Small Craft Advisory 631, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
413 PM AKDT Sat Aug 26 2017

.ANALYSIS AND UPPER LEVELS...
The long wave trough from the Arctic to the southwest mainland
continues to be the main driver of weather over the mainland.
Currently a deformation zone over the Kuskokwim Delta is
producing widespread rain and showers over the region. The upper
level low off Cape Newenham is dropping south. The surface low has
already dropped south of the Alaska Peninsula leaving the north
to northwest flow along the eastern half of the Bering Sea
coastline and Alaska Peninsula. Currently a low and front are in
the Gulf of Alaska and is primarily affecting the North Gulf
Coast. Satellite imagery shows this to be unstable and showery,
though we are not seeing any lightning along the Gulf Coast or
Prince William Sound so far.

The easterly flow across the North Gulf Coast and Prince William
Sound is holding showery weather and cloud cover over the south
central mainland today. Cross barrier flow is not very strong and downsloping
has not mitigated the intensity of the showers today, though
showers thus far have been relatively light.

The trough that moved across the Aleutians last night was
unstable enough to result in a band of thunderstorms moving across
the extreme western Aleutians out near Shemya. This has since
settled down and the thunderstorm activity has ended. A front is
now extending from the low near the Kurile Islands east along the
Aleutians. This front will continue to elongate to the east and
weaken and the triple point now near Adak will move off to the
east southeast becoming a low that will then interact with the
system we are expecting to move into the Gulf of Alaska tomorrow
and Monday.

&&

.MODEL DISCUSSION...
The models remain in good agreement with the NAM continuing to be
the outlier with respect to the low moving into the Gulf of
Alaska on Monday. The GFS has been consistent with it's solution
for this storm and the EC continues to align well with it. The
models are in excellent agreement with the formation then movement
of the triple point low over the central Aleutians into the Gulf
of Alaska storm.

&&

.AVIATION...
PANC...The onset of the southeast Turnagain Arm wind has improved
conditions to VFR in the area. The cross barrier flow this evening
and overnight will continue to dry out the low levels over the
terminal. The gusty southeast winds are expected to impact the
airport through this evening before abating after midnight. They
may redevelop again tomorrow (Sunday) afternoon, but should be
weaker than today.

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

The closed low in the gulf weakens tonight becoming an open wave
with the upper trough remaining centered across the mainland.
Showers will continue to develop across Southcentral and the gulf
as weak waves rotate along the broad trough. Isolated
thunderstorms are possible along the most unstable area of the low
center, which is across the central and northern gulf, through
Sunday morning. Thunderstorms are not anticipated elsewhere,
however small mesoscale changes across the mainland could produce
a strike or two in this conditionally unstable environment.

The main focus in the short term surrounds the next organizing
storm system expected to move up from the North Pacific on
Monday. The associated surface low moves a front into the southern
gulf early Monday morning spreading rain to Kodiak Island and the
gulf coast during the afternoon. Winds rapidly increase along the
front during the day with directional shifts as the front slams to
the coast. The last several models runs showed tighter pressure
gradient trends along the gulf; therefore, winds were increased in
the afternoon package with gale to storm force winds anticipated
with this next storm. The front pushes quickly inland Monday
night spreading rain throughout Southcentral into Tuesday along
with gusty winds, especially through east to west oriented gap
locations.

&&

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

The southwest mainland remains under the influence of a large
upper-level trough. Disturbances within the broad cyclonic flow
and diurnal instability will trigger numerous afternoon showers
coupled with some isolated thunderstorms across the Lower
Kuskokwim Valley Sunday afternoon. A pattern shift will occur
Monday evening as a gale force front over the western Gulf moves
inland over the Alaska Peninsula. The front will reach the Bristol
Bay area by Monday night and weaken as it pushes west throughout
the day. In addition to gusty winds, this front will bring a
swath of rain to the Bristol Bay area Tuesday night through
Wednesday morning.

&&

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

A frontal boundary associated with a low over the western Bering
Sea will stall and weaken overnight over the central Bering Sea.
A swath of gale force winds associated with the front over the
western Bering will weaken to small craft advisory-level winds
overnight as the front stalls and loses it's strength. A Kamchatka
low will move to the western Aleutians by Monday and will serve
as the focal point for the next round of showers to impact the
Bering Sea. Rain showers will shift east Tuesday as the low tracks
to the central Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term will begin mid week and will be characterized
primarily by a low south of Kodiak Island that will have moved
into the area from the south. It's front will push in over
Southcentral Alaska in the Monday night time frame. The low will
then slowly track north bringing continued southeast flow and rain
to southern Alaska, with the heaviest along the north Gulf coast.
On Friday the low will dissipate and a new low will push into the
Bering from the west.

&&

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

&&

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