Small Craft Advisory 672, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
430 PM AKDT WED AUG 26 2015

.ANALYSIS AND UPPER LEVELS...

BROAD BUT FLAT RIDGING OVER THE BERING SEA CONTINUES TO STEER
STORMS INTO THE NORTHERN HALF OF THE STATE. A STRONG 150+ KT ZONAL
JET MOVING INTO THE BERING IS MAKING FOR A VERY DYNAMIC AND FAST-
MOVING WEATHER PATTERN ACROSS MUCH OF THE STATE. SATELLITE IMAGERY
AT MIDDAY SHOWS A VIGOROUS SHORTWAVE MOVING EASTWARD ALONG THE
BROOKS RANGE...WITH AN ASSOCIATED BAND OF PRECIP EXTENDING
SOUTHWARD ACROSS THE COPPER RIVER BASIN AND INTO THE NORTHERN
GULF. WIDESPREAD RAIN HAS ENDED ACROSS SOUTHWEST ALASKA AND THE
COOK INLET AREA...WITH THE HIGHEST RAINFALL AMOUNTS CONFINED TO
THE NORTHERN SUSITNA VALLEY AND ON THE NORTH AND WEST SIDES OF THE
ALASKA RANGE. UNSEASONABLY STRONG AND WIDESPREAD WEST AND
SOUTHWEST WINDS ARE OCCURRING ACROSS ALMOST ALL OF SOUTHERN
ALASKA...AND WILL BE SLOWLY SWINGING AROUND TO A NORTHWEST AND
NORTHERLY DIRECTION OVER THE NEXT DAY OR SO. THESE STRONG
NORTHWESTERLY WINDS...PARTICULARLY THROUGH AND EAST OF THE ALASKA
RANGE...REPRESENT THE PRIMARY FORECAST CHALLENGE TODAY.

OVER THE BERING SEA...STRONG HIGH PRESSURE OVER THE NORTH PACIFIC
IS KEEPING GUSTY WEST WINDS AND RELATIVELY WARM TEMPERATURES IN
PLACE ACROSS THE ALEUTIANS AND SOUTHERN BERING SEA. WIDESPREAD
MARINE FOG AND STRATUS REMAIN IN PLACE...PARTICULARLY OVER THE
WESTERN ALEUTIANS. WITH THE AFOREMENTIONED STRONG JET LOCATED OVER
THE NORTHERN BERING SEA MOST OF THE ACTIVE WEATHER AND RAPIDLY
CHANGING CONDITIONS ARE CONFINED TO THE FURTHEST NORTH AND EAST
PORTIONS OF THE REGION.

&&

.MODEL DISCUSSION...

THE MODELS HAVE COME INTO A BIT BETTER AGREEMENT THAN YESTERDAY.
THE KEY EVOLUTION IN THE UPPER LEVELS OVER THE NEXT FEW DAYS WILL
BE WHAT IS NOW EXPECTED TO BE TWO SHORTWAVES THAT COME THROUGH
SOUTHCENTRAL ALASKA UNDER STRENGTHENING NORTHWEST FLOW. THE 12Z
ECMWF TUESDAY WAS THE FIRST MODEL TO SHOW A MUCH FASTER INITIAL
WAVE...HAVING BACKED OFF SLIGHTLY TODAY BUT BEING JOINED IN A
GENERALLY FASTER TRACK BY THE AMERICAN MODELS (NAM/GFS). THIS WILL
INITIATE THE GAP WIND/OUTFLOW PROCESS OVER THE KENAI PENINSULA AND
THROUGH THE ALASKA RANGE QUICKER THAN PREVIOUSLY EXPECTED...BUT IT
WILL NOT BE UNTIL THE 2ND AND STRONGER WAVE COMES THROUGH EARLY
THURSDAY EVENING THAT SURFACE CYCLOGENESIS WILL COMMENCE IN THE
GULF AND THE REST OF THE FAVORED GAPS ALONG WITH INTERIOR AREAS
LIKE ANCHORAGE AND THE SUSITNA VALLEY WILL EVEN GET IN ON THE
GUSTY NORTHWEST WINDS. ONCE NORTHWEST FLOW AT ALL LEVELS IS IN
PLACE ACROSS THE STATE ON FRIDAY THERE ARE A FEW MINOR MODEL
ISSUES WITH STRENGTH AND TIMING OF WEAK SUPPORTING SHORTWAVES
MOVING THROUGH THE OVERALL PATTERN...BUT PINNING DOWN THE DETAILS
AT THIS POINT MAY VERY WELL PROVE TO BE AN EXERCISE IN FUTILITY AS
TIMING OF WEAK SHORTWAVES MOVING THROUGH A 150 KT JET STREAM WILL
ALMOST CERTAINLY BOUNCE AROUND WITH EACH MODEL AND EACH RUN.
OVERALL...THE NAM WAS THE PREFERRED MODEL TODAY DUE TO SUPERIOR
RESOLUTION IN THE HANDLING OF EXPECTED GAP WINDS AROUND THE
REGION.

