Small Craft Advisory 625, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 PM AKDT Wed Aug 23 2017

.ANALYSIS AND UPPER LEVELS...
Upper levels show a longwave trough over the main portion of the
state with a lobe extending into the Bering Sea, where an upper
level low is moving slowly eastward. Satellites show that this
system is well developed, with a broad cirrus shield beginning to
slide over the Copper River Basin and Southcentral. At the
surface, radar shows showers moving through southwest Alaska just
ahead of the front. Upper level jet support in the form of a jet
streak moving south into the base of the trough is aiding the low
to strengthen as it moves eastward, which is reflected in near
Gale Force winds in the Central Aleutians.

&&

.MODEL DISCUSSION...
Models are in good agreement on low placement, although the 12z
NAM was slow on moving the front into Southcentral. Model data
also shows increased wind speeds in the west around the low center
as the low gains upper level support. In the west, a NAM/GFS blend
was chosen to increase speeds across the area, while in the east,
NAM was chosen for wind strength but timing was adjusted to match
current observations.

&&

.AVIATION...
PANC...Timing on the Turnagain Arm wind and downsloping remains
the big questions for today. The gradient looks to increase in the
afternoon as the front approaches the west, so am increasing the
winds even more after 06z. This should also impact -SHRA moving
over the airport as conditions will dry during the strongest
Turnagain winds. Cigs are dropping in SW flow ahead of the front,
and are not expected to hit MVFR conditions, but may cross the 050
ft mark which will impact airport ops. If southeasterly flow picks
up earlier ahead of the front, cigs may not drop at 00z and
showers will be negligible.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A very typical late-summer storm is taking shape which will bring
widespread rain and wind to Southcentral Alaska and the Gulf of
Alaska the next couple days (with the notable exception of the
Copper River Basin which will be much drier). A leading upper
level wave combined with instability in the lower to mid levels
is bringing showers to some areas this afternoon, particularly
south to west facing slopes which are benefiting from low level
upslope flow. A neutrally tilted short-wave trough over the
eastern Bering Sea will become negatively tilted as it enters the
western Gulf tonight, leading to development of widespread steady
rain. The mean low level flow will gradually back toward the
southeast and strengthen out ahead of a surface frontal system.
This often results in downslope drying from the western Kenai to
Anchorage and the Mat Valley. However, with strong upward vertical
motion, a more southerly component to mid level flow, and a bit
of remaining instability expect rain to fall in all of these
areas.

As the front makes its way toward the north Gulf coast a barrier
jet will form, bringing high end gale force winds to the immediate
coast. The northward progression of the front will rapidly stall
on Thursday as the upper-level short-wave trough rotates back
westward toward a new upper level low dropping into Bristol Bay.
Thus, expect the strongest winds to remain over southern Prince
William Sound (near interface with the Gulf). The approaching
front will also kick up wind through Turnagain Arm. Ageostrophic
flow will favor stronger winds in western Turnagain Arm tonight
while geostrophic flow will favor stronger winds through Portage
Valley/Eastern Turnagain Arm as the front arrives Thursday
afternoon.

The heaviest rain will fall in western Prince William Sound thanks
to persistent strong east to southeast low level flow, with 3 to 6
inches of rain possible through Friday afternoon. Do not expect
any flooding problems, though there will be some decent rises in
small streams of the eastern Kenai Peninsula. Rain will break up
into showers for inland areas Thursday afternoon as a now
vertically stacked low moves from Bristol Bay into the Gulf and
upper level flow becomes more easterly. The bulk of rain will then
remain along the Gulf coastal areas through Friday, with a chance
of showers over the rest of the area as weak upper waves head
inland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Fairly straightforward with rain moving in with the low entering
Bristol Bay. The rain will being to turn more showery Friday into
Friday night with the low moving farther east in bringing in
northerly flow on Saturday. This northerly flow will not be
sufficient to dry out the area and the upper level trough
remaining over Southwest mainland Alaska will keep showers in the
area into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Western and Central Bering Sea will have high pressure
building over them Thursday and then another low will enter the
Western Bering leaving the ridge over the Central Bering. This
will bring areas of low clouds and fog in the these areas under
the ridge as typically happens. The Eastern Bering Sea will see
marine Gale force winds develop between the low in Bristol Bay and
the ridge in the Central Bering Sea.

&&

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

Beginning the extended period forecast Saturday night, there will
be a small upper level low somewhere in the vicinity of eastern
Bristol Bay and the Alaska Peninsula with a departing weak
shortwave trough moving east toward the Yukon. There will be weak
upper level ridging over the northwest Bering Sea with an
undercutting Pacific jet stream crossing the Aleutian Chain.
Showers will mainly be impacting the Southcentral region with
drier air filtering over Southwest Alaska. A very brief drier
period will develop Monday as upper level heights build over
Alaska ahead of a deepening North Pacific low. Complex wave
mergers throw quite a bit of uncertainty in the overall forecast
of the low track, but it looks like a rather impressive surface
low will eventually develop and move north toward Kodiak Island
and the Alaska Peninsula region Monday night into Tuesday. With a
strong Northeast Pacific ridge axis in place, this low will have a
lot of potential for amplification along with heavy coastal rain
and strong winds. Expect rain to also impact most interior
locations Tuesday and Wednesday given the amount of moisture most
of the global models are progging. A lot of details remain
uncertain at this time, but the general idea is to expect a
dramatic uptick in wet weather along with potential higher impacts
to coastal regions. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...EZ
LONG TERM...JA