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Hawaiian Islands Synoptic Discussion and Guidance

083
FXHW60 PHFO 290200
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST SAT MAR 28 2015

.SYNOPSIS...
LIGHT TRADE WINDS WILL DELIVER A FEW WINDWARD SHOWERS OVERNIGHT...
WHILE ALSO ALLOWING LAND BREEZES TO CLEAR THE SKIES OVER LEEWARD
AREAS. TRADE WINDS WILL INCREASE SUNDAY AND MONDAY...BRINGING A
SLIGHT INCREASE IN WINDWARD SHOWERS...MAINLY ON KAUAI AND OAHU. A
DISTURBANCE PASSING NORTH OF THE ISLANDS ON TUESDAY WILL WEAKEN THE
TRADE WINDS AGAIN...AND MAY BRIEFLY ENHANCE SHOWERS. THE DISTURBANCE
WILL MOVE EAST OF THE AREA ON WEDNESDAY AND THURSDAY...WITH TYPICAL
TRADE WIND WEATHER EXPECTED THROUGH THE END OF NEXT WEEK.

&&

.DISCUSSION...
CURRENTLY...LIGHT TO MODERATE TRADE WINDS OVER NEAR SHORE WATERS ARE
BEING SUPPORTED BY A 1033 MB SURFACE HIGH CENTERED ABOUT 1650 MILES
NE OF HILO...AND ITS ASSOCIATED SURFACE RIDGE THAT IS NOW ABOUT 200
MILES N OF KAUAI. ALTHOUGH THE CENTER OF THE HIGH HAS BEEN NEARLY
STATIONARY OVER THE PAST 24 HOURS...THE WESTERN END OF THE RIDGE HAS
BEEN RETREATING EASTWARD AS A FRONT DRAWS CLOSER TO THE AREA FROM
THE NW. WITH THE RELATIVELY LIGHT WINDS...SKIES OVER LEEWARD AREAS
BECAME QUITE CLOUDY IN RESPONSE TO SEA BREEZE CIRCULATIONS THIS
AFTERNOON. HOWEVER...NEARLY ALL OF OAHU HAS BEEN PEPPERED WITH
CLOUDS AND SHOWERS ASSOCIATED WITH AN AREA OF MOISTURE IN THE TRADE
FLOW THAT COMBINED WITH SEA BREEZE CONVERGENCE.

TRADE WINDS WILL REMAIN ON THE LIGHT SIDE THROUGH TONIGHT AS THE
RIDGE TO THE N ERODES IN THE FACE OF THE ADVANCING FRONT...AND THE
LOCAL SURFACE PRESSURE GRADIENT LOOSENS A LITTLE MORE. THUS LAND
BREEZES ARE EXPECTED TO DEVELOP...AT LEAST OVER LEEWARD AREAS...
AND THE EVENING CLOUDS AND SHOWERS WILL GRADUALLY CLEAR. WINDWARD
AREAS MAY FOLLOW SUIT...DEPENDING ON THE STRENGTH OF LAND BREEZES...
BUT THE FORECAST CALLS FOR A FEW WINDWARD SHOWERS THROUGH THE NIGHT
HOWEVER.

ON SUNDAY MORNING...A NEW HIGH BUILDING NW OF THE AREA WILL START TO
TIGHTEN THE LOW LEVEL PRESSURE GRADIENT OVER THE W END OF THE
ISLAND CHAIN...WHILE THE FRONT AND ITS ASSOCIATED MOISTURE STALL
WELL N OF KAUAI. THIS NEW HIGH WILL TURN WINDS TO THE NE NEAR KAUAI
WHILE WINDS OVER THE E END OF THE STATE REMAIN FROM THE E...THUS
INDUCING A WEAK LOW-LEVEL TROUGH NEAR KAUAI AND OAHU. WITH THE NEW
HIGH BUILDING QUICKLY EASTWARD...THE TROUGH IS EXPECTED TO MOVE W
THROUGH SUNDAY NIGHT BEFORE DISSIPATING W OF KAUAI. WEAK LOW LEVEL
CONVERGENCE ASSOCIATED WITH THE TROUGH IS EXPECTED TO LEAD TO AN
INCREASE IN TRADE SHOWERS...ESPECIALLY NEAR ITS AXIS. THE ATMOSPHERE
WILL CONTINUE TO BE RELATIVELY STABLE SO THESE SHOWERS ARE NOT
EXPECTED TO BE HEAVY.

