/12z sounding shows 1 inch in the column. A
pretty strong southwesterly jet aloft stretches from the north
Pacific onto the mainland. There are several disturbances embedded
in the flow helping to blossom areas of rain over the Cook Inlet
region and others further upstream which will impact the area


Guidance is in good agreement with the breakdown of the highly
amplified trough. The consensus is that the two areas of ridging
in the Pacific will pinch off a cut off upper low in the central
Pacific, and the rest of the energy to the north will move as an
open wave from west to east across southern Alaska.


An upper level wave embedded in deep southwesterly flow aloft
will take advantage of an abundant supply of North Pacific
moisture to produce widespread heavy rain across much of
Southcentral AK tonight into Thursday morning. The heaviest rain
will fall over northeastern portions of Prince William Sound and
the Wrangell Mountains where 3-5 inches or rain will be possible
through Thursday evening, however most areas including Anchorage
will still likely see between one half and one inch of rainfall by
Thursday morning.

Attention then quickly turns to thunderstorm potential on Thursday
afternoon as a potent upper level trough moves in from the west
and partial clearing allows for increasing instability. The main
forecast challenge will be determining the degree of
destabilization that ultimately takes place within the short
window of time between the departure of the heavier stratiform
rain and the arrival of the trough in the afternoon, as lingering
cloud cover from overnight could inhibit development of more
robust instability. Given the favorable dynamic support, at least
isolated to scattered thunderstorm development is expected across
much of the area. This situation will bear watching through the
morning forecast package as finer details come into better focus.

Conditions will then improve rapidly from west to east beginning
Thursday night as high pressure builds in behind the departing
trough. This will yield pleasant conditions on Friday with
clearing skies and at least a temporary reprieve from the rainy



An upper level trough associated with relatively cold air aloft
will dominate the weather pattern across the Southwest mainland
over the next several days as the trough progresses eastward. On
Thursday, enough diurnal heating is expected across the Kuskokwim
valley and northern Bristol bay for a few thunderstorms to form in
the afternoon. By Thursday night, an upper level ridge will begin
to push inland across the Southwest mainland. This will support
broad southwesterly flow and help to push fog and stratus onshore
across the region.



A warm front currently stretching across the western Aleutians is
slowly progressing towards the east today towards the central
Aleutians. It is not a particularly potent front, and will support
generally light showers and winds below gale criteria. Meanwhile,
abundant stratus and fog sit beneath an upper level ridge over the
eastern and central Bering. &&

.LONG TERM FORECAST (Days 3 through 7)...

The pattern change will be well underway by Saturday with high
pressure building across the interior of Alaska. Models are still
struggling with just how dry the new air mass is going to be with
a very broad and expansive area of low pressure expending from the
Alaska Panhandle up through Prince William Sound. Nevertheless, it
appears as if much of Southwest and Southcentral Alaska will see
clearing skies and warming temperatures with this new pattern
through the weekend. The stacked low extending from the Alaska
Panhandle to Prince William Sound will be an issue through the
forecast period as it looks like several easterly waves will
rotate around the feature. Depending on the strength of the ridge
over western Alaska, a few showers could move through the Copper
River Basin and possibly the Anchorage area by midweek.

The Bering Sea will be quite quiet through much of the time
period with weak high pressure over the eastern half of the sea
while the rest of the area will remain under cyclonic flow. Given
how moist the air mass has been recently, it looks like fog will
remain an issue over the eastern Bering and the southwest
coastline through the weekend. Just like the rest of Alaska, a
pattern shift looks likely for the Bering Sea as well. However,
this will come in the form of a potent frontal boundary that will
enter the western and central Bering Sea Tuesday morning.
Increasing winds and rainfall will be likely with the frontal
boundary but strength and timing is a little rough to forecast at
this time.


PUBLIC...Flood Advisory 145.



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