Flash Flooding and Heavy Rain in Hawaii, August 22 to 26, 2015

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE HONOLULU HI
430 AM HST SAT AUG 22 2015

.SYNOPSIS...
MODERATE TO LOCALLY STRONG TRADE WINDS WILL TURN SOUTHEASTERLY AND
DIMINISH OVER THE WEEKEND. THE VERY MOIST AIR MASS OVER THE STATE
WILL BECOME MOISTER STILL...FUELLING LOCALLY HEAVY SHOWERS AND
THUNDERSTORMS. TROPICAL DEPRESSION KILO IS PREDICTED TO PASS SOUTH
OF THE STATE THROUGH THE WEEKEND...AND THEN TO STRENGTHEN AS IT
TURNS BACK TOWARDS THE ISLANDS EARLY NEXT WEEK. HOWEVER...THE
FORECAST WILL CHANGE SIGNIFICANTLY DEPENDING ON THE BEHAVIOR OF THIS
POTENTIALLY DANGEROUS STORM.

&&

.DISCUSSION...
TROPICAL DEPRESSION KILO CONTINUED TO PASS SOUTH OF THE MAIN
HAWAIIAN ISLANDS...TIGHTENING THE PRESSURE GRADIENT AND SPREADING
HIGH CLOUD LAYERS ACROSS THE STATE. THE INCREASE IN WIND SPEEDS WILL
BE SHORT-LIVED. ONCE KILO MOVES WEST OF THE STATE AND DRIFTS
NORTHWEST...IT WILL START TO ERODE THE SUBTROPICAL RIDGE TO THE
NORTH. HIGH CLOUD COVER WILL VARY DEPENDING ON THE TRACK AND
HIGH-LEVEL OUTFLOW OF KILO...BUT LOWER CLOUDS FORMING IN THE
EXTREMELY MOIST AIR MASS LIKELY WILL HIDE MUCH OF THE SKY ANYWAY.

SPEAKING OF THE AIR MASS...HUMIDITY WILL INCREASE AND THEN REMAIN
FAR BEYOND CLIMATOLOGY THROUGH AT LEAST NEXT WEEK...AND PERHAPS
THROUGH THE REST OF AUGUST. SATELLITE ESTIMATES SHOWED PRECIPITABLE
WATER TOTALS IN EXCESS OF TWO INCHES...AND EVEN UPWARDS OF TWO AND A
HALF INCHES...TO THE EAST THROUGH SOUTHEAST OF THE ISLANDS. GUIDANCE
BROADLY SUPPORTED THESE FIGURES...AND UNSURPRISINGLY PREDICTED THE
PREVAILING FLOW TO SPREAD THEM NORTHWEST OVER THE STATE THROUGH THE
NEXT 24 TO 36 HOURS.

HEAVY SHOWERS AND THUNDERSTORMS DEVELOPED OVER THE BIG ISLAND
OVERNIGHT...AND CONTINUED TO DO SO OVER WATERS JUST EAST OF MAUI AND
THE BIG ISLAND. OROGRAPHIC FORCING AND DIURNAL HEATING OVER THE
ISLANDS ALMOST CERTAINLY WILL SUPPORT GREATER DEVELOPMENT THIS
AFTERNOON...AND THEN AGAIN ON SUNDAY. AS THE PREVAILING FLOW TURNS
SOUTHEASTERLY...ISLAND LEE PLUMES WILL BE ABLE TO MOVE ASHORE ONTO
OTHER ISLANDS DOWNWIND...FURTHER ENHANCING CONVECTION AND
PRECIPITATION. THE FLASH FLOOD WATCH CURRENTLY IN EFFECT FOR THE
SOUTHEAST HALF OF THE STATE VERY LIKELY WILL BE EXPANDED TO THE
OTHER ISLANDS.

ALL THESE CONSEQUENCES RESULTED VIA ONLY INDIRECT INFLUENCE FROM
KILO. THE STORM ITSELF MAY BRING EVEN HEAVIER RAIN OVER ALREADY
SOGGY GROUND...TO SAY NOTHING OF DAMAGING WINDS AND BATTERING WAVES.
THE MOST RECENT FORECAST STILL PREDICTED KILO TO INTENSIFY AS IT
LOITERS TO THE SOUTHWEST OF THE STATE INTO NEXT WEEK...THOUGH BOTH
THE DETOUR TO THE WEST...AND THE INTENSIFICATION...WERE EXPECTED TO
TAKE LONGER THAN IN EARLIER FORECASTS. THUS...ALTHOUGH KILO WAS
EXPECTED TO TURN TOWARD ITS RIGHT...AND THUS KAUAI...EVENTUALLY...
THE PERIOD OF WAITING WAS EXPECTED TO LAST A BIT LONGER.

