Flash Flood and Heavy Rain in Hawaii, May 11 to 14, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
330 PM HST WED MAY 11 2011

.SYNOPSIS...
UNSETTLED WEATHER WILL CONTINUE TO EASE...ALTHOUGH HEAVY LOCALLY
HEAVY RAINFALL AND POSSIBILITY OF THUNDERSTORMS WILL PERSIST OVER
THE WESTERN THIRD OF THE STATE THROUGH THURSDAY. EAST-SOUTHEASTERLY
WINDS WITH PERIODS OF SEA AND LAND BREEZES WILL PREVAIL THROUGH THE
WEEKEND.

&&

.DISCUSSION...
THE GENERAL FORECAST REASONING REMAINS UNCHANGED FROM EARLIER IN THE
SHIFT. GOES WV IMAGERY DEPICTS STATIONARY LOW PRESSURE SITUATED NW
OF THE STATE WITH A WELL DEFINED SHORT WAVE UNDERCUTTING THE LOW AS
IT APPROACHES 170W. TPW IMAGERY DEPICTS A N-S ORIENTED TROPICAL
MOISTURE AXIS OVER THE FAR W PORTION OF THE STATE. INCREASING
DIFFLUENCE AND PVA AHEAD OF THE SHORT WAVE IS BEGINNING TO INTERACT
AND INTITIATE FRESH CONVECTION ALONG THE MOISTURE AXIS S-SW OF
KAUAI. HAVE ADDED HIGHER POPS AND SLIGHT CHANCE OF TSTORMS OVER
KAUAI FOR TONIGHT-THU AFTERNOON...AND FOR INTERIOR OAHU THU
AFTERNOON. ELSEWHERE A HYBRID DIURNAL/WEAK ESE TRADE PATTERN WILL BE
IN PLACE THU...WITH POSSIBLE ISOLATED CONVECTION OVER THE BIG ISLAND
SUMMITS AGAIN. VARIABLE AMOUNTS OF THIN DEBRIS CIRRUS WILL ALSO BE
CARRIED OVER THE AREA TONIGHT AND THU.

THE UPPER LOW WILL LINGER AROUND INTO THE WEEKEND WITH ITS
ASSOCIATED SURFACE TROUGH NEAR OR JUST W OF KAUAI. A HIGH AMPLITUDE
TROUGH IN THE ENPAC IS PREVENTING HIGH PRESSURE FROM ATTAINING ITS
TYPICAL MAY LOCATION/INTENSITY. THE POLAR JET WILL BECOME ZONAL
ALONG 40N INTO THE WEEKEND WHICH WILL FURTHER HINDER SUBTROPICAL
RIDGE DEVELOPMENT IN THE ENPAC. WITH PERSISTENT LOWER
HEIGHTS/PRESSURES W OF HAWAII...THIS SUGGESTS CONTINUED ESE FLOW
LOCALLY FOR QUITE SOME TIME PUNCTUATED BY PERIODS OF SEA/LAND
BREEZES. SEEMINGLY HOT MUGGY WEATHER MAY CONTINUE INTO NEXT WEEK
WITH AFTERNOON INTERIOR SHOWERS.

&&

.PREVIOUS DISCUSSION.../ISSUED 944 AM HST WED MORNING MAY 11/
STATIONARY MID/UPR LEVEL LOW PRESSURE RESIDES NW OF THE ISLANDS. A
WEAK ATTENDANT FRONTAL BOUNDARY/CONVERGENCE ZONE ALONG WITH
SUFFICIENT BUOYANCY IS MAINTAINING SCATTERED MODERATE SHOWERS OVER
THE WESTERN PORTION OF THE STATE. THE POSSIBILITY OF LOCALLY HEAVY
RAINFALL AND ISOLATED TSTORMS WILL CONTINUE FOR KAUAI/OAHU THROUGH
THIS AFTERNOON. THE BIG ISLAND/MAUI COUNTY ARE WEAKLY CAPPED BY A
MODEST ISOTHERMAL MIXED LAYER FROM 6-9 KFT. ISOLATED TSTORMS ALONG
THE BIG ISLAND SLOPES AND SNOW SHOWERS FOR THE SUMMITS ARE STILL
POSSIBLE THIS AFTERNOON.

