Flash Flood and Winter Storm in Hawaii, April 6 to 9, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST WED APR 6 2011

.SYNOPSIS...
A MOISTENING AIR MASS...TOGETHER WITH A VERY COLD LOW IN THE MIDDLE
ATMOSPHERE...WILL PRODUCE SHOWERS AND THUNDERSTORMS ACROSS THE
STATE...AND WINTRY WEATHER OVER THE BIG ISLAND SUMMITS...THROUGH
THURSDAY. LIGHTER BUT MORE WIDESPREAD RAIN WILL SET IN BY THE
WEEKEND...AND THEN LINGER INTO EARLY NEXT WEEK. WINDS WILL REMAIN
LIGHT FROM THE SOUTHEAST FOR THE MOST PART.

&&

.DISCUSSION...
LIGHT SOUTHEASTERLY WINDS PREVAILED ACROSS THE MAIN HAWAIIAN ISLANDS
UNDER A DEEPENING TROUGH ALOFT. RAINFALL ACROSS THE STATE WAS SPOTTY
OVERNIGHT...WITH THE MOST NOTABLE EXAMPLES BEING THE THUNDERSTORM
OVER MIDDLE TO UPPER WEST SLOPES OF THE BIG ISLAND ON TUESDAY
EVENING...AND THE TOWERING CUMULI POPPING UP OVER THE KOOLAU RANGE
ON OAHU SINCE MIDNIGHT. THE FORMER ENDED BEFORE IT COULD POSE MUCH
OF A RISK...BUT THE LATTER PHENOMENON COULD START TO CAUSE EXCESSIVE
RUNOFF IF IT CONTINUES MUCH LONGER INTO THE MORNING.

SIMILARLY...THERE HAS BEEN LITTLE CONVECTION OFFSHORE WITHIN 40
MILES OF THE ISLANDS OVER THE PAST SIX HOURS. ON THE OTHER HAND...
ENERGETIC...AND QUITE POSSIBLY SEVERE...THUNDERSTORMS HAVE BEEN
PRESSING UP AGAINST THE WEST AND SOUTHWEST OUTER BOUNDARIES OF THE
COASTAL WATERS...AND RECENTLY HAVE STARTED TO DEVELOP TO THE
NORTHEAST AS WELL. EVEN IGNORING LONGER-TERM AND LARGER-SCALE
SYNOPTIC CONCERNS...IT APPEARS INEVITABLE THAT ONE OR ANOTHER OF
THESE STORMS WILL AT LEAST ENTER THE COASTAL WATERS...IF NOT REACH
LAND AS WELL.

IN THE BROADER VIEW...A MID-LEVEL TROUGH DEEPENED RAPIDLY TO THE
NORTHWEST. A LOW NEAR 28N 163W REPRESENTED THE SURFACE REFLECTION OF
THE TROUGH ALOFT. THE SURFACE TROUGH WRAPPING AROUND THIS LOW
PROVIDED A FOCUS BOTH FOR CONVERGING MOISTURE...AND FOR TRIGGERING
DEEP CONVECTION. THE THUNDERSTORMS DEVELOPING ALONG THIS TROUGH WERE
WHAT SHOWED UP SO CLEARLY JUST OUTSIDE THE COASTAL WATERS TO THE
NORTHWEST.

SATELLITE ESTIMATES OF PRECIPITABLE WATER ALSO SHOWED A MOIST AIR
MASS JUST TO THE EAST THROUGH SOUTHEAST OF THE STATE. INCREASING
SOUTHEASTERLY FLOW AHEAD OF THE TROUGH WILL PULL THIS MOISTURE IN...
PROVIDING FUEL FOR THE CONVECTION.

GUIDANCE AGREES THAT THE MID-LEVEL TROUGH TO THE NORTHWEST WILL CUT
OFF SOON...AND THEN DRIFT SOUTH SLOWLY JUST WEST OF THE STATE
THROUGH THURSDAY. ASIDE FROM PROVIDING LIFT...THE LOW WILL BRING AN
EXTREMELY COLD POOL OF AIR...THUS FURTHER DESTABILIZING THE AIR
MASS. IN COMBINATION WITH SPRINGTIME TEMPERATURES NEAR THE
SURFACE...AND THE INCREASING ATMOSPHERIC MOISTURE AT LOW LEVELS...
THE RESULTING INSTABILITY WILL FURTHER EXPAND THE AREAS OF DEEP
CONVECTION INTO THURSDAY.

WITH WINDS PRETTY LIGHT THROUGH MOST OF THE ATMOSPHERE DURING THIS
TIME...IT IS UNLIKELY THAT THE RESULTING THUNDERSTORMS WILL PRODUCE
DAMAGING WINDS AT THE SURFACE. HOWEVER...RAINFALL HEAVY ENOUGH TO
CAUSE LOCALIZED FLOODING CERTAINLY WILL BE POSSIBLE. AT HIGHER
ELEVATIONS...PRECIPITATION LIKELY WILL FALL AS SNOW OR ICE
PELLETS...AND HAIL COULD EVEN REACH SEA LEVEL IN STRONGER STORMS.

THIS CONVECTIVE PRECIPITATION ALSO WILL BE QUITE SPOTTY...WITH GREAT
DIFFERENCES IN ACCUMULATION ACROSS SMALL DISTANCES. HOWEVER...GIVEN
THE COMBINATION OF MOISTURE FROM THE SURFACE TROUGH AND ADVECTED UP
FROM THE DEEP TROPICS BY SOUTHEASTERLY FLOW...IT IS FAIR TO SAY THAT
MID-WEEK LOOKS PRETTY WET FOR MOST AREAS.

WHAT COURSE THE WEATHER TAKES FROM FRIDAY INTO THE WEEKEND IS LESS
CERTAIN...MAINLY BECAUSE THE MOTION OF CUT-OFF LOWS CAN BE
NOTORIOUSLY DIFFICULT TO PREDICT...BUT IF THIS FEATURE DOES LINGER
SOUTHWEST OF THE STATE AS MOST SOLUTIONS EXPECT...THEN STRATUS RAIN
PROBABLY WILL REPLACE THE CONVECTION GRADUALLY ACROSS THE
ARCHIPELAGO.

THE SURFACE TROUGH TO THE WEST MAY TEMPORARILY TIGHTEN THE PRESSURE
GRADIENT ENOUGH LOCALLY TO BOOST WIND SPEEDS OVER THE NORTHEAST
COASTAL WATERS FROM FRIDAY INTO SATURDAY...AND THE CURRENT FORECAST
REFLECTS THIS POSSIBILITY. HOWEVER...A SLIGHT DIFFERENCE IN THE
TRACK OF THE LOW COULD EASILY THROW THIS PART OF THE FORECAST OFF.

EVENTUALLY...THE WINDS WILL ASSUME MORE TYPICAL SPEEDS AND AN
EASTERLY TO NORTHEASTERLY ORIENTATION...THE BETTER TO CARRY AWAY TO
THE WEST MOST OF THE CONVERGED MOISTURE. RECENT RUNS PREDICT THIS
CHANGE OCCURRING NO EARLIER THAN MONDAY OR TUESDAY...OR PERHAPS
LATER IF AN APPROACHING FRONT TURNS THE FLOW SOUTHEASTERLY AGAIN
AROUND NEXT TUESDAY.

&&

.MARINE...
A LARGE SWELL FROM THE NORTH NORTHWEST PEAKED TUESDAY...AND HAS BEEN
DECREASING SLOWLY SINCE...BUT REMAINED LARGE ENOUGH TO REQUIRE A
SHORT EXTENSION OF THE HIGH SURF AND SMALL CRAFT ADVISORIES. ANOTHER
MODERATE-SIZED NORTHWEST SWELL WILL ARRIVE THURSDAY AND FRIDAY...
POSSIBLY BUILDING SURF TO THE ADVISORY LEVEL ALONG NORTH SHORES.
ALSO...A SERIES OF SOUTH SWELLS WILL ARRIVE THROUGH THE REST OF THE
WEEK...LIKELY PRODUCING SURF REACHING OVERHEAD FOR SOME AREAS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH LATE THURSDAY NIGHT FOR ALL HAWAIIAN
ISLANDS.

HIGH SURF ADVISORY UNTIL NOON HST TODAY FOR NORTH-FACING AND
EAST-FACING SHORES OF KAUAI AND OAHU...AND FOR NORTH-FACING SHORES
OF NIIHAU.

WINTER WEATHER ADVISORY UNTIL 6 AM HST FRIDAY FOR BIG ISLAND
SUMMITS.

SMALL CRAFT ADVISORY UNTIL NOON HST TODAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-KAIWI CHANNEL.

&&

RYSHKO