Small Craft Advisory 703, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKDT Fri Sep 14 2018

.ANALYSIS AND UPPER LEVELS...
The blocking ridge across mainland Alaska is creeping eastward
while a Bering Sea low is moving northeast. The low is sending
waves of clouds and weak disturbances out ahead of the main
trough. For much of Southcentral, the dominant flow on the east
side of the ridge is keeping the sky clear.

Record heat continues for the region with new reports in the
Aleutians and Southcentral as the ridge axis moves over the state
today.

&&

.MODEL DISCUSSION...
Models are in general agreement though the short term. The main
challenge will be timing and extent of rainfall across
Southcentral this weekend. In the West, visibility and ceiling
forecasts will be the greatest concern as the next wave passes
through.

&&

.AVIATION...
PANC...VFR conditions and light winds are expected through Saturday
morning as surface ridging remains the dominant influence. The
Turnagain Arm wind is expected to increase, but don't expect it
to reach the airfield at this time. The wind should also limit any
shallow fog on Saturday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Monday)...

Another series of high temperature records were broken this
afternoon as the axis of the ridging responsible for this summer-
like weather moves over Southcentral this afternoon.
Unfortunately for those that like this weather, that means that
temperatures are all downhill from here. An upper level trough
currently centered west of Saint Paul will move over Southwest
Alaska and become negatively tilted by Saturday afternoon. This
will support lift and light rain that will develop out ahead of
the trough. Since the whole system will be moving northeast, the
first to see some rain will be Kodiak on Saturday morning, then
the rain will slowly progress northeastward, moving into the Kenai
Peninsula Saturday afternoon. Due to the effects of downsloping
and the dry air mass the rainfall with the next system is
replacing, the expected precipitation from Anchorage northward
will be light by normal September standards. Sunday looks to be
the most likely day for rain for nearly all of Southcentral, but
the amount that falls should stay under a tenth of an inch for all
areas outside of a possible heavier shower or two in a few
locations.

As the low approaches this evening, ridging out ahead of it will
develop across the Gulf and Prince William Sound, which in turn
will allow for the Turnagain Arm wind to increase. Turnagain Arm
and higher elevations are expected to see 20 to 30 mph winds with
occasional gusts to 40 mph starting tonight and continuing into
Saturday night before slowly diminishing on Sunday.

On Monday, the upper level trough causing the expected light rain
will stall west of the area meaning most of Southcentral stays in
the portion of the system where rain will be possible. However,
the trough will weaken in place, so while the chance of
precipitation will persist into Tuesday, most areas should stay
dry as any rain will largely favor the mountains.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Monday)

A weakening front is approaching the coast, with isolated to
widely scattered showers developing in advance of this feature
from Cape Newenham to near Sleetmute, and points west of this
line. The front is expected to stall this evening, as it awaits an
upstream trough, which will arrive during the day Saturday.
A fairly robust mid-level low will cross the region in the base
of the trough. This combined with any convergence left over from
the decaying front should foster increased shower development
during the afternoon/evening, before slowing down while
progressing inland eastward.

By Monday, a new low will form in the Gulf, with another low
located over eastern Siberia. These two systems will be connected
by a trough axis, with the initial system temporarily stalling
over the region in the middle of this axis. The net result will be
showers continuing through the day.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Monday)

More of the same best sums up the weather theme for the next few
days. A weakening surface low, currently north of the central
Aleutians, will continue to head northeast through the evening
while weakening. However, this low is associated with a deep upper
through that extends south from a low in Eastern Russia, with
another strong system in the base of this mid-level trough. The
latter feature will cross the Bering over the next 24 hours,
reaching the southwest Alaskan coast by later Saturday morning.
Rainfall coverage and intensity will be greatest near this
feature. For the remainder of the region, look for showers to
continue through Saturday as a broad cyclonic flow persists. A
ridge will attempt to build across the central and western
Aleutians for Sunday into Monday, with showers continuing to the
north and east.

&&

.LONG TERM FORECAST (Monday night through Friday)...
An upper level trough slowly dissipating over the South Mainland
will bring continued cloudiness and showery weather to most of the
area through early next week. Beyond that the confidence is increasing
that ridging from the Bering Sea will likely move over the South
Mainland beginning Wednesday. This will bring offshore flow and
sunny skies back to most areas for the middle to end of next week.
One notable difference will be the much more seasonable temperatures
as 850 mb temperatures under this ridge will be nearly 10 deg C
cooler than during the record warmth experienced this week. The
western Bering/Aleutians on the other hand will see more active
weather beginning on Wednesday into late next week as the storm
track off the Northwest Pacific will ride along the western edge
of the Bering ridge.

The biggest question in the long term forecast surrounds how long
the stretch of quiet weather that develops on Wednesday over much
of the forecast area will continue. One important difference in
the models in the last 24 hours has been for much stronger ridging
over the Bering, leading to much quieter weather from the Central
Bering eastward into the end of next week. If this trend of
stronger and less progressive ridging continues (as had been the
case with the current warm/dry spell) then it certainly is very
possible that more prolonged stretch of quiet weather could occur
for much of the forecast area again.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...DEK