Small Craft Advisory 638, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
850 PM AKDT Thu Aug 27 2020

.ANALYSIS AND UPPER LEVELS...

An active pattern is set up across Southcentral Alaska this
afternoon with a low in the southern Gulf. This is keeping a
showery pattern for the region with the last few easterly waves
moving in as the low tracks towards the eastern Gulf. Looking
westward, a strengthening low is moving moving into the western
Bering this afternoon centered near Shemya. This system has
already produced a few lightning strikes early this afternoon
south of the western Aleutians. South to southeasterly gale force
winds and widespread rainfall accompany this system as the
associated front pushes over the western to central Bering and
Aleutians.

&&

.MODEL DISCUSSION...

The deterministic models (ECMWF/GFS/NAM) are narrowing in a
solution with the remnants of Typhoon Bavi. It looks like the
low center will intensify as it tracks east across the Alaska
Peninsula on Sunday to near eastern Bristol Bay, perhaps far
western Gulf Sunday afternoon. Forecast package will be weighted
toward the 12z GFS model.

&&

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

&&

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

The radars at Middleton Island (PAIH) and Kenai (PAHG) have been
active today, especially over the Gulf Of Alaska. There is a
closed upper level low over the Gulf Of Alaska with numerous
shortwaves embedded in its circulation. The most potent shortwave
is over the southern GOA and is headed to SE AK. The little minor
shortwaves over Southcentral are just strong enough to sustain
isolated convection...keeping the atmosphere over Southcentral a
whee bit unstable. Just upstream, there is a minor ridge over the
Alaska Peninsula which will shunt the gulf energy eastward this
forecast period. Friday will be the driest day this forecast
package. All the while, a new mature low is churning over the
Western Aleutians and is expected to track eastward. The leading
edge of the precipitation shield will impact Kodiak Saturday. This
storm also has a tight pressure gradient associated with it and
the jet core steering this front will have a magnitude of 170
knots. Gusty winds will materialize at Whittier/Passage Canal,
Seward/Resurrection Bay, Turnagain Arm, Hillside and across the
Gulf Of Alaska. Stay tuned for more details. If the track of the
low adjusts it will definitely impact where hazardous winds could
develop. Very confident that unsettled conditions will be
widespread on Sunday.

&&

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

A few showers will persist through this evening as low level
cyclonic flow on the back side of a low pressure system persists
tonight. Showers will decrease as we head into Friday as an upper
level ridge moves towards the area. This will help to create
patchy fog for portions of the YK delta region. Friday will
generally be dry, however a weak area of low pressure will linger
north of the Bethel area which will kick off some showers mainly
over the Kuskokwim mountains.

Saturday a warm front will bring in rain and increasing east to
southeast winds. As we look towards Sunday rain will continue as
the front weakens over the land. An approached low pressure
system may bring stronger winds to the area and more rain, but
uncertainty on track of this system leads to lower forecast
confidence.

&&

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

Northwest flow will dominate the eastern Bering Sea and Aleutians
through Friday with generally dry weather, some patchy fog is
possible as an upper level ridge moves across the Bering Sea and
Aleutians through Friday morning. A low pressure system near
Shemya will gain strength and move east across the Aleutian chain
Friday and Saturday with gusty east to southeast flow and rain
along a frontal system ahead of the low. By Sunday the remnant
moisture and energy from Typhoon Bavi will interact with the upper
low over the western Aleutians this will allow for cyclogenesis
to occur with a strong low pressure moving across the central and
eastern Aleutians Sunday. Strong winds will be common with this
low pressure system with Gale to Storm Force winds along the
central and eastern Aleutians and eventually towards the AKPEN.
Some uncertainty still exists with this low pressure, but models
coming into better agreement, thus forecast confidence is
increasing.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

Model confidence has increased with a low approaching Bristol Bay
Sunday afternoon. The strength of this storm will produce storm
force winds, along the southern periphery of the low, south of the
eastern Aleutians. There is some uncertainty beyond this, with
the potential track up towards the northeast and gales across the
western Gulf Monday. After that, a fairly benign wind pattern is
expected across the Gulf and the Bering Sea for the remainder of
the forecast period. All in all, there will be plenty of
refinement associated with the latter part of this system as we
head towards the weekend, but mariners in the area should pay
close attention.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)...

A fairly active pattern is expected into the middle of next week.
A strong upper low ripping across the north Pacific Sunday moves
over the Southern Mainland Sunday evening then weakens. In its
wake numerous short waves will ripple eastward across the Alaska
Peninsula and Southern Mainland. The General mean position of the
nearly zonal upper jet remains over the northeast Pacific. After
the remnant of Typhoon Bavi makes landfall Sunday the pattern will
become more showery with support of upper short waves.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 170 173 175 177 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/BC
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...CK/BC