Small Craft Advisory 617, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
425 PM AKDT FRI AUG 7 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVELS...A BROAD UPPER RIDGE WAS EVIDENT FROM THE NORTH
PACIFIC...ACROSS THE AK PEN...TO OVER THE CENTRAL MAINLAND. THREE
MAIN UPPER LOW CENTERS WERE EVIDENT AROUND THE RIDGE...ONE NEAR
WRANGELL ISLAND...SE OF THE GULF OF ALASKA...AND A CENTER NEAR
THE WESTERN ALEUTIANS. A FAIRLY BROAD W-SW POLAR JET WAS OBSERVED
ACROSS THE NORTH PACIFIC TO JUST SOUTH OF THE ALEUTIANS. WATER
VAPOR IMAGERY SHOWS RELATIVELY DRY AIR ALOFT OVER THE SOUTHERN
MAINLAND AND FAIRLY MOIST CONDITIONS ASSOCIATED WITH A COMPLEX
FRONTAL STRUCTURE ACROSS THE BERING SEA AND ALEUTIANS.

.SURFACE...A VERY WEAK SURFACE TROUGH WAS EVIDENT ACROSS THE
NORTHERN GULF OF ALASKA WITH RIDGING OVER SW ALASKA. ABUNDANT LOW
CLOUDS AND FOG WERE EVIDENT ACROSS MUCH OF THE EASTERN BERING SEA
AND COASTAL SOUTHWEST ALASKA. A WEAKENING OCCLUDED FRONTAL
BOUNDARY WITH LIGHT RAIN WAS MOVING EAST TOWARD THE WESTERN AK PEN
AND THE PRIBILOF ISLANDS. DUEL LOW CENTERS WERE OBSERVED SOUTH OF
THE WESTERN ALEUTIANS AND A TRIPLE POINT NW OF ADAK. THE STRONGER
OF THE TWO CENTERS WAS SOUTH OF THE ALEUTIANS. WINDS ASSOCIATED
WITH THIS FEATURES WERE APPROACHING SMALL CRAFT STRENGTH ACROSS
THE ALEUTIANS.

&&

.MODEL DISCUSSION (SHORT TERM THROUGH SUNDAY AFTERNOON)... MODELS
INITIALIZED WELL AND FORECAST TRENDS ARE SIMILAR AND SYNOPTICALLY
IN GOOD AGREEMENT. TIMING OF UPPER CENTERS AND THE ONSET OF
SOUTHWESTERLY FLOW ACROSS THE SOUTHERN MAINLAND THIS WEEKEND ARE
SIMILAR. QPF PATTERNS ARE ALSO IN GOOD AGREEMENT. THE GFS WILL BE
USED IN GENERAL IN THE WESTERN DOMAIN AND THE NAM IN THE EAST.

&&

.FIRE WEATHER...
MINIMAL FIRE WEATHER CONCERNS FOR THE NEXT FEW DAYS. AN UPPER
LEVEL RIDGE WILL BRING WARMER AND DRIER CONDITIONS TO THE
AREA...ESPECIALLY THE COPPER RIVER BASIN BUT RELATIVE HUMIDITY
VALUES WILL REMAIN ABOVE 30 PERCENT...BUT WINDS WILL GET GUSTY
FROM THE SOUTH IN THAT AREA ON SATURDAY AFTERNOON. OTHERWISE
SUNDAY WILL BRING A WIDESPREAD WETTING RAIN TO MOST OF
SOUTHCENTRAL.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A PATTERN CHANGE IS UNDERWAY OVER THE NEXT 48 HOURS FOR
SOUTHCENTRAL ALASKA AS AN UPPER LEVEL SHORTWAVE ROTATING AROUND
THE BASE OF THE LOW CENTERED OVER THE NORTHERN ALASKAN COAST WILL
PUSH THROUGH SOUTH CENTRAL ALASKA. THE UPPER LEVEL RIDGE THAT IS
CENTERED OVER THE NORTHWESTERN PACIFIC OCEAN WILL FLATTEN OUT SOME IN
RESPONSE TO THE SHORTWAVE MOVING THROUGH...CREATING SOUTHWESTERLY
FLOW OVER THE SOUTHERN HALF OF THE ALASKAN MAINLAND. AS MOISTURE
STREAMS OVER TOP OF THE RIDGE...THE SOUTHWESTERLY FLOW ALOFT WILL
TRANSPORT THIS MOISTURE UP INTO SOUTHCENTRAL ALASKA AS THE
SHORTWAVE PASSES THROUGH. THE EFFECT OF THIS WILL BE FAIRLY
WIDESPREAD WETTING RAINS OVER THE REGION DEVELOPING LATE SATURDAY
NIGHT. ON SUNDAY...RAINFALL MAY BE HEAVY AT TIMES OVER PORTIONS
OF THE SUSITNA VALLEY AND ALONG PRINCE WILLIAM SOUND AS THE FLOW
ALOFT WILL HAVE THE BEST ORIENTATION TO THE TERRAIN TO PRODUCE
HEAVY RAINFALL. THUNDERSTORMS WILL BE LIMITED ON SATURDAY
AFTERNOON TO THE TALKEETNA MOUNTAINS AND THE ALASKA RANGE NORTH OF
GLENNALLEN AS CONDITIONS WILL REMAIN UNSTABLE ENOUGH BETWEEN
DAYTIME HEATING AND OROGRAPHIC LIFT TO PRODUCE A FEW THUNDERSTORMS
BEFORE THE APPROACHING WAVE STABILIZES THE ATMOSPHERE.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

