Small Craft Advisory 658, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
412 PM AKDT Sat Aug 25 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low continues to dominate the west. The low
is advancing northeastward advecting moist and warm air along the
coast line and throughout the Bering Sea. The low is sitting in
the left exit region of the jet streak at the end of a long-wave
trough causing the low to continue deepening. Jet stream winds
near the base of the trough are approaching 140 kts at 300 mb. On
the leading edge of the low, a warm front is draped over the
AKPen and southern Gulf of Alaska. Ahead of the front, a slight
ridge is building which is bringing a break in the clouds and
rain, resulting in a drier morning start through Southcentral
Alaska. However, the front can easily be seen moving northeastward
on radar as rain begins to move over the southern Kenai Peninsula
and the AKPen. As the warm front progresses northward, another
batch of rain is imminent throughout Southcentral Alaska.
Satellite imagery shows broad cloud cover over the most of the
Bering Sea and the Gulf, but there are a few breaks in the cloud
cover over Southcentral also aiding in the drier conditions.

&&

.MODEL DISCUSSION...

Models are in good agreement in regards to the synoptic features
through the early periods but begin to diverge around 36 hours out.
Guidance is in agreement in the depiction of the low in the
Bering Sea moving into the northern waters, in addition to the
front entering the Gulf of Alaska. There is general consensus that
there is a low advecting from the North Pacific however, the EC
continues to be the faster solution with the NAM and GFS lagging
behind. This pattern continues through Mon.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist tonight with a
few showers in the area. Southeast winds and rain are expected to
pickup in advance of a frontal passage Sunday which may push the
aerodrome into MVFR for a time.

&&

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

The main impacts, in the short range forecast through Tuesday
afternoon, is surrounded around the storm system currently moving
to the Southwest coast. Moderate to heavy rainfall and gusty
winds will impact Southcentral late Saturday night through early
Monday morning. Conditions from this storm improve inland on
Monday, however the Gulf and surrounding coasts will see the next
fast moving front enter the western gulf by early Monday morning.

Rain spreads across Southcentral from the Kenai this evening as
the frontal boundary extending from the Bering low is pushed
north. This low picks up momentum as it tracks toward the
Southwest coast overnight. Gusty winds are expected to increase as
the boundary moves through due to a pressure gradient squeeze
between a surface ridge building along the Northeast Gulf and the
low nearing the Kuskokwim Delta early Sunday. The front diminishes
late Sunday night as the associated surface low fills and it
tracks inland near the Seward Peninsula.

The next front moving into the western Gulf Monday morning is
projected to stay on an easterly track to the eastern Gulf by
early Tuesday. Rain from this system may spread into the Northeast
gulf, however inland locations should stay dry unless the low
center tracks further north.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Tuesday)

The tale of two storms best sums up the weather for the next few
days. Our first storm is currently northwest of the Pribilof
Islands, will move towards eastern Norton Sound by Monday night.
Ahead of this feature, a warm front was beginning to move ashore
across the coastal locations, with widespread rainfall preceding
the front. As both systems move northeast, rainfall will become
more showery in nature through the night. As both features pull
north of the area, moisture in the cyclonic flow will maintain a
chance of showers through Monday, especially during the afternoon
hours when instability increases slightly due to daytime heating.
high pressure builds across the region for Monday night and
Tuesday, with a gradual drying trend progressing from southwest to
northeast during this time.

Further south and east for the Alaska Range through the AKPEN, a
secondary system trekking parallel to and south of the peninsula
will allow some rain to continue as a deformation band associated
with the low skirts this region through Tuesday morning, with dry
advection taking hold by the afternoon.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Tuesday)

An active period is in store for parts of the region, as an area
of low pressure treks from the central Bering towards the western
coast of Alaska, with its trailing cold front, currently near
Nikolski, slowing its eastward progression over the next 24 to 36
hours. This is due to another area of low pressure forming a few
hundred miles south of the Aleutians, which will ride up the front
like a train on the tracks. this will bring a second round of
increased ascent as a deformation band takes shape, with the low
traveling south of and roughly parallel to the AKPEN.

Further north and west, cyclonic flow around the Bering system
will keep showers in the picture through Monday, before an area of
high pressure develops, helping to clear the skies some while
significantly reducing any precipitation potential.

&&

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

By Tuesday, ridging over Southwest Alaska will have pushed much of
the precipitation out of Southern Alaska for a brief drier midweek
break. The upper level ridge will amplify slightly Tuesday night
as a strong north Pacific low crosses the western Aleutians and
tracks into the Bering. The ridge axis will shift east over
Southcentral Alaska on Wednesday as the frontal system
accompanying the Bering low approaches the Kuskokwim Delta coast.
The front will push inland over Southwest Wednesday evening and
then be followed by the associated upper level trough on Thursday.
The front will continue east into Southcentral Alaska on Thursday
with the upper trough following Thursday night and Friday.
Additional frontal systems with moist tropical origins will likely
follow over the weekend, although model uncertainty remains high
regarding their timing and track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 131 132

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JR