Small Craft Advisory 590, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT Wed Aug 1 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper low continues over the NE Bering Sea with a
negatively oriented ridge extending from the NE Pacific to over
the Yukon Territory. Satellite imagery shows upper waves and
moisture caught in a broad nearly zonal jet stream moving eastward
from the Western Bering Sea/Aleutians to across Southcentral
Alaska. At the surface, a stacked low continues near Saint
Matthew Island with surface front/troughs bringing abundant cloud
cover from the Western Aleutians to over the Mainland. Rain was
observed from the western Aleutians to over mainland. Rain ended
across much of Southcentral this morning just after transiting to
drizzle. Low stratus and patchy fog across the Eastern Bering
Sea/SW Alaska, and portions of Southcentral make for challenging
TAF forecasts.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement through the short term. The
GFS is generally bringing easterly gales into the northern Gulf
late Thursday into Friday as the surface low intensifies over the
Gulf. Will be leaning toward max small craft winds for this
package.

&&

.AVIATION...
PANC...The ceiling should remain VFR this evening, but the
terminal could see MVFR ceiling again Thursday morning as a front
brings in rain. Winds should be fairly light through tonight, but
possibly a little stronger out of the southeast Thursday
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (through Saturday
afternoon)...
Despite a cooler air mass overhead, this evening should be fairly
pleasant with some breaks appearing around the Cook Inlet region.
A weakly unstable air mass lingers over the Copper River Basin
where isolated thunderstorms are possible early this evening. The
pattern changes very quickly overnight as upper-level heights fall
and a trough moves overhead. Widespread rain will move into Cook
Inlet Thursday morning. As the surface low pressure moves into the
Gulf, a period of easterly cross-barrier flow will set up, which
could limit rain amounts in the normal downslope areas. The
Turnagain wind will tick up at this time, but shouldn't impact the
Anchorage Bowl much at all. Rain will linger along the north Gulf
coast on Friday morning, but otherwise the low will exit to the
southeast fairly quickly. Friday has the potential to be dry and
warm as the offshore flow establishes. Saturday will feature some
middle and high clouds streaming overhead from the northwest but
should otherwise be a warm late-summer day.

&&

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

As the main upper-level low continues to track north toward the
Bering Strait a more progressive weather pattern is taking shape
across the southern Bering. This pattern features a series of
quick-moving troughs and fronts, the first of which will continue
to produce rain across the Y-K Delta tonight as it lifts farther
into the mainland interior. The second, a warm front situated over
the eastern Aleutians with a trailing cold front, will push
another slug of clouds and moisture across the Alaska Peninsula
and through Bristol Bay overnight tonight into Thursday. Rain and
clouds continue through early Friday morning as a surface low
positioned south of Norton Sound dives south along with its upper-
level counterpart in the northern Bering. Conditions improve
briefly for Friday as high pressure builds over the southwest
mainland. This will be short-lived, however, as a weak warm front
approaches the forecast area Friday night with increasing clouds
and a chance of rain for the start of the weekend.

&&

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

It will remain quite active over the Bering through the forecast
period. To begin, a vertically stacked low continues to lift
northwest toward Saint Matthew Island overnight. Farther south, a
a warm front will continue to produce widespread rain over the
eastern Aleutians through the evening hours. Rain and fog will
also persist over the central and western Aleutians through
tomorrow as a shortwave rides the boundary of an upper-level low
to the north and a building ridge to the south. The building ridge
takes hold over the central Aleutians by Thursday afternoon and
remains through Saturday with low stratus and patchy fog likely
under the ridge. The resultant northwesterly flow upstream of the
ridge should keep some clouds and showers over the eastern
Aleutians through Friday. To the west, a storm system will ride up
and over the ridge with a warm front entering the western Bering
Thursday reaching the eastern Bering Friday evening. The trailing
cold front will produce some gales across the western Aleutians
late Thursday then weaken and stall while keeping things unsettled
through Friday.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning this weekend a stagnant pattern will begin to setup as a
cutoff low from the Gulf moves southward into the North Pacific
and becomes nearly stationary. This will allow for a ridge of high
pressure to ride up along a broad trough that will setup over the
Northern Mainland, providing persistent zonal flow for most of
the Bering and South Mainland into early next week. This will
allow for a series of frontal systems from the Bering to move
through the South Mainland for later this weekend into the early
part of next week, allowing for a wet start to next week for most
areas.

By the middle of next week there uncertainty begins to develop
with how deep a trough diving from Eastern Russia will move into
the Bering. This will erode away the ridging that will setup over
the Bering, and allow for continued active and likely wet weather
for most of the area into the later portions of wet week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 178.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...DEK