&&

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

RAINFALL HAS SHIFTED EAST INTO THE COPPER RIVER BASIN AND PRINCE
WILLIAM SOUND THIS AFTERNOON AS THE UPPER LEVEL DISTURBANCE
CONTINUES TO SHIFT EASTWARD AT A RAPID PACE. ISOLATED SHOWERS
WILL REMAIN POSSIBLE THROUGH THE EVENING HOURS ACROSS ANCHORAGE
AND THE MATANUSKA VALLEY AS ONE MORE WEAK UPPER LEVEL DISTURBANCES
PUSHES THROUGH. THE COPPER RIVER BASIN AND PRINCE WILLIAM SOUND
AREAS WILL LIKELY SEE A CONTINUATION OF LIGHT RAIN THROUGH
THURSDAY AFTERNOON AS THE STRONG ZONAL JET WILL PUSH TWO STRONG
WAVES THROUGH ON THURSDAY AND COMBINE WITH ENOUGH RESIDUAL
MOISTURE CURRENTLY BEING ADVECTED INTO THE AREA. RAIN CHANCES THEN
BEGIN TO TAPER OFF SIGNIFICANTLY ON FRIDAY AS COLDER AND MUCH
DRIER AIR MOVES BACK INTO THE REGION WITH STEEP NORTHWESTERLY FLOW
RESIDING OVER THE AREA.

AN UNSEASONABLY STRONG WIND EVENT LOOKS POISED TO IMPACT SOUTHCENTRAL
ALASKA THURSDAY NIGHT THROUGH EARLY ON SATURDAY. THE COMBINATION
OF DECENT COLD AIR ADVECTION ON THE BACK SIDE OF THE UPPER LEVEL
TROUGH AND THE LOCATION OF THE UPPER LEVEL JET CREATES A VERY
FAVORABLE ENVIRONMENT FOR OUTFLOW WINDS. WINDS WILL STRENGTHEN TO
GALE FORCE ACROSS THE BARREN ISLANDS THIS EVENING AND CONTINUE TO
OVERSPREAD THE GULF OF ALASKA COME THURSDAY NIGHT. TYPICAL OUTFLOW
AREAS(SEWARD...WHITTIER...VALDEZ...AND THOMPSON PASS WILL BE
SLIGHTLY DELAYED AS THE STRONGER WINDS WONT DEVELOP UNTIL
THURSDAY NIGHT. WHILE NO HIGH WIND WARNING OR WATCHES ARE
ANTICIPATED AT THIS TIME...WIND GUSTS ANYWHERE FROM 50 TO 65 MPH
ACROSS THE AFOREMENTIONED AREAS APPEAR LIKELY. THE STRONGEST WINDS
WILL BE THROUGH THOMPSON PASS AND NEAR WHITTIER WHILE SEWARD AND
VALDEZ WILL LIKELY BE CAPPED AROUND 50 MPH. THE REST OF
SOUTHCENTRAL ALASKA WILL NOT BE IN THE CLEAR AS WINDS 10 TO 20 MPH
WITH GUSTS UP TO 35 MPH IN SOME LOCATIONS (INCLUDING THE ANCHORAGE
BOWL) WILL BE POSSIBLE. THIS SET UP IS MORE TYPICAL OF A LATE
SEPTEMBER OR EARLY OCTOBER PATTERN...ESPECIALLY WITH THE PUSH OF
COLDER AIR BEING ADVECTED SOUTH.

