Small Craft Advisory 423, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 PM AKDT Fri May 25 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough stretches from just south of the AK
Peninsula into the Gulf, bringing widespread cloud cover and
rainfall to much of the Gulf Coast today. This trough has largely
remained far enough to the south to limit any cloud cover to just
thin high level clouds to the Anchorage Bowl and Matanuska Valley.
Further north over the Susitna Valley and Kuskokwim Valley skies
have remained mostly sunny today, allowing for the development of
some diurnal rain showers to begin to developing. Over the Bering
Sea, the remnants of the easterly wave that brought rainfall from
the Southwest Mainland into the Pribilof Islands since yesterday
is beginning to diminish as high pressure from the Bering Sea
begins to move in. While this ridge is allowing for overall
diminishing clouds, widespread low stratus has developed under the
ridge and has spread throughout a majority of the Bering Sea.

&&

.MODEL DISCUSSION...
Model agreement remains generally good through this weekend with
both the location and strength of the thermal trough that will
move through the South Mainland this weekend. In particular, there
is increasing agreement that this trough will spread even further
southward into the Chugach, allowing for chances for thunderstorms
brought further southward, including into mountains surrounding
the Anchorage Bowl. Model agreement is also very good with the
timing of the next low to push through the Bering for late this
weekend. In general forecast confidence for most of the area is
above normal.

&&

.AVIATION...
PANC...VFR conditions will persist through Saturday with the
primary concern being the timing and intensity of Turnagain Arm
winds moving into the area this evening. There is stronger down-
inlet flow that yesterday which should prevent these winds from
developing as strong as they did on Thursday, however likely not
strong enough to limit them from moving into town at all. Winds
are then expected to remain light and variable for Saturday as the
pressure gradient along the Gulf Coast weakens.

&&

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

A decaying surface trough extending from Marmot Bay south of
Prince William Sound will continue to drift slowly westward
throughout the evening hours. The widespread showers associated
with this trough will lessen Saturday morning as a drier,
offshore flow develops. At the upper levels, a building ridge
over southwest Alaska will shift the thermal trough currently in
place over central Alaska south of the Alaska Range. This sagging
trough, combined with daytime surface heating, will build the
convective potential over the region. Scattered showers and
isolated thunderstorms are expected to develop over the Talkeetna
and Alaska Ranges Saturday afternoon and evening, with the
Chugach and Kenai Ranges getting in on the action Sunday afternoon
and evening. An isolated shower or thunderstorm is not out of the
question for the Anchorage bowl and the Mat- Su Valleys late
Sunday as a northeasterly steering current aloft could drive cells
developing off the mountains into the valleys.

The threat for showers and thunderstorms diminishes on Monday as
westerly flow develops ahead of the next trough. This will usher
in more maritime air, inhibiting convective potential.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 3)...
Diurnal convection will focus along a weak thermal trough from the
Kuskokwim Valley to Bristol Bay this afternoon and evening.
Maintained a slight chance of thunderstorms, but don't think
conditions are quite unstable. This is evident by the lack of
vertical growth in clouds as indicated by the relatively warm
Infrared Satellite cloud top temperatures. The thermal trough will
strengthen on Saturday, so expect an increase in both showers and
thunderstorms. The storm motion will be from north to south on
Saturday, so any thunderstorms which do form in the more unstable
interior portions of Bristol Bay may drift into the inner portion
of the Bay, between Dillingham and King Salmon.

Meanwhile, cooler and more stable air will advect southward along
the east side of a ridge over the Bering Sea. This air mass will
move into the Kuskokwim Delta Saturday then continue eastward
across Southwest Alaska on Sunday. This will confine any showers
and thunderstorms to right along the west side of the Alaska Range
on Sunday. The ridge will move overhead on Monday leading to a dry
mostly sunny day across most of the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 3)...
A ridge of high pressure will continue to build eastward across
the Bering Sea and Aleutians tonight through Saturday resulting in
benign conditions. Cold advection on the front side of the ridge
will lead to gusty northerly winds across the far eastern bering
Sea southward to the eastern Aleutians and southern Alaska
Peninsula. This ridge will be transitory, continuing on to the
eastern Bering Sea on Sunday.

A fairly weak frontal system will move to the western Aleutians
Saturday night. However, a digging trough over the northwest
Pacific will quickly lead to a strengthening upper level jet
and surface cyclogenesis. This will serve to both invigorate the
frontal system along the Aleutians and track the new stronger
storm to the central Aleutians on Sunday. Have trended stronger
with winds in the new forecast, with gales likely across the
central to eastern Aleutians Sunday night into Monday. The larger
frontal system will continue eastward across the Bering Sea
through Monday, bringing widespread rain and winds to the entire
region.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Memorial Day, models are in good agreement in the
overall longwave pattern with a ridge of high pressure over
Southcentral while a low pressure system is located over the
Western Bering with its associated front moving east across the
Central and Eastern Bering Sea. Therefore, there is decent
confidence that most of mainland Alaska will see pleasant weather
for the holiday with the exception of diurnally driven convection
over higher terrain and the Northern Susitna Valley.

The frontal system out west looks to move over the west coast and
AKPEN by Monday night spreading a wide swath of rain and wind across
the area. The front looks to shear apart and weaken by the time
it crosses the Alaska Range and moves into Southcentral Tuesday as
it runs into an area of high pressure to the east. Typical
upslope/windward locations along the Gulf Coast and Kodiak should
still see precipitation with downslope areas seeing lighter precip
or even none at all. Looking past mid-week, there is very little
model consensus in the sensible weather at the surface with models
becoming completely out of phase with each other by the end of
the week (trough over ridge). Thus, the long term forecast stuck
with the WPC solution which keeps an upper level low over the
Bering and unsettled weather over most of the region.

&&

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

&&

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