Flash Flooding and Heavy Rain in Hawaii, December 18 to 23, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT DEC 18 2010

.SYNOPSIS...
RAIN WILL SPREAD ACROSS THE MAIN HAWAIIAN ISLANDS FROM TONIGHT
THROUGH SUNDAY NIGHT. HEAVIER SHOWERS IN THE RAIN RAISE THE RISK OF
FLASH FLOODING ANYWHERE IN THE STATE...AND A WINTRY MIX OF RAIN AND
SNOW MAY CAUSE PROBLEMS ON THE BIG ISLAND SUMMITS. THE RAIN WILL
TAPER OFF BY THE MIDDLE OF NEXT WEEK...MOVING TO WINDWARD SLOPES AS
TRADE WINDS RETURN.

&&

.DISCUSSION...
OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS CONTINUED TO REPORT
GENERALLY LIGHT AND VARIABLE WINDS...AND PRESSURE TRENDS SHOWED
LITTLE CHANGE FROM 24 HOURS AGO. SOUNDINGS REVEALED A WEAKENING
INVERSION AND INTRUDING MOIST LAYERS AT MULTIPLE LEVELS...BUT
CLEARLY THE BULK OF THE PRECIPITABLE WATER REMAINED OFFSHORE TO THE
SOUTHWEST OF THE STATE...WHERE THE RADAR DATA DEPICTED WIDESPREAD
RAIN WITH EMBEDDED SHOWERS TRAINING FROM THE SOUTH.

GUIDANCE PREDICTS DEEP RIDGING FROM THE SURFACE THROUGH THE MIDDLE
ATMOSPHERE TO WEAKEN FURTHER TONIGHT AS A DEEPENING STACKED LOW TO
THE WEST TURNS THE WINDS MORE SOUTHERLY AT MOST LEVELS. THIS
PERSISTENT SOUTHERLY FLOW WILL ADVECT IN AN INCREASINGLY MOIST AIR
MASS FROM THE DEEP TROPICS. THE RIDGING AND DEEP WARM ADVECTION
TOGETHER WILL KEEP TEMPERATURES ALOFT A BIT TOO HIGH FOR OPTIMAL
THUNDERSTORM DEVELOPMENT...BUT THE COPIOUS MOISTURE MAY PROVE
SUFFICIENT ON ITS OWN...SO THE POSSIBILITY OF DEEP CONVECTION
EMBEDDED IN THE SURROUNDING STRATUS CANNOT BE EXCLUDED.

THE CURRENT FORECAST SCENARIO PREDICTS RAIN TO MOVE NORTHEAST OVER
KAUAI AND THE BIG ISLAND THIS EVENING...AND THEN ACROSS THE REST OF
THE STATE THROUGH SUNDAY NIGHT. ONCE THE STRATUS REACHES THE BIG
ISLAND IN STRENGTH...IT SHOULD BE DEEP ENOUGH TO COVER THE SUMMITS.
WITH TEMPERATURES NEAR FREEZING AT THOSE ALTITUDES...WINTER WEATHER
CONDITIONS CERTAINLY WILL BE POSSIBLE. PREDICTING ACCUMULATION OF
SNOW OR ICE IS PROBLEMATIC BECAUSE EXTENSIVE WARM ADVECTION WILL
TEND TO PRODUCE PERIODS OF MELTING...ESPECIALLY BY DAY. ON THE OTHER
HAND...SOME FROZEN OR MIXED PRECIPITATION AT THE SUMMITS APPEARS
LIKELY...ESPECIALLY AT NIGHT...AND MAY BE SUFFICIENT TO REQUIRE A
WINTER WEATHER ADVISORY SOMETIME BETWEEN TONIGHT AND MONDAY...WITH
THE PEAK SNOW OR ICE ACCUMULATION LIKELY OCCURRING SUNDAY NIGHT.

FROM SUNDAY NIGHT THROUGH THE FIRST HALF OF NEXT WEEK...A WEAKENING
FRONTAL CLOUD BAND FINALLY MAY PRESS SOUTH TOWARD THE STATE.
HOWEVER...THIS FEATURE IS EXPECTED TO WEAKEN CONSIDERABLY BEFORE
REACHING THE ISLANDS...SO THE MOIST AIR MASS AND STRATUS LAYERS
EASILY COULD LANGUISH OVER THE ISLANDS PAST WEDNESDAY. THE CURRENT
FORECAST PACKAGE FOLLOWS A MORE PROGRESSIVE SCENARIO IN WHICH THE
STRATUS LAYERS BREAK UP GRADUALLY AS THE TRADE WINDS PUSH THE
REMNANTS OF THE FRONT OVER THE STATE AROUND TUESDAY...ALLOWING THE
RAIN TO BECOME MORE SHOWERY AFTERWARD...AND THEN TAPER OFF TO MORE
TYPICAL TRADE-WIND LEVELS BY THE END OF THE COMING WEEK.

&&

.MARINE...
A LARGE NORTH SWELL WILL BUILD OVERNIGHT...REQUIRING A HIGH SURF
ADVISORY FOR NORTH-FACING SHORES...AND FOR EAST-FACING SHORES
EXPOSED TO THE NORTH SWELL...UNTIL TUESDAY MORNING. THE SWELL ALSO
WILL PRODUCE HARBOR SURGES IN NORTH-FACING HARBORS...INCLUDING
KAHULUI AND HILO HARBORS.

&&

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

A HIGH SURF ADVISORY IS IN EFFECT FOR SHORES FACING NORTH OR EAST
UNTIL TUESDAY MORNING.

&&

RYSHKO