Flash Flood and Lightning Strike in Hawaii, May 1 to 3, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST SUN MAY 1 2011

.SYNOPSIS...
A LOW ALOFT WILL PRODUCE UNSETTLED WEATHER THROUGH THE EARLY PART OF
THE WEEK...WITH HEAVY SHOWERS AND THUNDERSTORMS POSSIBLE. A SURFACE
RIDGE TO THE NORTH WILL PRODUCE TRADE WINDS...BUT THE TROUGH ALOFT
WILL KEEP THE TRADES MODERATE THROUGH TUESDAY. A HIGH FAR NORTHEAST
OF THE AREA WILL BUILD SOUTHWEST STARTING TUESDAY NIGHT AND BRING
BREEZY TRADE WINDS.

&&

.DISCUSSION...
SURFACE ANALYSES SHOW A 1026 MB HIGH ABOUT 1100 MILES NORTH
NORTHEAST OF HONOLULU NEAR 36N153W. THE HIGH WILL MOVE EAST
NORTHEAST AND STRENGTHEN OVER THE NEXT FEW DAYS. THIS HIGH MIGHT
NORMALLY BE EXPECTED TO PRODUCE FRESH TRADE WINDS OVER THE MAIN
HAWAIIAN ISLANDS BUT A SURFACE TROUGH NORTHEAST OF THE ISLANDS IS
WEAKENING THE PRESSURE GRADIENT ENOUGH TO KEEP TRADE WINDS MODERATE
FOR THE NEXT FEW DAYS.

WATER VAPOR IMAGERY SHOWS A LOW ALOFT 380 MILES SOUTHWEST OF KAUAI
NEAR 18N164W. THE LOW HAS BEEN MOVING EAST NORTHEAST FOR THE PAST
SEVERAL HOURS. THE GFS AND ECMWF MODELS SHOW THE LOW CURVING NORTH
NORTHEASTWARD AND MOVING RIGHT OVER THE ISLANDS. THIS APPROACHING
LOW WILL KEEP THE ATMOSPHERE OVER THE ISLANDS QUITE UNSTABLE FOR AT
LEAST THE NEXT 24 HOURS. THE 200 AM BALLOON SOUNDING AT LIHUE SHOWED
A LIFTED INDEX...LI...OF -7.5 AND A CAPE OF 3041 J/KG. THE
ATMOSPHERE OVER THE ISLANDS RARELY GETS THIS UNSTABLE. SURFACE
OBSERVATIONS SHOW FAIRLY QUIET TRADE-WIND TYPE WEATHER OVER THE MAIN
HAWAIIAN ISLANDS...BUT RADAR AND SATELLITE IMAGERY SHOW HEAVY
SHOWERS AND THUNDERSTORMS OFFSHORE TO THE SOUTH OF OAHU AND SOUTH
THROUGH WEST OF KAUAI. AS THE LOW ALOFT MOVES OVER THE ISLANDS THE
CHANCE OF HEAVY SHOWERS AND THUNDERSTORMS WILL INCREASE.

THE ATMOSPHERE OVER THE ISLANDS WILL PROBABLY BE MOST UNSTABLE TODAY
BUT MOISTURE...AS INDICATED BY FORECASTS OF PRECIPITABLE WATER
VALUES...WILL PROBABLY PEAK ON TUESDAY. TRADE WINDS MAY BE LIGHT
ENOUGH TODAY FOR CONVECTION TO BE MOST ACTIVE OVER LEEWARD AREAS
WITH SEA-BREEZE CONVERGENCE. GRADUALLY STRENGTHENING TRADE WINDS AND
LESS INSTABILITY STARTING TONIGHT MAY FOCUS MORE CONVECTION OVER
WINDWARD AREAS TONIGHT THROUGH TUESDAY. THE LATEST COMPUTER MODELS
SUGGEST THE GREATEST RAINFALL ACCUMULATIONS WILL BE MONDAY NIGHT
THROUGH TUESDAY. IF THE MODELS MAINTAIN THIS TREND WE MAY ISSUE A
FLASH FLOOD WATCH. WITH THE COLD TEMPERATURES ALOFT AND UNSTABLE
CONDITIONS...WINTRY WEATHER IS POSSIBLE ON THE SUMMITS OF MAUNA KEA
AND MAUNA LOA. WINTER WEATHER ADVISORIES OR WARNINGS MAY BE NEEDED
AS THE LOW ALOFT GETS CLOSER TO THE BIG ISLAND.

THE LATEST GFS SHOWS THE LOW ALOFT WEAKENING MONDAY. ON TUESDAY THE
LOW IS FORECAST TO MERGE WITH A DEVELOPING EAST TO WEST TROUGH...
THE KIND OFTEN CALLED A TUTT...TROPICAL UPPER TROPOSPHERIC TROUGH.
THE ATMOSPHERE OVER THE ISLANDS WILL BECOME LESS UNSTABLE...BUT
THERE WILL STILL BE SOME LINGERING INSTABILITY ASSOCIATED WITH THE
TUTT. THE GFS IS NOW SHOWING A NEW LOW ALOFT FORMING WITHIN THE TUTT
EAST NORTHEAST OF THE ISLANDS ON WEDNESDAY...THEN MOVING WEST
SOUTHWEST AND CREATING MORE UNSTABLE CONDITIONS OVER THE ISLANDS
STARTING AROUND FRIDAY MORNING. THE ECMWF MODELS ALSO SUGGESTS A
TUTT DEVELOPING BUT DIFFERS IN THE DETAILS. IT IS TOO SOON TO SAY
HOW THIS WILL TURN OUT.

&&

.MARINE...
WINDS AND SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY...SCA...
CRITERIA THROUGH THE DAY ON TUESDAY. TRADE WINDS WILL LIKELY
STRENGTHEN ENOUGH TUESDAY NIGHT TO REQUIRE A SCA FOR THE WINDIER
AREAS AROUND THE BIG ISLAND AND MAUI. THE SCA MAY REMAIN UP THROUGH
THE COMING WEEKEND. NO LARGE SWELLS ARE EXPECTED THROUGH NEXT
WEEKEND BUT A MODERATE NORTHWEST SWELL WILL ROLL THROUGH WEDNESDAY
NIGHT THROUGH FRIDAY. THE STRONGER TRADES NEXT WEEK MAY PRODUCE
ENOUGH ROUGH SURF ALONG EAST FACING SHORES TO REQUIRE A HIGH SURF
ADVISORY.

&&

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

&&

DONALDSON