Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 250006
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 PM AKDT SAT SEP 24 2016
.ANALYSIS AND UPPER LEVELS...
A neutrally tilted trough is positioned along the western coast
of Mainland Alaska with a flat ridge upstream over the Bering
Sea. Radar imagery shows unorganized widely scattered showers
ahead of the trough axis over Southwest Alaska. Much more
impressive activity is ongoing over the northern Gulf where an
upper short-wave is headed toward the northeast Gulf coast and a
weak surface low is backing into Prince William Sound. Warmer low
level temperatures (and sea surface temperatures) are leading to
steeper lapse rates compared with Southwest Alaska, contributing
to heavier more widespread showers and a few thunderstorms. Weaker
impulses embedded within the broader trough are also producing
isolated to scattered convection along the Cook Inlet to Susitna
Models remain in excellent synoptic agreement for the next couple
days and forecast confidence is generally high. The main forecast
challenge is location/areal coverage of showers over Southcentral
Tonight through Sunday. First a series of mostly weak waves lift
northward this afternoon and evening. The western Alaska trough
then arrives Sunday and undergoes weakening through the day as
anticyclonic flow strengthens upstream. Models vary on the
details associated with these features, so will focus on general
trends of convective initiation and advection.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Cloudy and showery will be the forecast for most areas around
Southcentral Alaska. Cold air aloft continues to promote shower
development, and will be the case through Sunday. The forcing for
showers however, is a little more sparse. One disturbance in the
flow is in the northern Gulf, while the synoptic scale trough is
nearing the Alaska Range. These will promote development along the
north gulf coast as well as the western Cook Inlet region. The
trough will continue to the east tonight into Sunday keeping
showers going in most areas, with a resurgence off the mountains
Sunday afternoon. The trough clears the area late Sunday into
Monday ushering in a quieter period of weather. Some cold air
accompanies this trough, which will allow overnight temperatures
to drop into the 30s once it passes, but maximum afternoon
temperatures will be very seasonable.
.SHORT TERM FORECAST SOUTHWEST ALASKA...
A trough pushing through Southwest Alaska will bring showers to
the area tonight, but begin to diminish towards tomorrow morning
as high pressure pushes in from the west. A low will then track
southeast through the eastern Bering bringing rain to areas along
the coast through Sunday night. Then Monday night another ridge
will move in over Southwest Alaska.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Weak northwest flow will dominate the Bering and Aleutians until
Sunday afternoon when a low over the northern Bering pushes
southeast through the eastern Bering. This low will bring rain to
the Pribilof Islands, eastern Aleutians and Alaska Peninsula
Sunday into Monday morning. Then ridging will move across the
Bering from the west followed by a new system moving into the
northwestern Bering Tuesday.
.LONG TERM FORECAST (Days 3 through 7)...
The extended period will be a more progressive pattern than
currently in place with a transition from a low amplitude longwave
pattern to a much more amplified longwave pattern. A series of
very potent shortwaves will move along the periphery of an arctic
trough anchored over far eastern Russia and significantly amplify
through trough as each wave passes. Models, however, are struggling
a lot with how much jet energy will be associated with each shortwave
which looks to have significant impact on how the longwave pattern
(and therefor surface pressure fields) evolve. The GFS is by far
the most aggressive with a jet streak near 100 kt at 500 mb and
150 kt at 250 mb developing a very strong closed low over the
western Bering Sea. This solution is by far the outlier as most
other guidance, including ensembles, keep the upper low much
further north over Kamchatka/Siberia and weaker. All of the
aforementioned solutions amplify the longwave trough significantly
more which correlates to longwave ridging building over the
mainland of Alaska. This should allow the Alaska Mainland to
remain largely on the dry side through much of the upcoming week
before rain chances increase by next weekend as a potent front
pushes across the Bering Sea.
SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK