Heavy Rain in Hawaii, September 30, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1035 AM HST MON SEP 30 2013

.SYNOPSIS...
AN UPPER LEVEL LOW NORTH OF THE STATE WILL CONTINUE TO BRING
UNSTABLE CONDITIONS TO THE ISLANDS THROUGH TUESDAY. EXPECT SOME
LOCALLY HEAVY RAINFALL ACROSS THE ISLANDS...AND THE CHANCE FOR
THUNDERSTORMS OVER KAUAI AND OAHU. ADDITIONAL THUNDERSTORMS ARE
POSSIBLE OVER THE BIG ISLANDS LEEWARD AND INTERIOR SECTIONS DURING
THE AFTERNOON. MORE STABLE TRADE WIND WEATHER IS EXPECTED FOR THE
SECOND HALF OF THE WEEK.

&&

.UPDATE...
FLASH FLOOD WATCH HAS BEEN ISSUED FOR KAUAI AND OAHU THROUGH TUESDAY
AFTERNOON.

&&

.DISCUSSION...
AN UPPER LEVEL LOW NORTH OF THE ISLANDS HAS THUNDERSTORMS ASSOCIATED
WITH IT...AND IS BRINGING AN UNSTABLE AIRMASS TO THE ISLANDS. THE
OVERNIGHT SOUNDINGS FROM LIHUE AND HILO BOTH SHOW A MOIST UNSTABLE
AIRMASS. THE KAUAI SOUNDING IS MORE UNSTABLE AND HAS HIGHER
PRECIPITABLE WATER /PW/ VALUES...DUE TO BEING CLOSER TO THE UPPER
LEVEL LOW. MODELS SHOW HIGHER PW MOVING IN TONIGHT.

THE UPPER LEVEL LOW IS FORECAST TO SLOWLY MOVE TO THE WEST...WHICH
WILL KEEP UNSTABLE WEATHER OVER THE ISLANDS THROUGH TOMORROW
AFTERNOON. AT THE SOUTHERN END OF THE ISLAND CHAIN...MORE MORNING
SUNSHINE WILL LEAD TO THE POSSIBILITY OF AFTERNOON CONVECTION OVER
THE LEEWARD AND UPPER SLOPES OF THE BIG ISLAND. AS A RESULT...HAVE
MODIFIED THE BIG ISLAND WEATHER TO BETTER REFLECT THE AREAS MOST AT
RISK FOR AFTERNOON THUNDERSTORMS TODAY AND TOMORROW. HAVE INCREASED
POPS OVER THE INTERIOR AND INTO THE SUMMIT AREAS OF THE BIG ISLAND
AS WELL.

WE ARE SEEING SOME SOUTHEAST FLOW...ALONG WITH LIGHT WINDS. THIS
WILL LIKELY MANIFEST ITSELF IN ADDITIONAL SHOWERS IN AREAS SUCH AS
THE KOOLAU RANGE...ON THE EAST SIDE OF THE KOOLAU RANGE...AND ON THE
SOUTHEAST FACING SLOPES OF HALEAKALA. RAINFALL OVER THE KOOLAU RANGE
ON OAHU HAS PICKED UP SINCE DAYBREAK...WITH RADAR SHOWING HEAVY
RAINFALL AS OF 930 AM.

A DRIER...MORE STABLE AIRMASS IS EXPECTED FOR THE SECOND HALF OF THE
WEEK.

&&

.MARINE...
WITH LIGHT WINDS INTO THE MIDDLE OF THE WEEK...AND LIGHT TO MODERATE
TRADES FOR THE SECOND HALF OF THE WEEK...WINDS ARE EXPECTED TO
REMAIN BELOW THE SMALL CRAFT ADVISORY /SCA/ CRITERIA THROUGH THE
FORECAST PERIOD.

THE CURRENT SMALL NORTHWEST SWELL WILL CONTINUE TO SLOWLY SUBSIDE
TODAY. A NEW NORTH SWELL ARRIVING LATE TUESDAY WILL PRODUCE MODEST
SURF ALONG NORTH FACING SHORES OF THE ISLANDS WEDNESDAY AND
THURSDAY. THIS SWELL WILL SLOWLY SUBSIDE THURSDAY NIGHT AND FRIDAY.
A NORTH NORTHWEST SWELL ARRIVING IN THE ISLANDS EARLY SATURDAY
MORNING WILL PRODUCE MODEST SURF ALONG MOST NORTH AND WEST FACING
SHORES OF THE SMALLER HAWAIIAN ISLANDS NEXT WEEKEND.

THE TRADE WIND SWELL WILL DIMINISH ALONG THE WINDWARD SHORELINES AND
REEFS DUE TO THE WEAKER TRADE WINDS TODAY AND TUESDAY. ELSEWHERE...
SMALL SWELLS ORIGINATING FROM THE SOUTHERN HEMISPHERE WILL MAINTAIN
SMALL BACKGROUND SURF ALONG SOUTH FACING SHORES THIS WEEK.

&&

.AVIATION...
AN UPPER LEVEL LOW ALMOST DUE NORTH OF KAUAI HAS BROUGHT QUITE A BIT
OF INSTABILITY AS WELL AS MOISTURE TO THE ISLANDS EARLY THIS
MORNING. KAUAI IS EXPERIENCING MOST OF THE CLOUDS AND SHOWERS AT
TIME...AND IS EXPECTED TO CONTINUE TODAY WITH MORE WIDESPREAD MVFR
CEILING CONDITION. WILL ISSUE A MTN OBSC AIRMET FOR THAT ISLAND IN
THE COMING HOURS. ELSEWHERE MAINLY VFR CONDITIONS PREVAIL...THOUGH
BRIEF MVFR CEILING AND SHOWER MAY AFFECT ALL SITES AT TIMES AS
ISOLATED CONVECTION PASS ACROSS THE ISLANDS. THERE IS ALSO A SLIGHT
CHANCE OF THUNDERSTORMS FOR KAUAI TODAY...AND FOR LEEWARD AREAS OF
OAHU AND THE BIG ISLAND IN THE AFTERNOON.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH TUESDAY AFTERNOON FOR NIIHAU-KAUAI-OAHU.

&&

DISCUSSION/MARINE...M BALLARD
AVIATION...BRENCHLEY