THE POTENTIAL FOR A BONA FIDE DISASTER APPEARED TO RECEDE
SLIGHTLY...BUT THE TRACK THAT KILO ENDS UP TAKING WILL INFLUENCE
ITS DEVELOPMENT GREATLY. THIS CYCLE OF FEEDBACK WILL MEAN THAT SMALL
UNCERTAINTIES IN CURRENT MEASUREMENTS WILL GROW WITH TIME INTO LARGE
UNCERTAINTIES IN OUR FORECASTS FOR THIS SYSTEM. IF KILO TAKES A
SHARP...EARLY TURN TO THE RIGHT...THEN MOST OF THE SMALLER ISLANDS
WILL BE AT RISK FOR DAMAGING WINDS AND RAIN. IF KILO TURNS RIGHT
SLOWLY ENOUGH...OR CONTINUES STRAIGHT TO THE WEST...THEN IT MAY MISS
THE ISLANDS AND COASTAL WATERS ALTOGETHER...SENDING MAINLY LARGE
SWELLS IN OUR DIRECTION. MOST NUMERICAL GUIDANCE SUGGESTED OUTCOMES
BETWEEN THESE EXTREMES.

SPEAKING PLAINLY...THE RISKS REMAINED HIGH ENOUGH SO THAT ANYONE
EXPECTING TO BE IN OR NEAR THE MAIN HAWAIIAN ISLANDS THROUGH THE
NEXT WEEK SHOULD PAY CLOSE ATTENTION TO THIS DEVELOPING SITUATION.
PLEASE REVIEW YOUR PLANS AND GATHER SUPPLIES YOU MAY NEED TO RIDE
OUT SEVERAL DAYS OF DAMAGING WIND AND SURF...AND VERY HEAVY RAIN.
THE NATIONAL WEATHER SERVICE AND THE HAWAII EMERGENCY MANAGEMENT
AGENCY OFFER HELPFUL SUGGESTIONS ALONG THESE LINES ON OUR WEB PAGES.

&&

.MARINE...
THE TRADE WINDS WILL REMAIN STRONG ENOUGH TO REQUIRE A SMALL CRAFT
ADVISORY FOR TYPICALLY WINDY AREAS AROUND MAUI AND THE BIG ISLAND
THROUGH ABOUT TONIGHT. FROM LATE IN THE WEEKEND INTO NEXT WEEK...THE
BACKGROUND FLOW WILL SLOW DOWN AND BECOME SOUTHEASTERLY.

A WEST SWELL FROM FORMER SUPER TYPHOON ATSANI WILL CONTINUE TO
PRODUCE LOW SWELLS THIS WEEKEND...WHICH SHOULD ARRIVE EARLY NEXT
WEEK ALONG WEST-FACING SHORES.

SWELLS FROM TROPICAL DEPRESSION KILO WILL START TO ARRIVE ALONG
SOUTH-FACING SHORES ON SUNDAY. DEPENDING ON THE TRACK AND INTENSITY
OF KILO...SURF COULD REACH WARNING LEVELS ON SOUTH- OR WEST-FACING
SHORES BY MID-WEEK.

MARINERS MAY ENCOUNTER HEAVY SHOWERS OR THUNDERSTORMS AS A VERY
MOIST AIR MASS PUSHES OVER THE AREA FROM THE SOUTHEAST.

SINCE KILO WILL DOMINATE THE FORECAST OVER HAWAIIAN WATERS WELL INTO
NEXT WEEK...EVEN SMALL CHANGES IN THE TRACK OR INTENSITY OF THIS
STORM MAY LEAD TO BIG CHANGES IN THE FORECAST FOR ANY GIVEN AREA.
MARINERS SHOULD MONITOR THE LATEST UPDATES TO STAY INFORMED ABOUT
THIS RAPIDLY DEVELOPING...AND POTENTIALLY DANGEROUS SITUATION.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH MONDAY AFTERNOON FOR MAUI-BIG ISLAND.

&&

RYSHKO