THE DEPARTURE OF THE TROUGH WILL ALLOW A TYPICAL MODERATE TRADE WIND
FLOW TO PREVAIL ON MONDAY...WITH FEWER SHOWERS EXPECTED. ON TUESDAY
A PROGRESSIVE MID-LEVEL SHORT WAVE TROUGH WILL PASS N OF THE AREA...
AND INDUCE THE DEVELOPMENT OF ANOTHER SURFACE TROUGH...THIS TIME N
OF THE ISLANDS ALONG THE STALLED FRONTAL BOUNDARY. THIS WILL ACT TO
WEAKEN ISLAND WINDS TO THE POINT THAT THEY BECOME LIGHT AND
VARIABLE...WITH GFS AND ECMWF NOW IN BETTER AGREEMENT. THE BULK OF
THE INSTABILITY ASSOCIATED WITH THE PASSING SHORT WAVE WILL REMAIN N
OF THE AREA...BUT MAY BE NEAR ENOUGH TO KAUAI AND OAHU TO HELP
TRIGGER SOME SHOWERS ALONG SEA BREEZE CONVERGENCE ZONES TUESDAY
AFTERNOON. THE MID-LEVEL TROUGH WILL TRACK PROGRESSIVELY E INTO
WEDNESDAY AS THE LOW LEVEL TROUGH MOVES W...AND TRADE WIND SPEEDS
WILL BEGIN TO REBOUND...WITH TYPICAL TRADE WIND WEATHER EXPECTED
INTO THE END OF NEXT WEEK.

&&

.AVIATION...
PREDOMINANTLY E TO SE WINDS WILL CONTINUE TO BRING PASSING SHOWERS
ACROSS MOST WINDWARD AND MOUNTAIN AREAS TONIGHT. TEMPO MVFR EXPECTED
THROUGH THE REST OF THIS AFTERNOON AND EVENING OVER WINDWARD AREAS
AND SELECT LEEWARD LOCATIONS...INCLUDING PHNL. CONDITIONS ARE
EXPECTED TO IMPROVE LATER THIS EVENING...ESPECIALLY OVER LEEWARD
AREAS AS LAND BREEZES DEVELOP. AIRMET SIERRA IS IN EFFECT OVER
MOUNTAIN AND N THRU E SLOPES OF OAHU AND FROM KEANAE TO HANA TO
KAUPO IN MAUI. OTHERWISE...VFR WILL PREVAIL.

&&

.MARINE...
NO MARINE WARNINGS OR ADVISORIES ARE CURRENTLY POSTED. TRADE WINDS
ARE ON A SHORT TERM WEAKENING TREND...AND ALTHOUGH THEY ARE EXPECTED
TO INCREASE A NOTCH ON SUNDAY INTO MONDAY...WINDS ARE EXPECTED TO
REMAIN BELOW SMALL CRAFT ADVISORY /SCA/ SPEEDS BEFORE WINDS DIMINISH
AGAIN ON TUESDAY.

SMALL BUT LONG-LIVED SWELLS ARRIVING FROM THE S WILL PROVIDE SMALL
SURF TO S FACING SHORES THROUGH MID-WEEK...WITH SURF WELL BELOW
ADVISORY LEVELS. A LONG-PERIOD WNW SWELL WILL ARRIVE LATE TONIGHT
AND PEAK LATE SUNDAY...WITH PEAK SURF HEIGHTS NEARING ADVISORY
LEVELS ALONG EXPOSED N AND W SHORES. THIS SWELL WILL GRADUALLY
DIMINISH IN HEIGHT AND PERIOD THROUGH TUESDAY. AT ITS PEAK...THIS
SWELL WILL BRING COMBINED SEAS CLOSE TO THE 10 FT THRESHOLD FOR SCA
ISSUANCE. WILL BE MONITORING OVERNIGHT SWELL/SEA HEIGHTS AT BUOY
51101...WHERE LONG-PERIOD FORERUNNERS ARE NOW ARRIVING...TO AID IN
DECISION MAKING WITH THESE POTENTIAL ADVISORIES.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

DISCUSSION/MARINE...BIRCHARD
AVIATION...REYNES/EATON



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