Small Craft Advisory 603, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
355 AM AKDT Tue Aug 15 2017

.ANALYSIS AND UPPER LEVELS...

The upper levels consist of an upper trough over the Central
Bering Sea with a mobile shortwave trough along the subtropical
jet stream propagating eastward, just to the south of Kodiak
Island. Weak, almost flat ridging, has built in across
Southcentral behind a weakening trough over the Yukon. Beyond some
rogue showers over the the interior mountains across Southwest
Alaska and along the Alaska Range/Talkeetna Mountains, rather
quiescent weather dominates the southern mainland of Alaska. Weak
upper flow and the presence of an inversion, however, is keeping a
rather extensive deck of mid level stratus (5000-8000 ft) over
most of the region.

&&

.MODEL DISCUSSION...
The suite of dynamic numerical weather models are in excellent
agreement with all the main synoptic features through Thursday
evening. Forecast confidence is very high. The main theme to this
forecast is wetter weather will once again ensue as a deep
longwave trough forms over Alaska.

&&

.AVIATION...
PANC...VFR conditions will persist before rain begins to move in
overnight and Wednesday morning with CIGS dropping to 3000-5000 ft.
The Turnagain arm wind will continue to impact the terminal with
gusty winds through this morning before diminishing this afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Weak upper waves embedded in the southwesterly flow aloft will
give rise to a few showers across Southcentral today. However,
there will also be numerous breaks in the cloud cover allowing
afternoon temperatures to rise into the 60s in spots inland.

A large upper trough moves over the Southwest mainland late
tonight then takes on a negative tilt as it digs southeast over the
western Gulf late Wednesday. Although not as potent as last
weekends system, broad southerly flow with this feature ushers in
widespread rain/showers to Southcentral on Wednesday. Rainfall
amounts of .1-.5 inches inland (higher on upslope areas in the
Susitna Valley) and 1-2 inches along the coasts are possible by
the end of the day Wednesday. As the surface trough moves east,
pressure gradients will respond and increase southerly flow over
the western Gulf and ramp up gap winds across Turnagain Arm and
the Copper River Basin on Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cloudy and unstable conditions will continue across much of the
Southwest mainland for the short term, with the exception of
onshore flow bringing more stable conditions and fog into the
Kuskokwim Delta and Bristol Bay area this morning. A fairly weak
upper low continues to support some lingering light showers
across areas surrounding the Kilbuck mountains. More widespread
rainfall is anticipated Wednesday through Thursday as the upper
low over the central Bering pushes eastward across the Alaska
Peninsula, and a separate upper level wave dives southward towards
the Kuskokwim Delta.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Throughout the short term, surface high pressure will maintain
hold over the southern and western Bering, while a trough will
remain over the eastern Bering and Bristol Bay area. A low level
jet will develop between these two features and support some gusty
winds winds and cold air advection. This will keep the lower
boundary layer well mixed and prevent fog from becoming too
widespread or dense over the eastern Aleutians/Bering and Alaska
Peninsula. By Wednesday morning, northwesterly flow along the
western periphery of the trough will begin to increase, with some
gale force winds developing over parts of the central and western
Bering Sea through Thursday morning.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The models are still struggling with the timing of a closed upper
level low as it drops southward into the Southwest Alaska region
by approximately Thursday morning. The GFS models is the outlier
with this synoptic feature, and very progressive with its speed
of this system as it tracks eastward. By Thursday evening the
aforementioned closed upper level low is located over the western
Gulf before moving the lower Southeast Panhandle by Friday
afternoon. Meanwhile, upstream another shortwave drops down off
the Eastern Siberian region into the Southwest Alaska area. This
synoptic feature moves eastward before developing into a closed
upper level low over the northern Gulf by Sunday evening before
elongating, and extending into the lower Southeast Panhandle
region by Tuesday morning. As these aforementioned synoptic
features track through the AOR, look for the shortwaves to produce
a showery weather regime. Therefore, the forecast confidence is
below average with the continued timing, and intensity issues with
the closed upper level lows/troughs. Otherwise, look for
continued cooler, and wet conditions through the early part of
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 411 412.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher (JA)
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...PD

SPECIAL WEATHER STATEMENT...RESENT
National Weather Service ANCHORAGE AK
1106 AM AKDT Tue Aug 15 2017

AKZ145-152000-
Susitna Valley-
Including the cities of Talkeetna, Willow, and Cantwell
1106 AM AKDT Tue Aug 15 2017

...HIGH WATER ON THE BELUGA RIVER IN WESTERN COOK INLET...

High water on the Beluga River is resulting in significant debris
in the river and could accumulate at powerline bridge. A glacial
dammed lake release on Strandline Lake is unconfirmed at this time
but may be a contributing factor to the high water levels. The
more likely cause is heavy rain in the basin. While river levels
may drop some through Tuesday, additional rain beginning
Wednesday could cause the river to rise higher than the current
levels.