Small Craft Advisory 425, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 AM AKDT Sat May 26 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough is in place over the Gulf this
morning. Behind the trough, a large ridge extends across the
Bering. The ridge axis, oriented from north to south, is currently
centered over the Aleutians. Stout northerly winds are observed
this morning in the eastern Aleutians and AK Pen with higher gusts
occurring through the typical gaps and passes.

For Southcentral, a front associated with a low in the eastern
Gulf is moving inland, generating persistent rain showers
overnight along the northern Gulf coast. Showers continue to
spread in this morning, traveling westward across the Kenai
Peninsula and Cook Inlet. As the low becomes vertically stacked,
it will remain nearly stationary in the eastern Gulf through the
weekend.

&&

.MODEL DISCUSSION...
Model guidance has been consistent over the last 24 hours. The
GFS was the preferred model for the morning forecast package,
though all models agree fairly well on the strength, timing, and
placement of the low in the Gulf. The same is true for the
building ridge over the Bering. Models also agree on a thermal
trough crossing over the Southern Mainland this weekend, bringing
a chance of isolated thunderstorms to the Copper River Basin,
Susitna Valley, and as far south as the Chugach Mountains around
Anchorage. Toward the end of the weekend, models diverge on the
placement of an incoming Kamchatka low over the western Bering.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level trough over the Kenai Peninsula will weaken and
drift south today. As a result, shower activity will diminish
this morning. This will also allow upper level heights to build
over the area, and the thermal trough that has been north of the
Alaska Range will push south into the area this afternoon. This
will increase the instability and cause diurnal showers to
develop, mainly over the higher terrain. Isolated thunderstorms
are possible this afternoon and evening over much of the Copper
River basin, the Talkeetna mountains, and the northern Susitna
valley. On Sunday afternoon and evening, the possibility of
thunderstorms will move south and west, extending from the western
Copper River basin to the southern Susitna valley, and southward
to the interior Kenai Peninsula. Further east, an upper level
short-wave trough will move into the eastern Copper River basin
and Prince William Sound from northwest Canada tonight into
Sunday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
With the thermal trough strengthening in Southwest Alaska today,
the pattern looks good for isolated to scattered thunderstorms
forming along and near the thermal trough axis running from north
to south across the Kuskokwim Valley and into the greater Bristol
Bay area. Along the coast, a building ridge and breezy north to
northwesterly winds will bring cooler, more stable conditions. By
Sunday, as the upper level ridge builds more firmly over eastern
Bering, the breezy northwesterly winds will spread further inland
bringing a cooling trend and pushing the thermal further to the
north and east limiting the thunderstorm potential to the western
Alaska Range. On Monday, the upper level ridge will weaken but
shift more onshore further stabilizing conditions over all but the
Alaska range and interior to the north. An approaching frontal
system will reach the Southwest Alaska coast Monday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Ridging building over the Bering today will strengthen as it
shifts east through the weekend in response to a deepening North
Pacific low. As the ridge shifts east, the northerly flow over the
eastern Bering and channeling through the gaps and passes in the
Alaska Peninsula and eastern Aleutians will turn progressively
more northwesterly Saturday night and Sunday before diminishing
Sunday night as the upper level ridge shifts overhead. On the
upstream side of the ridge, an initial weak frontal system will
reach the western Aleutians this afternoon and evening. A
developing frontal wave well to the south will strengthen and
merge with the front over the central Aleutians on Sunday as the
frontal system strengthens to Gale force. The front will track
from the central to eastern Bering Sunday night and Monday, then
begin to shear apart and weaken as it approaches the Southwest
Alaska coast Monday night.

&&

.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...KO
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
LONG TERM...KVP