ONE LAST THING...IT LOOKS AS IF TEMPERATURES ACROSS EAST ANCHORAGE
AND PARTS OF THE MATANUSKA VALLEY WILL DROP NEAR FREEZING THURSDAY
NIGHT AND AGAIN FRIDAY NIGHT. THIS WOULD BE UNSEASONABLY EARLY FOR
THESE LOCATIONS TO DROP BELOW FREEZING. RIGHT NOW TEMPERATURES ARE
FORECASTED BETWEEN 34 AND 36 DEGREES FOR THE MOST VULNERABLE
PLACES...SO NO FREEZE WARNING HAS BEEN ISSUED AT THIS TIME.
HOWEVER...IF TEMPERATURES ARE EXPECTED TO DROP BELOW FREEZING...A
FREEZE WARNING WILL BE ISSUED.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A SERIES OF UPPER LEVEL DISTURBANCES WILL TRACK THROUGH THE
SOUTHWEST MAINLAND OVERNIGHT TONIGHT AND THROUGH TOMORROW
AFTERNOON...KEEPING SCATTERED SHOWERS OVER MUCH OF THE SOUTHWEST
MAINLAND AND STEADIER RAINFALLS ALONG THE UPSLOPE AREAS OF THE
ALASKA RANGE. THE INITIAL DISTURBANCE WILL BE SLIGHTLY DRIER IN
COMPARISON TO THE PREVIOUS SYSTEM THAT EXITED THE SOUTHWEST
MAINLAND THIS AFTERNOON...WITH A LESS IMPRESSIVE MOISTURE FEED
FROM THE WEST. THE SECOND OF THESE DISTURBANCES WILL USHER IN A
DRYING AND COOLING TREND AS NORTHWEST FLOW TAKES HOLD HEADING INTO
THE END OF THE WEEK. AS A RESULT...WIDESPREAD GUSTY WINDS WILL
CONTINUE AND EVENTUALLY PICK UP ALONG A TIGHTENING NORTHWEST
GRADIENT OVER THE MAINLAND THURSDAY MORNING...AND PERSIST THROUGH
LATE FRIDAY MORNING.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
PRECIPITATION WILL GRADUALLY DIMINISH IN COVERAGE ACROSS THE
BERING THROUGH THURSDAY NIGHT AS THE PATTERN SLOWLY AMPLIFIES AND
THE NORTH PACIFIC RIDGE RETROGRADES WESTWARD. THIS WILL KEEP MOST
OF THE RAINFALL CENTERED OVER THE WESTERN TIP OF THE ALEUTIANS AS
DEEP SUBTROPICAL MOISTURE CONTINUES TO STREAM NORTHWARD ON THE
WESTERN PERIPHERY OF THE RIDGE. HOWEVER...SOME OF THIS MOISTURE
AND ENERGY MAKES IT OVER THE RIDGE AND INTERACTS WITH THE
NORTHERN ENERGY STREAM...BRINGING SOME GUSTY WINDS AND SHOWERS
OVER THE CENTRAL BERING FRIDAY. A FEW SHOWERS WILL ALSO DEVELOP
ALONG THE ALASKA PENINSULA AND EASTERN ALEUTIANS IN BRISK
NORTHWEST FLOW ACROSS THE EASTERN BERING. OTHERWISE...WIDESPREAD
STRATUS AND FOG WILL CONTINUE TO IMPACT MUCH OF THE REGION THROUGH
THE END OF THE WEEK BENEATH THE STRONG HIGH.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

BEGINNING THE EXTENDED PERIOD FRIDAY NIGHT...THE REGION WILL BE
UNDER THE INFLUENCE OF AN ELONGATED TROUGH EXTENDING FROM THE
NORTH SLOPE OF ALASKA TO THE COAST OF BRITISH COLUMBIA. THE
OUTFLOW WIND EVENT WILL BE WINDING DOWN FRIDAY NIGHT AS THE GULF
LOW CONTINUES TO WEAKEN AND DRIFT SOUTHEAST. EVENTUALLY WITH
TIME...A SHORTWAVE TROUGH IN NORTHWEST FLOW ORIGINATING FROM
EASTERN SIBERIA WILL MERGE WITH THE AFOREMENTIONED ARCTIC TROUGH
OVER THE NORTH SLOPE SATURDAY AND SUNDAY. THERE UNFORTUNATELY
REMAINS MUCH UNCERTAINTY WITH THE DEVELOPMENT OF THIS LOW AS IT
MOVES FROM THE NORTHERN BERING SEA TO THE GULF OF ALASKA. THE
UNCERTAINTY SEEMS TO LIE IN HOW MUCH SUBTROPICAL ENERGY OVER THE
WESTERN BERING MERGES WITH THE DIGGING TROUGH. THE GFS FAVORS A
STRONGER SOLUTION AND MORE INTACT MERGER...WHICH BRINGS MORE
PRECIPITATION INTO SOUTHWEST ALASKA FRIDAY NIGHT INTO
SATURDAY...AND EVENTUALLY INTO SOUTHCENTRAL SATURDAY AFTERNOON AND
EVENING. THE WEAKER SOLUTIONS SUCH AS THE ECMWF AND CANADIAN GDPS
FAVOR A FARTHER SOUTH TRACK OF THE LOW...AND WEAKER DEVELOPMENT IN
THE GULF OF ALASKA. FOR THIS FORECAST...A CONTINUED CONSERVATIVE
FORECAST WAS TAKEN FAVORING A WPC ENSEMBLE BLEND ALONG WITH SOME
ECMWF. THIS WILL KEEP LOWER END CHANCES FOR PRECIPITATION OVER
MAINLAND LOCATIONS...WITH HIGHER CHANCES REMAINING LARGELY OVER
THE BERING AND GULF OF ALASKA AS THE LOW SKIRTS THE REGION.
THEREAFTER...THERE IS HIGHER CONFIDENCE LEADING INTO MIDWEEK AS
SOUTHCENTRAL REMAINS UNDER THE INFLUENCE OF DRY AND COOLER
OFFSHORE FLOW...WITH A SLOWLY DEVELOPING FLAT RIDGE OVER THE
BERING. PRECIPITATION CHANCES INCREASE AGAIN TOWARDS THE MIDDLE TO
END OF NEXT WEEK...WITH A SLOW REBOUND IN TEMPERATURES AFTER THE
COOL PERIOD THIS WEEKEND AND EARLY NEXT WEEK.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA