Small Craft Advisory 601, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKDT Sun Aug 5 2018

.ANALYSIS AND UPPER LEVELS...

With the continued departure of a vertically stacked low over the
North Pacific, an upper level ridge over the western Pacific has
briefly attempted to build in over Southcentral. However, a
surface low centered over Norton Sound has stifled this attempt
by generating a sequence of shortwaves across the southern half
of the state this morning.

Out west, a strong upper level jet is stretched across the
Bering, oriented from the southwest to the northeast. This 140
knot jet continues to bring plenty of moisture onshore. Radar
imagery shows rain falling over Norton Sound with bands of
showers also beginning to spread in over Southwest Alaska.
Precipitation from this band has also reached the northern
Susitna Valley with returns visible on radar this morning.

&&

.MODEL DISCUSSION...

Models remain in good agreement with the overall synoptic pattern
through Mon night. Uncertainty arises with the timing of an upper
level trough developing across the state Tue night in response to
the remnants of an upper level Kamchatka low moving ashore on the
northern Mainland Mon afternoon. The GFS and EC are in agreement
with the location of the trough, but the NAM solution is slightly
offset to the west at this time.

&&

.AVIATION...
VFR conditions will persist through most of the day but the big
impact to terminal ops will be increased southerly winds. Winds
will initially start out from the south before becoming more
southeasterly as the Turnagain Arm wind kicks in by noon. Winds
will curve north over the terminal leading to gusts up to 30 kts
in the afternoon. Gusty winds should persist through most of the
night before tapering off some by Monday morning. Rain approaching
the area this evening will likely bring in some MVFR ceilings to
the terminal as well.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Tuesday)...
The shift to a much wetter pattern with persistent zonal flow has
begun as the first in a series of lows from the Bering is moving
through the South Mainland. Impacts from this system are already
being felt with gusty S-SE winds around Cook Inlet and through
Turnagain Arm, which will only intensify and spread eastward today
as the pressure gradient tightens along the North Gulf Coast. This
low will also bring widespread rains to the Mat-Su Valleys and
Copper River Basin today, and possibly further south into the
Anchorage Bowl as some of the guidance is beginning the suggest.
While winds will begin to diminish late tonight as the pressure
gradient weakens behind the weakening low, clouds and chances for
rain will continue through the day Monday as numerous weak
shortwaves move through the area. Forecast confidence in the
timing of these waves and resulting rainfall forecast for Monday
is much lower than today however, so the forecast was left mostly
as is until there is better agreement from the models.

Starting Tuesday the next system from the Bering will take a much
more southerly track into the Gulf, which will bring widespread
rain, which could be heavy at times, back to much of the Gulf
Coast and Southcentral. While there remains good confidence in wet
conditions for most of the area on Tuesday, the one piece of good
news is that the models have trended more progressive with the
front. This will help prevent widespread and persistent heavy
rains to the Gulf Coast and Susitna Valley, but there is good
consensus in around in inch of additional rainfall for many areas.
This second round of rains over the Susitna Valley will likely
bring water level rises and possibly flooding through the middle
of this week, especially to watersheds draining from the Alaska
Range and Talkeetna Mountains.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A low amplitude shortwave trough and associated compact surface
low is currently tracking through Norton Sound which is bringing
rain to parts of Southwest along with gusty southwest winds. The
bulk of the rain with this system will be over the Kuskokwim Delta
and Lower Kusko Valley through tonight. Thereafter, rain threats
will partially diminish and favor coastal locations and the
higher mountains. The next significant low will track into Bristol
Bay then move south over the Alaska Peninsula Monday night into
early Tuesday, bringing rain mainly to the AK Range and Bristol
Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A broad upper ridge over the Bering will slowly shift east and
flatten as a series of shortwaves moves over top. The front will
make eastward progression with the zone of 20-30 knot winds
shifting east through Monday night. Areas of fog and rain will
also shift east away from the Bering by Tuesday as the warm front
moves east into the Gulf of Alaska.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)...

After a very wet day Tuesday, Wednesday will start out with the
remnant precipitation from the low ongoing across the eastern half
of Southcentral, with rainfall rates decreasing the further west
you go. With decent model agreement that the upper level trough
will be nearly overhead, it can be expected that off-and-on
showers generated from the abundant moisture and instability will
continue through the day. Behind the low, northwest winds will
keep southwest Alaska and Kodiak in an offshore, downsloping wind
regime, which will help dry out that area. The instability may
remain prevalent enough that even Bristol Bay and the Lower
Kuskokwim Valley see some isolated to scattered showers,
especially Wednesday afternoon. That same northwesterly wind flow
will keep the Kuskokwim Delta largely socked in cloudiness with
likely showers in the Kuskokwim Mountains.

Most of Southcentral starts to dry out on Thursday, with mountain
showers likely ongoing once again. For Southwest Alaska,
precipitation chances will largely depend on the behavior of an
Arctic low which is not being well handled by the models at this
point. Those disagreements become even worse for Friday,
depending on how a semi-tropical system currently over northern
Japan is handled. There is some agreement that Saturday will be a
very wet day for Southwest Alaska with a subtropical moisture
feed. Depending on the speed and strength of the parent low, some
of that moisture may get into Southcentral as well.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 139 140.

&&

SYNOPSIS AND MODEL DISCUSSION...KO/KVP
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JW