Flash Flooding and Heavy Rain in Hawaii, December 26 to 28, 2008

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
1010 PM HST THU DEC 25 2008

.UPDATE...
LATEST QUIKSCAT PASS INDICATES WIDESPREAD 25 KNOTS ACROSS NEARLY ALL
COASTAL WATERS AND UPWARDS OF 30 KTS IN MOST CHANNELS. HAVE
THEREFORE EXPANDED THE SMALL CRAFT ADVISORY TO COVER ALL HAWAIIAN
WATERS AND INCREASED WINDS OVER ALL BUT THE KAUAI CHANNEL TO 25 TO
30 KT. WIND WAVES HEIGHTS HAVE ALSO BEEN INCREASED IN RESPONSE TO
THE INCREASE IN WINDS.

&&

.SYNOPSIS...
AN UPPER LEVEL LOW SOUTHEAST OF THE BIG ISLAND IS MOVING SLOWLY
WESTWARD. THIS LOW WILL CONTINUE TO BRING UNSETTLED WEATHER MAINLY
TO THE BIG ISLAND AND MAUI OVERNIGHT. THE UPPER LEVEL LOW WILL
WEAKEN TO AN UPPER LEVEL TROUGH OVER THE STATE LATER THIS WEEKEND.
THEREFORE...EXPECT WET TRADES ACROSS THE REMAINDER OF THE ISLAND
CHAIN.

&&

.DISCUSSION...
AN UPPER TROPOSPHERIC LOW CENTERED ABOUT 190 MILES SOUTHEAST OF THE
BIG ISLAND IS MOVING TOWARD THE WEST SOUTHWEST AT 15 MPH. AT THE
SURFACE...A 1034 MB HIGH PRESSURE LOCATED NEAR 36N 144W IS MOVING
TOWARD THE SOUTHEAST AT ABOUT 10 MPH. MODERATE TO FRESH TRADE WINDS
ARE BLOWING SOUTH OF THIS HIGH.

RADAR REFLECTIVITY DATA AND SATELLITE OBSERVATIONS SHOW BANDS OF
WESTWARD MOVING SHOWERS...SOME WITH LOCALLY HEAVY RAINFALL MOVING
ONSHORE ALONG THE EASTERN BIG ISLAND AND MAUI EARLY THIS CHRISTMAS
EVENING. THUNDERSTORMS EMBEDDED WITHIN THESE SHOWER BANDS HAVE BEEN
MOVING ONSHORE ALONG THE WINDWARD COAST OF THE BIG ISLAND BASED ON
OBSERVATIONS FROM THE HILO AIRPORT AND A WEATHER SPOTTER LOCATED
JUST NORTHWEST OF HILO. RAINFALL HAS BEEN BRIEFLY HEAVY...WITH A FEW
RAIN GAGES ON THE BIG ISLAND REPORTING UP TO ONE-HALF OF AN INCH
DURING SOME 15 MINUTE INTERVALS. NESDIS JUST ISSUED A SATELLITE
PRECIPITATION ESTIMATE INDICATING THAT THE PW HAS BEEN INCREASING
JUST EAST OF THE BIG ISLAND AND THERE HAS BEEN A CONSIDERABLE
EXPANSION OF MODERATE MOISTURE WITH 1.7 INCH MAX PW VALUES NOW
OVERSPREADING THE EAST COAST OF THE BIG ISLAND. NESDIS ALSO
INDICATES THE EAST SIDE OF MAUI WILL CONTINUE TO HAVE A THREAT FOR
LOCALLY HEAVY RAIN MOST OF TONIGHT.

AS THE UPPER LEVEL LOW MOVES CLOSER TO THE BIG ISLAND...THE SUMMITS
WILL CONTINUE TO HAVE THE POTENTIAL FRO ADDITIONAL SNOW. AS A
RESULT...THE WINTER WEATHER ADVISORY REMAINS IN EFFECT UNTIL 500 AM
HST FRIDAY FOR THE BIG ISLAND SUMMITS. THIS ADVISORY MAY NEED TO BE
EXTENDED INTO FRIDAY MORNING. ALSO...WINDS ON HALEAKALA CONTINUE
NEAR THE WIND ADVISORY CRITERIA. THEREFORE...THE WIND ADVISORY WILL
CONTINUE THERE UNTIL NOON FRIDAY.

