Small Craft Advisory 662, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 PM AKDT Tue Sep 5 2017

.ANALYSIS AND UPPER LEVELS...
The upper level longwave trough stretches out of southwest
mainland Alaska and extends well south of 40N. A secondary upper
level low is in the western Bering Sea and the jet stream is
moving south of these lows and then turns sharply northward where
it is rapidly spinning up the large surface low in the Gulf of
Alaska. This surface low is the main weather maker for southern
mainland Alaska through tomorrow and will be the source of the
strong winds and some periods of heavy rain.

&&

.MODEL DISCUSSION...
The 12z models came into good agreement with the strong low in
the Gulf heading northwestward through tomorrow. 18Z model runs
are matching very close to the 12z runs which is a good sign that
they have locked on to this system, which does increase the
forecast confidence. The large low near Attu island is also
handled in a similar manner by all models which was something
that was a question mark as it is a former tropical storm from
the western Pacific.

&&

.AVIATION...
PANC...Rain will continue into the evening potentially bringing
some occasional MVFR conditions. Of greater concern will be the
low level wind shear that develops in the evening as winds aloft
increase from the southeast while surface winds remain northerly.
Late in the evening the surface winds should switch to the
southeast and get strong and gusty which will persist through
most of the night before diminishing some. Winds will remain gusty
from the Southeast through the day tomorrow...just not as strong
as they will be overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...The main
focus for the forecast will be timing and strength of winds this
evening and overnight. Steady moderate rain will continue until
the winds kick up later this evening. The High Wind Warning still
looks on track with the approaching low pressure system. The
further south track of the surface low toward Kachemak Bay will
keep pulling the wind down-Inlet and could keep the strongest
winds bending into the Arm and away from the higher elevations of
Anchorage. However, the wind will still come into all areas once
the surface low crosses into Cook Inlet around midnight. The
window for maximum winds still looks the same from about midnight
to 4 AM. While gusty winds will continue on Wednesday, they will
slowly taper down from the early morning maximum.

Behind the departing system the upper level pattern stagnates for
a couple days with no distinct features to key in on. Showers will
continue along the northern Gulf while a cooler air mass sits over
most of Southcentral Alaska. Generally light winds will exist with
partly cloudy conditions and the chance for showers.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Satellite imagery this evening depicts a rapidly deepening and
already powerful cyclone moving northwest toward the southern tip
of the Kenai Peninsula. This low will eventually move into the
Lower Kuskokwim Valley, bringing rain as far west as the Kilbuck
Mountains and interior Kuskokwim Delta. Rather strong southeast
winds will also impact the far northern part of the Alaska Range
as the low moves into interior Alaska tonight and Wednesday
morning. Wednesday night and Thursday, weak shortwave ridging
will build behind the departing upper shortwave trough, keeping
southwest onshore flow and showery conditions through Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A gale force low is now centered over Shemya and will continue to
slowly move east along the chain and weaken. Widespread light
rain and showers will accompany the low along with broad northerly
winds as the low moves east. The low will reach the Alaska
Peninsula by Thursday night with an expected trend toward drying
conditions.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The synoptic pattern for the long range forecast remains wet and
unsettled as broad cyclonic flow remains steadfast over the 49th
State. Anticipate that the active pattern will hold through the
weekend. Upstream, the models bring a ridge of high pressure over
eastern Russian and the western Aleutians and it will slowly spread
eastward. By Sunday, the models have a deep trough axis from the
interior bisecting the Gulf and and spanning into the North
Pacific and the ridge stalls out before making it to the AK
Peninsula. The models have been struggling with run to run
continuity in the short term and the vagaries of the models'
performance is only magnified in the long term. This is a low
confidence forecast package and expect updates as newer model
runs come in for evaluation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101.
MARINE...Gale Warning 119 120 121 125 129 130 131 132 139 140
                      177 178.

&&

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