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Hawaiian Islands Synoptic Discussion and Guidance

372
FXHW60 PHFO 270148
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST THU FEB 26 2015

.SYNOPSIS...
A SURFACE TROUGH NEAR KAUAI AND OAHU WILL SLOWLY DRIFT EASTWARD
FRIDAY AND SATURDAY. THE TROUGH WILL ACT AS A FOCUS FOR CLOUDS AND
SHOWERS...AND LOCALLY HEAVY RAIN WILL BE POSSIBLE FROM FRIDAY
THROUGH AT LEAST SATURDAY AS THE ATMOSPHERE BECOMES INCREASINGLY
MOIST AND UNSTABLE. TRADE WINDS MAY REBUILD ON SUNDAY...THOUGH THE
UNSETTLED WEATHER WILL LIKELY CONTINUE INTO NEXT WEEK.

&&

.DISCUSSION...
A COMPLEX...UNSETTLED WEATHER PATTERN CONTINUES TO SLOWLY EVOLVE
OVER THE STATE. AN ERODING RIDGE ALOFT IS HANGING ON OVER THE
ISLANDS...BUT AFTERNOON SOUNDINGS ARE SHOWING SEVERAL DEGREES OF MID
LEVEL COOLING AS A DEEP MID TO UPPER LEVEL TROUGH IS PUSHING OVER
THE ISLANDS FROM THE WEST. THIS FEATURE IS SUPPORTING A DECAYING
SURFACE FRONT ROUGHLY 275 MILES NORTHWEST OF KAUAI WITH AREAS OF
HIGH CLOUDS BREAKING OFF AND PASSING NEAR THE GARDEN ISLE. A
SEPARATE...SOMEWHAT WEAK SURFACE TROUGH IS STALLED IN THE KAUAI
CHANNEL...WHILE A COMPACT UPPER LEVEL LOW AND AN ASSOCIATED SURFACE
TROUGH CENTERED ABOUT 600 MILES EAST-SOUTHEAST OF HILO ARE DRIFTING
TOWARD THE STATE. ALL OF THESE FEATURES WILL COME TOGETHER OVER THE
ISLANDS AT VARIOUS TIMES DURING THE NEXT FEW DAYS...LEADING TO
PERIODS OF UNSETTLED WEATHER.

IT HAS BEEN A QUIET DAY OVER THE EASTERN END OF THE ISLAND CHAIN...
WITH SPOTTY AFTERNOON SHOWERS MAINLY CONFINED TO THE HIGHER TERRAIN
ON KAUAI AND OAHU. THE SURFACE TROUGH IN THE KAUAI CHANNEL IS
RESPONSIBLE FOR INCREASED MOISTURE OVER THE WESTERN ISLANDS AND FOR
DISRUPTING THE LOW LEVEL BACKGROUND FLOW ACROSS THE SMALLER ISLANDS.
AS A RESULT...AFTERNOON SEA BREEZES ARE WIDESPREAD ON THE SMALLER
ISLANDS...BUT ONLY KAUAI AND OAHU HAVE ENOUGH MOISTURE TO SUPPORT
SHOWER DEVELOPMENT. A DRIER EAST-SOUTHEAST LOW LEVEL FLOW REMAINS IN
PLACE OVER THE BIG ISLAND.

THE HIGHEST CHANCES FOR ENHANCED SHOWER ACTIVITY WILL REMAIN OVER
THE WESTERN END OF THE STATE TONIGHT THEN SPREAD AS FAR EAST AS MAUI
ON FRIDAY. THE SURFACE TROUGH WILL MOVE LITTLE OVERNIGHT...LEAVING
SOME ENHANCED MOISTURE NEAR KAUAI AND OAHU. MEANWHILE...THE FRONT TO
THE NORTHWEST OF KAUAI WILL BE WEAKENING AND PUSHING EASTWARD...AND
THE DIFLUENT PORTION OF THE JET STREAM ALOFT WILL BE MOVING OVER THE
STATE. IT DOES NOT LOOK LIKE FORCING WILL BE SIGNIFICANT ENOUGH TO
TRIGGER HEAVY SHOWERS OVERNIGHT...BUT WE HAVE ADDED THEM OVER THE
SMALLER ISLANDS FOR FRIDAY AFTERNOON. THE ASSUMPTION IS THAT LOW
LEVEL MOISTURE ASSOCIATED WITH THE SURFACE TROUGH WILL SPREAD
EASTWARD...AND DAY TIME HEATING COMBINED WITH COOLING ALOFT WILL
ACT AS A TRIGGER. WE CANNOT RULE OUT A THUNDERSTORM ON FRIDAY
AFTERNOON...BUT CHANCES ARE CONSIDERED TOO LOW TO MENTION AT THIS
TIME.

