Small Craft Advisory 383, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 AM AKDT Fri May 12 2017

.ANALYSIS AND UPPER LEVELS...
The low in the southern Bering is weakening with its front draped
across the southwest Alaska coastline and extending through the
Alaska Peninsula and over the southern portion of Kodiak Island.
It is a weak front that is stalling and falling apart but is still
bringing some rain to the area. Ahead of the front, ridging
remains over the Gulf.

&&

.MODEL DISCUSSION...
The NAM, GFS, and EC are in good agreement with the main synoptic
features, adding confidence to the forecast. However, there are
some differences with instability values over the mountains
around Southcentral Alaska with the NAM being more in favor of
thunderstorm development than the GFS. The NAM looks to be
somewhat overdone in this pattern so the GFS solution was
preferred.

&&

.AVIATION...
PANC...VFR conditions persist. Winds should remain relatively
light with afternoon and evening sea breezes.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak upper level trough over south central Alaska will slowly
drift north today and high pressure will build behind it. This
will shift any convection northward, thus any chances of
thunderstorms are limited to the Copper River basin and the higher
elevations of the Susitna River basin. A Bering Sea low will push
into southwest Alaska on Saturday, bringing increasing chances of
showers and rain. This low will then move into the Gulf of Alaska
Saturday night. Showers will decrease Sunday morning as the upper
low moves towards the panhandle.

&&

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

An active weather pattern is in store for the next couple days as
a front and it's attendant low pressure system move across the
area. The front is already over Bristol Bay stretching
northwestward along the Kuskokwim Delta coast. The front is
quickly falling apart as it shears out. The low will be making
its way towards the coast today with associated precipitation
reaching there later on this afternoon. In the meantime, scattered
showers with plenty of dry periods in between will be the norm as
the front causing the precipitation dies out.

Showers will overspread all of Southwest Alaska tonight in
association with the low pressure system itself. The center of
the low is not expected to reach the coast near Goodnews Bay
until Saturday morning. Thus, all areas, including inland locales
should get some amount of precipitation through Saturday. The
passage of the upper level low pressure center during the day on
Saturday should result in the termination of organized shower
activity from west to east through the day. Upslope enhancement
along the Alaska Range will cause the highest precipitation
amounts with the storm to be found there. In the wake of the storm
Saturday night, clearing skies, diminishing winds, and mid-level
warm advection will set up the conditions necessary for areas of
fog to form in many parts of Southwest Alaska.

&&

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

The low moving across the Bering this morning will continue to
weaken as it moves eastward towards the Pribilofs today. It will
move into Southwest Alaska on Saturday morning. A few localized
gales are expected today, especially on the low's southern side.
After today, continued weakening of the low should mean only small
craft conditions will be generated from it. Behind the low,
precipitation chances fall to just isolated threats for most of
the Bering as high pressure builds in. Such will not be the case
for much of the Aleutians as the next Pacific low begins making
its presence known over the western and central Aleutians tonight,
then overspreads the rest of the chain gradually through Sunday.

Unlike the low currently over the Bering, which has long since
occluded, has become vertically stacked, and is now surrounded by
cool air on all sides, this next low will have a slug of warm,
juicy air with it. The warm front extending out ahead of the low
will elongate and stall along the Aleutians through the weekend.
This will mean an extended period of light to occasionally
moderate rain is in the forecast as the warm, moist air
continually lifts above the cooler air mass in place across the
Bering.

&&

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

The upper-low and attendant surface trough will continue its
eastward progress through the weekend. Confidence is increasing as
the majority of the models converge on a solution that brings the
low center from the western Gulf Saturday night to the
southeastern Gulf Sunday night. As the low moves out of
Southcentral Alaska, a brief drying trend will begin Sunday
morning over the Mainland as offshore flow sets up behind the low
and a ridge of high pressure builds in the upper-levels. Meanwhile
behind the ridge, the next longwave trough will dig through the
north central Pacific, encompassing the Aleutians as the next work
week begins. A series of surface low pressure systems will
develop within the longwave trough south of the western and
central Aleutians, with eastward progress limited by the
downstream ridge. The net result at the surface will be a
continuation of the showery conditions over the Aleutians and
southern Bering Sunday through Tuesday. The ridge that will build
in over the southern Mainland is progged to shift east Monday
evening, allowing for the surface lows and associated fronts over
the Aleutians to move to the southwest Mainland Monday night. This
will bring a return of widespread rain chances for the Mainland
starting Monday night and continuing through the middle of the
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales...170 173 414.
FIRE WEATHER...NONE.

&&

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