THE MODELS STILL INDICATE THE UPPER LEVEL LOW WILL OPEN UP INTO AN
UPPER LEVEL TROUGH AS IT CONTINUES MOVING WESTWARD OVER THE ISLANDS
DURING THE NEXT 24 TO 36 HOURS. THE TROUGH THEN BEGINS TO MOVE BACK
TO THE EAST BY LATE SUNDAY AND MONDAY AS A NEW UPPER LEVEL LOW FAR
NORTHWEST OF THE ISLANDS BEGINS PUSHING TOWARD THE SOUTHEAST. THE
ECMWF AND GFS DEVELOP A SURFACE TROUGH OR A WEAK LOW ALONG THE
TROUGH TO THE NORTHWEST OF KAUAI BY WEDNESDAY OR THURSDAY.
HOWEVER...THE LATEST GUIDANCE APPEAR TO KEEP THE SURFACE SYSTEM
WELL WEST OF THE ISLANDS.

EXPECT WET TRADES TO CONTINUE THROUGH THE WEEKEND FOR THE REMAINING
ISLANDS WITH THE GREATEST RAINFALL OCCURRING OVER WINDWARD FACING
SLOPES. THE WINDS ACROSS THE SMALLER ISLANDS WILL REMAIN BREEZY...
BUT ARE EXPECTED TO REMAIN BELOW ADVISORY CRITERIA.

&&

.MARINE..
WIND WAVES PRODUCED BY FRESH TRADES COMBINED WITH AN EASTERLY SWELL
WITH A PERIOD OF AROUND 10 SECONDS WILL CONTINUE TO BRING WAVES NEAR
THE HIGH SURF ADVISORY THRESHOLD TO THE EAST FACINGS SHORES OF ALL
OF THE MAIN HAWAIIAN ISLANDS...EXCEPT NIIHAU...KAHOOLAWE AND LANAI.
THE MOKAPU BUOY NEAR KAILUA ON OAHU SHOWS A SIGNIFICANT WAVE HEIGHT
OF 9 TO 10 FEET WITH A PERIOD OF 10 SECONDS FROM OF A DIRECTION OF
AROUND 75 DEGREES. SURF OBSERVATIONS FROM WINDWARD OAHU AND THE EAST
COAST OF THE BIG ISLAND ALSO SUPPORT THE HIGH SURF ADVISORY. THIS
ELEVATED SURF IS EXPECTED TO CONTINUE THROUGH FRIDAY AFTERNOON.

ELSEWHERE...VERY SMALL SWELLS FROM BOTH THE NORTH AND NORTHWEST WILL
PERSIST THROUGH MONDAY NIGHT. A NEW MODERATE SIZED WEST NORTHWEST
SWELL IS EXPECTED TO ARRIVE DURING THE DAY TUESDAY. THIS SWELL IS
EXPECTED TO THEN BUILD WEDNESDAY...AND MAY PRODUCE SURF NEAR THE
HIGH SURF ADVISORY THRESHOLD ALONG NORTH AND WEST FACING SHORES OF
KAUAI AND NIIHAU THAT DAY. HOWEVER...THIS SWELL APPEARS TO INITIALLY
BE COMING IN FROM A DIRECTION OF AROUND 290 DEGREES ACCORDING TO THE
LATEST RUN OF THE WAVE WATCH III MODEL. THIS MAY CAUSE ENOUGH
SHADOWING OF THE SWELL BY KAUAI AND NIIHAU TO KEEP IT FROM BUILDING
TO HIGH SURF LEVELS ALONG NORTH AND WEST FACING SHORES OF THE OTHER
SMALLER ISLANDS DURING THE DAY WEDNESDAY. HOWEVER...WEST FACING
SHORES OF THE BIG ISLAND MAY SEE SURF APPROACHING THE HIGH SURF
ADVISORY THRESHOLD WEDNESDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 600 PM HST FRIDAY FOR EAST FACING SHORES
OF KAUAI OAHU MOLOKAI MAUI AND THE BIG ISLAND.

WINTER WEATHER ADVISORY UNTIL 600 AM HST FRIDAY FOR BIG ISLAND
SUMMITS.

WIND ADVISORY UNTIL NOON HST FRIDAY FOR HALEAKALA SUMMIT.

SMALL CRAFT ADVISORY UNTIL 600 PM HST FRIDAY FOR ALL HAWAIIAN WATERS.

&&

HOUSTON/TANABE