SHOWERS THIS EVENING WILL BE THE END OF CONVECTIVE ACTIVITY
FOR SEVERAL DAYS. THE APPROACHING FRONT WILL BEGIN TO SPREAD
RAIN OVER THE KUSKOKWIM DELTA TONIGHT. THIS WILL SPREAD ACROSS
SOUTHWEST AK ON SATURDAY AND REACH THE ALASKA AND ALEUTIAN
RANGES BY SATURDAY NIGHT. STEADY RAINFALL WILL LAST THROUGH
SUNDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

RAIN WITH SOME BREEZY CONDITIONS THROUGH CHANNELED TERRAIN WILL
PERSIST ACROSS THE ALEUTIANS THROUGH SAT. PRECIPITATION WILL THEN
TAPER OFF TO SHOWERS ON SUN.

&&

.LONG TERM FORECAST (MONDAY THROUGH FRIDAY)...

AS THE LARGE RAINMAKER THIS WEEKEND MOVES OUT OF THE AREA ON
MONDAY...THE BERING SEA/ALEUTIANS AS WELL AS THE MAINLAND WILL
COME UNDER THE GENERAL INFLUENCE OF A BROAD LONGWAVE TROUGH
CENTERED OVER THE FAR WESTERN BERING. THIS WILL SET THE AREA UP
UNDER WESTERLY OR WEST/SOUTHWESTERLY FLOW FOR NEXT WEEK. THIS IS
USUALLY A PRETTY PROGRESSIVE PATTERN OVERALL...WHICH MEANS THAT
INDIVIDUAL STORMS/WAVES TEND TO BE ON ON THE FASTER AND WEAKER END
OF THE SCALE (RELATIVELY) FOR THE VOLATILE NORTH PACIFIC. THE
UPCOMING WEEK LOOKS LIKE IT WILL FOLLOW THIS GENERAL
TEMPLATE...WITH SENSIBLE WEATHER DICTATED LARGELY BY AN
ALTERNATING SEQUENCE OF SHORTWAVE TROUGHS AND SHORTWAVE RIDGES.
THE BIGGEST AREA OF UNCERTAINTY IS THE EVENTUAL FATE OF A TROPICAL
DISTURBANCE CURRENTLY SOUTHEAST OF JAPAN. GLOBAL MODELS HAVE BEEN
STRUGGLING WITH WHETHER TO BRING THE REMNANTS INTO THE ALEUTIAN
REGION OR KEEP IT SOUTH...WITH THE LATEST AND GREATEST GUIDANCE
SUGGESTING IT WILL STAY SOUTH AND OUT OF OUR AREA. THERE SHOULD BE
SOME BREAKS OF SUN BY MIDWEEK ACROSS THE MAINLAND...BUT NO
HEATWAVES OR HUGE RAIN-OUTS ANTICIPATED AT THIS TIME.
OVERALL...CONSIDERING THAT AUGUST IS HISTORICALLY THE RAINIEST
MONTH FOR MOST LOCATIONS IN SOUTHERN ALASKA...NEXT WEEK SHOULD BE
NICER THAN NORMAL ACROSS MAINLAND LOCATIONS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...RF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...AMD