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000
FXAK68 PAFC 211244
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT SUN AUG 21 2016

.ANALYSIS AND UPPER LEVELS...

A large trough is in the process of amplifying across the eastern
Bering Sea this morning as two shortwave troughs, now clearly
evident on satellite imagery, phase together. The stronger of
these two, which is connected to the Pacific jet stream, is
tracking into Bristol Bay. The associated surface low is centered
just south of Nunivak Island, with strong southerly winds
impacting the coast from Cape Newenham eastward. The southern
stream shortwave is also supporting a broad area of moderate to
heavy rain across most of southwest Alaska this morning. The
Pacific jet streak will continue to dig southeast as it tracks
across the Alaska Peninsula through the day, bringing rain and
clouds eastward into Southcentral.

&&

.MODEL DISCUSSION...

The numerical weather models are in excellent agreement with the
development of the upper level trough across the eastern Bering
Sea this morning. They remain in good agreement with the eventual
eastward progression into Southcentral tonight through Tuesday.
All the numerical models have continued to trend slower with the
eastward progression (which can be verified by satellite and RADAR
trends), but they do continue to differ slightly with the timing
of smaller scale shortwave troughs embedded within the flow.
However, overall forecast confidence is very high through the
short term as these differences can be easily accounted for within
the forecast package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Wet and windy remains the headline for the next couple days across
Southcentral and the Gulf of Alaska as a large trough and surface
frontal system move from the west. The overall forecast remains on
track and have made minor adjustments with the latest forecast
package. Most notably, have continued trend of bumping up winds
most places as confidence increases in model pressure gradient
forecast and strength of low level jet out ahead of the trough.
In addition, it now looks like a surface wave will lift northward
across the Gulf Sunday Night out ahead of the main frontal system.
This will push a northwest to southeast oriented front up to the
north Gulf coast which will lead to the development of a barrier
jet. With strong southerly flow behind the front expect gale
force winds to make it well into Prince William Sound on Monday.

The heaviest rain over the next couple days will fall in upslope
areas of the Susitna Valley and the north Gulf Coast/Prince
William Sound. Rainfall estimates for the next 48 hours are 1 to 3
inches in the northern to western Susitna Valley and 2 to 5 inches
along the north Gulf Coast/Prince William Sound. Features will
generally stay in motion and not focused on the same location for
very long. Thus, generally thinking flood potential is low. Will
monitor rainfall amounts as this event unfolds, particularly in
the Susitna Valley and Seward areas. Other areas of Prince William
Sound can handle a lot of rain without causing any problems.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
Troughing will linger over southwest Alaska through Tuesday,
keeping rain over the area through that time. However, as the
trough weakens early in the week the rain will be less consistent
and lighter than it was over the weekend. Winds will be strongest
around the Alaska Peninsula today as the front pushes through but
are expected to weaken late Monday when a trough pushes in from
the northwest bringing stronger winds to the Kuskokwim Delta
coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Westerly flow behind the front will dominate the area bringing
light rain to areas around the Bering and Aleutians until late
Monday when ridging builds in over the western Bering. The ridging
will continue to build in through Tuesday pushing out over the
central Bering. Late on Tuesday another front will push in behind
the ridge over the western Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active weather pattern continues to headline the extended
forecast through much of next week. A stalling broad upper level
trough stretching from the Central Bering/Aleutians eastward to
the southwest mainland, will begin to elongate Monday and split
the energy north and south by Tuesday. The northern portion will
remain north of Norton Sound and head back into the Arctic by
Wednesday. The focus shifts to the energy that splits off to the
south, which models continue to show multiple waves associated
with this upper low tracking back northward into the Gulf and
southern mainland mid to late next week. There still remains some
uncertainty with the multiple waves tracking northward into the
southern mainland and Gulf mid to late next week. These waves will
bring more rainy and windy conditions to the southern mainland
during this time frame, but the finer details are still being
hammered out regarding the timing and placement.

A front associated with a low near the Kamchatka Peninsula will
track into the western Bering Wednesday and weaken as it tracks
eastward into the eastern Bering by the end of the week. This will
bring another round of wet weather to the area as it traverses
through the Bering late next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 125 130 131 132 139 150 155 .

&&

$$

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DWK
LONG TERM...Pepe