Small Craft Advisory 542, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 PM AKDT Tue Jul 18 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low centered just off the coast of the
Kuskokwim delta, moving towards the southeast. There is a surface
low just to the southwest of the upper low, and another over
Kodiak Island. A front associated with the upper low stretches
from near Skwentna to Seward and then south across the Gulf of
Alaska. A surface ridge is along the north Gulf coast east of the
Kenai peninsula. There is an upper level ridge building over the
central and western Bering Sea and the Aleutians. A weak and
dissipating front is over the western and central Aleutians
associated with an upper low off the coast of Kamchatka.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through the
short term portion of the forecast (Thursday afternoon). There are
some disagreements on the movement and location of the low in the
Gulf by Thursday, but impacts on the sensible weather forecasts
are minimal. Forecast confidence is near normal.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Rain should
taper off to showers late tonight as the flow aloft becomes more
easterly.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The short term forecast remains focused on the front tracking
into the western Gulf and Cook inlet region this afternoon and
evening, with radar imagery from this afternoon showing
widespread rainfall filling in over much of the forecast area.
Rainfall is expected to persist over much of the area through
Thursday morning, with the highest rainfall totals expected over
the eastern Kenai peninsula and Prince William sound area. The
exception to this will be the increasing southeasterly flow
Wednesday morning through Thursday, limiting rainfall amounts
over the western Kenai, Anchorage bowl, and Matanuska Valley.
Wind gusts associated with this system will begin to increase
tonight across Prince William Sound and Turnagain Arm and
continue throughout the day Wednesday. Winds will begin to
diminish Thursday as the parent low tracks further south into
the North Pacific and the pressure gradient over Turnagain arm
slackens.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There is a 998 mb low track just west of Nunivak Island this
evening, and tracks along the Southwest Alaska coastline. By
Wednesday morning this system is located in the Bristol Bay
before crossing the Alaska Peninsula just south of Kodiak Island
by early Wednesday evening. This synoptic feature is vertically
stacked with a closed low at 500 mb. There will be shortwave
spinning off the upper level closed low bringing more wetting rain
to the region. Meanwhile on the backside of this low pressure the
precipitation changes over to a showery regime as the low exits
the region. The heaviest precipitation occurs along the Aleutian
and the Alaska ranges due to the onshore southwesterly winds
resulting in orographic lift along the mountains.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Western Aleutians/Bering Sea has an upper level ridge located
in the Bering that flattens out as it moves eastward. A weak low
pushes into the western Bering tonight, and fills before
dissipating by Thursday afternoon, while a ridge of high pressure
builds off the Kamchatka Peninsula, and extends into the northern
Bering. Therefore, look for stratus accompanied by fog with this
synoptic feature. Otherwise, the rest of the Bering will have rain
out ahead of the low tracking along the Southwest Alaska
coastline before crossing the Alaska Peninsula by Wednesday
evening. Look for a showery regime on the backside of this system
for the northern to eastern Bering through Wednesday time-frame.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The stacked low pressure system over the Alaska Peninsula late on
Thursday will continue to drop south into the North Pacific and
Western Gulf of Alaska by Friday morning. Models begin struggling
on how quickly this feature shifts eastward as well as how far
north the feature tracks. At the time, the subtropical jet looks
rather weak which tends to support a slower movement (depicted by
the GFS) which keeps the low pressure system will away from the
northern gulf coast. However, the ECMWF and some of the ensemble
members show a jet streak developing within the subtropical jet
which could push this system toward the Panhandle by the end of
the weekend. With poor model agreement, forecast confidence with
this is quite low. However, teleconnections and models are
hinting at longwave ridging developing over the Bering Sea and
nudging into the Alaska mainland throughout the upcoming weekend.
The amplitude of the ridge is rather unimpressive but the
wavelength hints at the potential for long duration ridging over
the Bering Sea. This should allow for some clouds to clear across
Alaska and the Bering Sea through the weekend although fog may
become an issue at times, especially with the wet pattern
currently unfolding. The low amplitude ridge will allow for some
weak upper level disturbances to push inland through early next
week which could bring a few periods of showers but confidence
with any given disturbance is low 5 to 7 days out.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...MC