THE UPPER LOW WILL LINGER NW OF THE STATE HAVING VARIABLE EFFECTS ON
RAINFALL COVERAGE/INTENSITY. THE ASSOCIATED SURFACE
REFLECTION/TROUGH W OF THE STATE AND HIGH PRESSURE TO THE NE WILL
MAINTAIN E-ESE FLOW WITH PERIODS OF LIGHT WINDS THROUGH THE WEEKEND.
SHOWERS SHOULD REMAIN PRIMARILY FOCUSED ON WINDWARD TERRAIN WITH
ISOLATED AFTERNOON LEEWARD/INTERIOR CONVECTION. ENHANCED
CONVERGENCE AND LINGERING INSTABILITY OVER THE FAR WESTERN PORTION
OF THE STATE/COASTAL WATERS COULD STILL PRODUCE PERIODS OF
WIDESPREAD MODERATE RAINFALL THROUGH SUN.

UNCERTAINTIES IN THE GFS/ECMWF SOLUTIONS APPEAR LATE IN THE WEEKEND
REGARDING THE PERSISTENCE/LOCATION OF THE SURFACE TROUGH W OF
HAWAII. THIS HAS SIGNIFICANT EFFECTS ON THE WESTWARD
EXTENT/STRENGTH OF THE SURFACE RIDGE AND RESULTANT LOCAL TRADE
FLOW. STILL EXPECTING LIGHT ESE FLOW TO PREVAIL INTO NEXT WEEK WITH
A HYBRID DIURNAL/TRADE POP REGIME. WILL BE MAKING SOME SUBTLE
ADJUSTMENTS TO THE GRIDS TO ACCOUNT FOR MODEL TRENDS.

&&

.MARINE...
WEATHER OVER THE WRN COASTAL ZONES REMAINS UNSETTLED. AN INCOMING
SHORT WAVE TROUGH FROM THE W WILL TRIGGER MORE HEAVY RAIN AND
TSTORMS OVERNIGHT THROUGH THU AFTERNOON. ANY THUNDERSTORMS MAY
PRODUCE DANGEROUS LIGHTNING...WATERSPOUTS AND BRIEF PERIODS OF ROUGH
HIGHER SEAS. HAVE ISSUED A MARINE WX STATEMENT HIGHLIGHTING THESE
THREATS.

ELSEWHERE NO MARINE HAZARDS ARE EXPECTED. SURF WILL STAY ON THE
SMALLER SIDE FOR ALL SHORES OVER THE NEXT SEVERAL DAYS.

&&

.AVIATION...
IN THE NEAR TERM...MAIN HAWAIIAN ISLAND TAFS LOOK FINE WITH VFR
CONDITIONS CONTINUING AT ALL TERMINALS EXCEPT LANAI. HILO WILL
EXPERIENCE LOCAL CEILINGS JUST BELOW 3000 FEET...BUT NOT ENOUGH TO
WARRANT A CHANGE GROUP UNTIL THIS EVENING AND TONIGHT WHEN COOLING
AND LAND BREEZE CONVERGENCE WILL HELP PRODUCE LOWER CEILINGS AND
INCREASED SHOWERS. ASIDE FROM PHNY AND PHTO...THE ISLAND TERMINALS
WILL REMAIN VFR FOR THE REST OF TODAY AND TOMORROW WITH LOCALIZED
MVFR VISIBILITY POSSIBLE WITH ANY SHOWERS...AND THESE SHOWERS ARE
MOST LIKELY OVER INTERIOR KAUAI AND OAHU DURING THE AFTERNOON AND
EVENING HOURS.

&&

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

&&

PUBLIC...DEJESUS
MARINE...DEJESUS
AVIATION...BRENCHLEY