Small Craft Advisory 665, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT Tue Aug 28 2018

.ANALYSIS AND UPPER LEVELS...

The fast-moving upper level trough that crossed over the northern
AKPEN yesterday is now located over the northeastern Gulf and
continuing to move east. Residual rain associated with this
feature continues to fall over the eastern coast of the Prince
William Sound this morning.

Behind this departing trough, a ridge of high pressure is building
over the eastern Bering into Mainland Alaska, delivering the
typical subsidence-induced low level stratus and patchy fog to
the eastern Aleutians and coastal areas of Bristol Bay. Areas of
patchy fog are also visible on satellite this morning over Cook
Inlet and Prince William Sound.

Further west, a 998 mb North Pacific low is currently located
south of Adak while the associated front extends over the central
Aleutians.

&&

.MODEL DISCUSSION...

Model solutions are in good synoptic agreement through Wed. Only
minor differences in the placement of the Bering Sea low arise by
Thu morning, but this has little significance for the forecast, as
all models currently agree on the leading front becoming occluded
and lifting over Southwest late Wed and approaching the Interior
Thu morning.

&&

.AVIATION...

Clearing skies overnight have led to the development of patchy
low stratus over the Anchorage Bowl this morning. Web cams and
local METARS show only broken areas of MVFR to IFR stratus over
the bowl though there is likely lower clouds/fog hovering over the
Knik Arm/Cook Inlet. Stratus should burn off completely by mid
morning leading to VFR conditions and light winds for the rest of
the day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Thursday)...

After rainfall from the Cordova area into the Eastern Copper River
Basin diminishes this morning, most of the focus today will shift
to increasing outflow winds behind the departing Gulf low. These
winds are already being observed early this morning in
Resurrection Bay and Passage Canal and are expected to spread
eastward into the Valdez area later this morning. The strongest
winds are likely to occur around Seward where gusts will approach
gale force this morning. Another short term forecast concern will
surround the patchy fog/stratus that developed as expected around
Cook Inlet overnight and the possibility of its redevelopment
Wednesday morning. So far, most of the fog has occurred along
Kenai Peninsula coast from Kenai to Homer, however with the low
levels approaching saturation around the Anchorage Bowl and
Matanuska Valley, fog development during the morning commute this
morning cannot be ruled out. One limiting factor for this
development will be from mid-level cloud cover that is rapidly
approaching the area from the Susitna Valley. Once any fog/stratus
burns off this morning, generally sunny, warm, and pleasant
conditions are expected for much of Southcentral for the next 2
days.

This period of quiet weather then looks to end late Wednesday as
a weakening frontal system from the Bering Sea moves over the
area. This will bring increasing cloud cover back to the area on
Wednesday evening followed by the next chance of rain to the
region as a lee side low develops off the Kenai Peninsula and
moves through the North Gulf. At this point the best chance of
rain exists to the north of the Kenai Peninsula, including the
Anchorage Bowl and Matanuska Valley. One limitation to widespread
rainfall around the lee of the Chugach Mountains will be as gap
winds along Turnagain Arm and the Knik River Valley develop, some
low level drying should occur from downsloping. This may limit
rainfall amounts in particular around south/west Anchorage,
however other areas away from this drying influence will likely
see widespread wetting rains for Thursday afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Wednesday
Night)...

Surface and upper level ridging is building across Southwest this
morning, although low stratus continues to move inland as well.
There should be enough heating and weak northerly/offshore flow in
the afternoon for some partial clearing, especially over inland
areas.

Attention then turns to a Bering low and the associated occluded
front. This will move inland Wednesday bringing light rain to the
entire region. The front quickly moves into the Gulf of Alaska
with onshore flow keeping a moist and cooler pattern in place
heading into Thursday

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Wednesday
Night)...

A subtropical warm front is in the process of merging/phasing with
an upper level baroclinic trough over the Western Aleutians. The
front will quickly move north through the Bering Sea today and
tonight with a swath of gale force winds associated with it. The
low will occlude Wednesday then slowly drift east. A swath of
strong westerly gale force cold-air advection winds will
accompany this eastward progress heading into Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through
Monday)...

Thursday will start out with an area of precipitation associated
with a front north of the area over Interior Alaska.  An area of
low pressure located west of Nunivak Island Thursday morning will
track southeastward along the Bering Coast through the day
Thursday into Thursday night. This will keep unsettled weather
ongoing along the coast, while the rest of the moisture
associated with the front continues eastward into the Copper River
Basin by Friday morning. The low will cross the Alaska Peninsula
during the day Friday and weaken into a surface trough by Friday
evening.

As the trough continues southeastward into the Gulf of Alaska and
away from mainland Alaska, an expansive area of high pressure out
over the Bering will build in for the weekend and into Monday.
This should allow much of Southern Alaska to get a welcome break
from all of the wet weather. While this will be the case for most
of the area there are a couple of exceptions. First, on Saturday
afternoon and evening, a weak cold front will track southward out
of Interior Alaska and into the Copper River Basin. This could
mean a brief period of rainfall for portions of that area Saturday
evening. The models differ on whether or not any rainfall is
produced by this feature at all. Without any moisture source, that
fact is certainly not surprising. The other exception will be the
end of the dry weather as a strong cold front moves into the far
western Bering on Sunday, and reaches the Kuskokwim Delta as early
as Monday evening. Unsurprisingly, there are differences in
timing. Everywhere else, a dry weekend with seasonable
temperatures is expected. Enjoy!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130 173 174 175 176 185 412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...KO/KVP
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JW