Small Craft Advisory 454, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKDT TUE MAY 31 2016

.ANALYSIS AND UPPER LEVELS...
The two main features this morning are an upper level low over the
Central Aleutians, and an upper level high centered over northeast
Russia. As a result, clear skies dominate much of mainland Alaska,
while stratus and light rain encompass much of the Aleutian Islands
and the Bering. Meanwhile, an upper level wave is beginning to dig
southward across the northern shores of Alaska. This wave and
associated cold air, will be the main weather driver across the
Southcentral and Southwest Mainland the next few days.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement in the short term as the
the upper level high slips away to the northwest further into
Russia, and an upper level low and its associated wave dive
southward from the Arctic to the Southwest and Southcentral
Mainland. Models differ more in the intensity and location of
convection across the Southwest and Southcentral mainland in
response to the upper level wave towards the end of the short
term.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Southcentral Alaska remains in an area of weak upper level flow
between a trough in the gulf of Alaska and an upper ridge over
northern Alaska. A weakening frontal system across the western and
central Gulf will continue to bring precipitation and some breezy
winds to the Gulf coastal waters, Kodiak Island and the southeastern
Kenai Peninsula. Marginal instability and the interior thermal
trough sagging south will bring isolated to scattered afternoon
and evening showers Wednesday and Thursday with a slight chance of
thunderstorms mainly along the mountains surrounding the Susitna
Valley and Copper River Basin. Surface ridging along the northern
Gulf Coast will increase southeasterly gap winds through Turnagain
Arm, the Knik River Valley and through the Copper River Valley
Thursday afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weak disturbance will bring a chance for a few isolated
thunderstorms to the Lower Kuskokwim Valley and inland portions of
the Kuskokwim Delta through this evening. Afternoon and evening
convective activity will remain the primary forecast challenge
across the Southwest mainland through Thursday as a positively
tilted upper level trough digs over the Y-K Delta region starting
Wednesday evening, bringing colder air aloft and helping to
steepen mid level lapse rates. This will support the development
of numerous showers and isolated thunderstorms centered across the
Kuskokwim Valley but also extending towards Bethel and into
western portions of greater Bristol Bay. There is potential for
this convection to continue into the overnight hours of Wednesday
as the digging trough provides extra forcing. This will complicate
the forecast for thunderstorm coverage on Thursday afternoon, as
any lingering cloud cover from overnight convective activity could
serve to inhibit diurnal heating and limit instability.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The quasi-stationary low centered west of Dutch Harbor will keep
windy and rainy conditions over the southern half of the Bering
through Wednesday, with conditions beginning to improve on
Wednesday night as the low finally drops south into the Pacific.
This will yield partially clearing skies in northerly flow through
Thursday. Patchy fog development will be the main issue as high
pressure builds over the western Bering, with a transition to a
more showery regime across the southeast Bering and Alaska
Peninsula as temperatures cool aloft.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The preferred EC/NAM solution was used for day 3, bringing a cool
airmass and associated area of low pressure south through the
Bering Sea to Bristol Bay. The eastern flank of this system will
interact with the thermal ridge draped near the AK Range. Showers
are expected over much of mainland AK through Thu evening with
thunderstorms west of the AK and Aleutian ranges as well as over
the Copper River Valley and Talkeetna Mountains.

As the cold air filters into the Bering, models become more varied
in their solution. The remainder of the forecast through Tue
utilizes the ensemble approach of WPC. This solution weakens what
Fri into Sat is a residual Bristol Bay low. This is also the end
of the main interaction with the thermal trough. On Sat the low
continues to weaken and merges with a low south of the Aleutians.
By Mon this low weakens but the next north pacific low migrates
east across the chain through Wed. While this is happening, a
dirty ridge builds across the mainland.

Friday afternoon and evening expect showers again with some
thunderstorms west of the AK and Aleutian ranges as well as over
the northern Copper River Valley and Talkeetnas. Sat through Wed
some showers can be expected beneath the dirty ridge over the
mainland. High pressure associated with the cool airmass over the
Bering will keep most of the Aleutian chain fairly dry through
Sat. Moisture from the lows passing south of the chain will bring
showers back to the islands late this weekend, and persist through
midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 131 138.
FIRE WEATHER...None.

&&

SYNOPSIS AND MODEL DISCUSSION...EN
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...DS