Heavy Rain in Hawaii, May 5, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1010 AM HST SUN MAY 5 2013

.SYNOPSIS...
A FRONT NEAR KAUAI WILL MOVE EAST OVER OAHU AND MAUI TODAY...
BRINGING CLOUDY AND SHOWERY WEATHER...AND THE THREAT OF LOCALLY
HEAVY SHOWERS. THE FRONT IS EXPECTED TO STALL AND GRADUALLY
DISSIPATE NEAR MAUI AND THE BIG ISLAND ON MONDAY AND TUESDAY. WINDS
WILL REMAIN LIGHT FOR MOST OF THE WEEK...WHICH WILL RESULT IN MOSTLY
DRY NIGHTS AND MORNINGS...WITH CLOUDS AND SHOWERS FAVORING INTERIOR
AND MAUKA AREAS DURING AFTERNOONS AND EVENINGS. TRADE WINDS ARE NOT
EXPECTED TO RETURN UNTIL THE END OF THE WEEK...AND MAY BE RATHER WET
WHEN THEY DO RETURN.

&&

.DISCUSSION...
A SOMEWHAT UNSEASONABLE WEATHER PATTERN WILL PREVAIL FOR MOST OF THE
UPCOMING WEEK...WITH AN ABSENCE OF TRADE WINDS FOR SEVERAL
DAYS...AND THE THREAT OF HEAVY RAIN THROUGH TONIGHT.

LATEST RADAR SATELLITE AND SURFACE OBSERVATIONS HELP TO DELINEATE A
SHARP NNE-SSW ORIENTED SURFACE BOUNDARY...ANALYZED AS A FRONT...
STRADDLING KAUAI THIS MORNING...WITH MODERATE N TO NE WINDS W OF THE
BOUNDARY...AND LIGHT S TO SE WINDS E OF THE BOUNDARY. SHOWERS
EMBEDDED WITHIN THE MOIST SOUTHERLY FLOW E OF THE FRONT ARE BEING
ENHANCED BY A SHARP N-S ORIENTED UPPER TROUGH...THAT IS ALONG THE
LONGITUDE OF OAHU/MAUI COUNTY...BUT STRONGER N OF THE ISLANDS. THE
COMBINATION OF THE SURFACE CONVERGENCE ASSOCIATED WITH THE FRONT
AND THE INSTABILITY ASSOCIATED WITH THE TROUGH ALOFT IS LEADING TO A
WIDESPREAD AREA OF SHOWERS FROM KAUAI TO MAUI...WITH SOME EMBEDDED
HEAVY SHOWERS AND ISOLATED THUNDERSTORMS. FORECASTS FOR TODAY HAVE
ALREADY BEEN TWEAKED TO ACCOUNT FOR LATEST OBSERVATIONS...BUT MAY
NEED TO BE TWEAKED FURTHER...ESPECIALLY IF THE UPPER TROUGH
PRODUCES STRONGER THAN EXPECTED WINDS OVER THE BIG ISLAND SUMMITS.

GIVEN LATEST TRENDS...AND CONSIDERING THE LOCALIZED HEAVY RAINFALL
THAT OCCURRED ON OAHU YESTERDAY...HAVE DECIDED TO ISSUE A FLASH
FLOOD WATCH FOR OAHU AND MAUI COUNTY THROUGH TONIGHT. GREATEST
THREAT ON OAHU APPEARS TO BE THROUGH THIS EVENING...WHILE THE
GREATEST THREAT FOR MAUI COUNTY APPEARS TO BE LATER TODAY AND
OVERNIGHT.

THE FORECAST CALLS FOR THE FRONT TO MOVE E TO MAUI COUNTY TONIGHT...
AS A WEAK SURFACE LOW DEVELOPS ON THE FRONT NE OF THE ISLANDS...AND
THEN STALL NEAR MAUI AND THE BIG ISLAND THROUGH TUESDAY AS THE LOW
TRACKS AWAY TO THE NE. AS THE FRONT DISSIPATES NEAR THE ISLANDS OVER
THE COURSE OF THE UPCOMING WEEK...THE LOW LEVEL GRADIENT WILL REMAIN
WEAK...INITIALLY SUPPORTING A LIGHT N FLOW OVER KAUAI AND OAHU...A
LIGHT SE FLOW NEAR THE BIG ISLAND...AND NEARLY CALM WINDS OVER MAUI
COUNTY. THE LIGHT WINDS WILL ALLOW ISLAND SCALE CIRCULATIONS TO
DOMINATE THE WEATHER OVER THE ISLANDS...WITH MOSTLY DRY WEATHER
NIGHTS AND MORNINGS...WITH CLOUDS AND SHOWERS DEVELOPING OVER THE
INTERIOR AND MAUKA PORTIONS OF THE ISLANDS DURING AFTERNOONS AND
EVENINGS.

THE UPPER TROUGH SUPPORTING THE THREAT OF HEAVY RAIN THROUGH TONIGHT
WILL BE EXITING AND WEAKENING TOMORROW...AND THE FLOW ALOFT WILL
BECOME ZONAL FOR THE BULK OF THE WEEK...THUS ONLY MODERATE
INSTABILITY AT BEST TO SUPPORT THE CONVECTIVE SHOWERS THIS WEEK. BY
THE END OF THE WEEK AND NEXT WEEKEND...A SHEAR LINE MOVING OVER THE
ISLANDS FROM THE NW WILL USHER TRADE WINDS BACK IN...SUPPORTED BY A
WEAK TROUGH ALOFT TO BRING WHAT COULD BE A WET TRADE WIND PATTERN.

&&

.AVIATION...
AIRMET SIERRA FOR MTN OBSC IS IN EFFECT AND IS EXPECTED TO REMAIN
THROUGH THE FORECAST PERIOD. THIS AIRMET IS CURRENTLY FOR OAHU AND
MOLOKAI AND WILL LIKELY EXPAND TO INCLUDE LANAI AND MAUI AS THE
FRONT CONTINUES ITS EASTWARD MOVEMENT.

WITH THIS FRONT...WIDESPREAD MVFR AND ISOL IFR CONDITIONS ARE
EXPECTED FOR OAHU AND SPREADING TO MAUI THROUGH TODAY...WITH
CONDITIONS IMPROVING FOR OAHU THIS AFTERNOON. OTHERWISE...VFR
CONDITIONS WILL PREVAIL AT KAUAI AND BIG ISLAND WITH BRIEF MVFR IN
SHOWERS AND CLOUDS.

&&

.MARINE...
SMALL NORTHWEST...SOUTH AND SOUTHWEST SWELLS WILL CONTINUE TO BRING
MODEST SURF...AT BEST...ALONG EXPOSED SHORES OF THE ISLANDS THROUGH
THE FORECAST PERIOD. WITH SMALL SWELLS AND LIGHT WINDS IN THE
FORECAST...NO SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED THROUGH
NEXT THURSDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH 6 AM HST MONDAY FOR OAHU MOLOKAI MAUI
LANAI AND KAHOOLAWE.

WIND ADVISORY THROUGH 6 AM HST MONDAY FOR BIG ISLAND SUMMITS.

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...LUTU-MCMOORE