THE POSSIBILITY OF UNSETTLED WEATHER WILL PERSIST ACROSS PORTIONS OF
THE STATE THROUGH THE WEEKEND. THE UPPER LEVEL TROUGH WITH COLD
TEMPS ALOFT WILL REMAIN OVERHEAD THROUGH SUNDAY. THE LOW LEVEL
TROUGH WILL LIKELY CONTINUE TO BE THE FOCUS FOR CLOUDS AND SPOTTY
HEAVY SHOWERS ACROSS THE CENTRAL ISLANDS ON SATURDAY. THE GFS AND
ECMWF SHOW THE LOW LEVEL TROUGH WASHING OUT BY SUNDAY WITH TRADE
WINDS RETURNING...BUT BOTH MODELS ARE HINTING THAT THE AREA OF LOW
LEVEL MOISTURE...CURRENTLY ABOUT 800 MILES TO THE SOUTHEAST OF THE
STATE...WILL PASS NEAR OR OVER THE ISLAND CHAIN FROM EAST TO WEST ON
SUNDAY.

UNCERTAINTY INCREASES ON MONDAY. THE GFS SHOWS A GREATER DEGREE OF
INTERACTION BETWEEN THE MOISTURE COMING IN FROM THE EAST AND THE MID
TO UPPER LEVEL TROUGH PARKED OVERHEAD. AS A RESULT...THE GFS
DEVELOPS ANOTHER SURFACE TROUGH OVER THE STATE MONDAY THROUGH
TUESDAY...WHILE THE ECMWF SUGGESTS A WET TRADE WIND WEATHER PATTERN.
FOR NOW...WE HAVE OPTED TO GO WITH THE MORE CONSERVATIVE ECMWF
SOLUTION. BOTH MODELS HINT AT A DRIER AND BREEZY TRADE WIND FLOW BY
THE END OF NEXT WEEK.

&&

.AVIATION...
LAND BREEZES ARE EXPECTED ACROSS MOST OF THE STATE TONIGHT. A FEW
SHOWERS MAY CARRY PERIODS OF MVFR...MAINLY OVER THE SMALLER ISLANDS.
INTERIOR BUILD UP ON THE BIG ISLAND SHOULD BEGIN TO CLEAR THIS
EVENING AND OVERALL THE BIG ISLAND WILL REMAIN FAIRLY QUIET TONIGHT.

AFTERNOON INTERIOR SHOWERS OVER KAUAI THIS AFTERNOON WERE
ACTIVE...AIRMET SIERRA REMAINS OVER N THRU E SECTIONS. A SLOW
MOVING FRONT IS ENTERING THE WATERS WEST OF KAUAI. THIS AREA IS
DOMINATED BY MULTIPLE CLOUD LAYERS UP TO FL300.

&&

.MARINE...
NO MARINE WARNINGS OR ADVISORIES ARE IN EFFECT. LIGHTER WINDS WILL
PREVAIL TODAY THROUGH SATURDAY AS A SURFACE TROUGH LINGERS OVER THE
ISLANDS. TRADE WINDS SHOULD BUILD BACK IN ACROSS THE STATE DURING
THE WEEKEND INTO EARLY NEXT WEEK...WITH SOME CHANCE THAT WINDS MAY
APPROACH ADVISORY LEVELS ACROSS THE WINDIER ZONES. ADVISORY LEVEL
WINDS ARE MORE LIKELY LATER NEXT WEEK.

A SMALL INCREASE IN FRESH...SHORT-PERIOD EASTERLY SWELL IS DUE
TONIGHT AND THURSDAY WITH EAST SHORE SURF EXPECTED TO REMAIN BELOW
ADVISORY LEVELS. THIS SWELL HAS ALREADY ARRIVED AT WINDWARD BUOYS...
WITH PERIODS OF 11 SECONDS. A NORTH-NORTHWEST SWELL MAY PUSH NORTH
SHORE SURF TO THE ADVISORY LEVEL SUNDAY NIGHT AND MONDAY. SEE THE
COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST /SRDHFO/ FOR MORE
DETAILS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

&&

$$

DISCUSSION/MARINE...WROE
AVIATION...EATON



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