Heavy Rain in Hawaii, March 28, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1010 AM HST SAT MAR 28 2015

.SYNOPSIS...
LIGHT TRADE WINDS WILL DELIVER A FEW WINDWARD SHOWERS TODAY...BUT
WILL BE LIGHT ENOUGH TO ALLOW CLOUDS AND ISOLATED SHOWERS TO DEVELOP
OVER LEEWARD AREAS THIS AFTERNOON. 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...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 1032 MB SURFACE HIGH CENTERED ABOUT 1500 MILES
NE OF HILO...AND ITS ASSOCIATED SURFACE RIDGE THAT EXTENDS TO ABOUT
375 MILES N OF KAUAI. ALTHOUGH THE CENTER OF THE HIGH IS NEARLY
STATIONARY...THE WESTERN END OF THE RIDGE IS WEAKENING AND
RETREATING EASTWARD AS A FRONT DRAWS CLOSER TO THE AREA FROM THE NW.
IN RESPONSE...TRADE WINDS HAVE WEAKENED TO THE POINT THAT LAND
BREEZES DEVELOPED IN MANY AREAS OVERNIGHT. HOWEVER...TRADE WIND FLOW
IS STILL MANAGING TO CARRY SOME SHOWER-BEARING CLOUDS INTO WINDWARD
AREAS...WITH THE GREATEST AMOUNTS OF SHOWERS CURRENTLY NEAR OAHU AND
THE BIG ISLAND...WHILE MOSTLY SUNNY SKIES CURRENTLY PREVAIL OVER
KAUAI. THE EXPECTATION FOR THE REMAINDER OF TODAY IS THAT WINDWARD
SHOWERS WILL DECREASE INTO THE AFTERNOON...WHILE THE LIGHT TRADE
WINDS ALLOW LEEWARD SEA BREEZE CIRCULATIONS TO DRIVE CLOUD AND
ISOLATED SHOWER FORMATION OVER LEEWARD SLOPES IN THE AFTERNOON.

TRADE WINDS WILL REMAIN ON THE LIGHT SIDE THROUGH MOST OF 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. THUS
LEEWARD AREAS ARE EXPECTED TO BECOME MOSTLY CLEAR...AND WINDWARD
AREAS MAY FOLLOW SUIT...DEPENDING ON THE STRENGTH OF LAND BREEZES.

BY LATE TONIGHT...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 N
OF THE ISLANDS. 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 WINDS TO THE POINT THAT THEY BECOME LIGHT AND VARIABLE...WITH
GLOBAL MODELS TRENDING TOWARD LIGHTER WINDS SINCE YESTERDAY. 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...
MOSTLY VFR CONDITIONS ARE EXPECTED TO PREVAIL AT ALL TERMINALS
TODAY. PREDOMINANTLY E TO SE WINDS WILL BRING PASSING SHOWERS
ACROSS MOST WINDWARD AND MOUNTAIN AREAS TODAY. CURRENTLY...THERE
ARE NO AIRMETS IN EFFECT...HOWEVER...AN ISSUANCE OF AIRMET SIERRA
CAN NOT BE RULED OUT OVER MOUNTAIN AND NORTH THRU EAST SLOPES
TODAY. SOME OF THE LARGER SHOWERS MAY REACH LEEWARD AREAS AT
TIMES.

&&

.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...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 ON 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 SWELL/SEA HEIGHTS AT BUOY 51101...ESPECIALLY
TONIGHT...TO AID IN DECISION MAKING WITH THESE POTENTIAL ADVISORIES.

&&

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

&&

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