Small Craft Advisory 781, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKDT Sun Oct 15 2017

.ANALYSIS AND UPPER LEVELS...
There is a weak upper low over Southcentral Alaska with a weak
high over southwest Alaska. There are some lingering showers over
southwest Alaska, and some showers over the far eastern Copper
River basin. There is an upper level low over the northwest
Bering Sea. Fast, zonal flow is over the northern Pacific with the
jet stream well south of the state. There is a weak low over the
central/western Aleutians moving rapidly eastward. A strengthening
low is located to the southwest of the western Aleutians, which
is beginning to "buckle" the zonal flow.

&&

.MODEL DISCUSSION...
The numerical models are struggling with the development of the
low currently southwest of the Aleutians as it moves across the
southern Bering Sea and then crosses the Aleutians (Monday night)
and then into the Gulf of Alaska. The differences consist of the
speed, depth of the low, and the details of possible triple-point
development. The NAM can be safely discarded, as it is a definite
outlier. The ECMWF is showing good run-to-run consistency and is
somewhat similar to the GFS Ensemble mean. The GFS is a nice
compromise with the other models, and with the previous forecast.
Forecast confidence is below normal with this feature.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through Monday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A broad low pressure system centered just south of Cordova will
continue to drive the sensible weather over Southcentral as it
shifts east through Monday night. Northerly offshore flow behind
the low will persist, setting up a gradual cooling and clearing
trend over the Susitna Valley south through the Cook Inlet area.
With partly to mostly clear skies forecasted tonight, the
atmosphere over Southcentral will be able to efficiently radiate
heat out. Coupled with a slightly colder airmass in place than
last night, expect to see lows tonight at or slightly below last
night's lows. Meanwhile, onshore southerly flow to the east of the
low will keep cloudier, showery conditions from Cordova east.
Cold air advection on the back side of the low is supporting gusty
northwesterly outflow winds over Kamishak Bay through the Barren
Islands. The gusty winds will continue through Monday and peak
Monday evening with a swath of small craft winds extending from
near Augustine through the central Gulf. By Tuesday morning, the
low will have largely dissipated and weakening pressure gradients
will allow for a slackening of winds throughout the Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon and Tue)...
Showery and cooler conditions will persist through Mon before a
brief drying trend sets up for Tue. Weak west to northwest flow
will allow slightly cooler air to filter across much of the area
on Monday. This will make for yet another chance for patchy fog
for much of Bristol Bay through the morning hours. More widespread
fog is expected in the YK-Delta, with the lowest visibilities
occurring near Bethel once again. Showers will persist through
Mon with the best likelihood of them along the coast and NW facing
terrain. By Tue, light offshore will develop as the area will be
stuck between several systems. This should cause the showers to
abate, however, it will allow for some colder air to move in from
Interior AK.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Mon and
Tue)...Some of the busiest weather in the state will be right
along the Aleutians to start the week. A very strong 180 kt jet
will continue to channel plenty of energy right along the chain.
Several lows will quickly develop, reach maximum intensity, and
then weaken as they track along the chain. The first is currently
passing through the Central Aleutians and will continue to move
quickly to the east-southeast. The next system will move into the
Western Aleutians tonight. It will bring gale-force winds,
moderate rain, and a slight chance for some thunderstorms on its
eastern flank Mon. By Mon afternoon-evening, some colder air will
start to wrap in the western side of this system. This will bring
some storm-force winds near Kiska and pushing towards Adak. There
is an outside chance of some hurricane-force gusts. This low will
slowly weaken as another triple-point low forms south of the
chain and moves out of the area on Tue. For the northern Bering,
winds will remain below small craft out of the west for much of
Mon. They will then turn in a counter-clockwise fashion late Mon-
Tue as the lows pass to their south.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The mid to long term forecast beginning Tuesday night shows an
active storm track that will primarily stay south of the southern
mainland. This will bring more rain and winds to the Aleutians
through much of the week. There remains disagreement for the
exact track and timing of the low that moves through late in the
week but all guidance keeps it south of the southern mainland
until the weekend. That would leave mainly dry and cold for the
southern mainland until Saturday where there becomes a fair amount
of uncertainty as to what the low will do as it moves toward the
Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 173-175 178 413.
         Storm 176 177.
&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...RD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...DWK