Flash Flood and Heavy Rain in Hawaii, December 31, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST TUE DEC 30 2008

.SYNOPSIS...
A WEAK HIGH PRESSURE RIDGE NORTH OF THE STATE WILL MAINTAIN LIGHT
TRADE WINDS THROUGH WEDNESDAY. A WEAK UPPER TROUGH NEAR THE ISLANDS
WILL PROVIDE INSTABILITY FOR WETTER THAN NORMAL CONDITIONS FAVORING
WINDWARD AND MAUKA SECTIONS THROUGH THE NEW YEAR. A NEW UPPER LEVEL
TROUGH WILL DEVELOP WELL TO THE WEST OF THE STATE WEDNESDAY NIGHT
AND THURSDAY AS SURFACE HIGH PRESSURE BUILDS TO THE NORTH. THIS WILL
BRING AN INCREASE IN THE TRADE WINDS ACROSS THE STATE THROUGH THE
COMING
WEEKEND.

&&

.DISCUSSION...
RATHER ACTIVE WEATHER CONTINUES ACROSS THE ISLANDS THIS EVENING.
MOIST TRADE FLOW COMBINED WITH THE INSTABILITY FROM THE UPPER LEVEL
TROUGH CONTINUE TO BRING CLOUDS AND LOCALLY HEAVY PRECIPITATION TO
OUR ISLANDS. IN FACT...ISOLATED THUNDERSTORMS WERE REPORTED ACROSS
THE STATE EARLIER THIS EVENING. ROUNDS OF PRECIPITATION CONTINUE
OVER THE WINDWARD AND SOUTH BIG ISLAND AS WELL AS MOST OF EAST MAUI.
THE CONTINUOUS SNOW SHOWERS OVER THE SUMMITS OF MAUNA KEA AND MAUNA
LOA HAS PROMPTED THE ISSUANCE OF WINTER STORM WARNING FOR THOSE
SUMMITS UNTIL 6 AM WEDNESDAY MORNING. DO EXPECT THE UNSETTLED
WEATHER TO PREVAIL THROUGH THE NIGHT INTO WEDNESDAY MORNING. HAVE TO
KEEP A CLOSER WATCH ON WINDWARD BIG ISLAND AS THE AREA SHOULD BE
SATURATED FROM PREVIOUS BOUTS OF RAIN. LATEST RAINFALL SUMMARY
SHOWED GOOD AMOUNT OF RAIN ACROSS THE STATE...WITH SELECTED WINDWARD
BIG ISLAND LOCATIONS REPORTING ABOUT AN INCH OF RAIN IN THE PAST SIX
HOURS.

LATEST FORECAST CHARTS HAVE BEEN CONSISTENT IN THE PENDING ARRIVAL
OF ANOTHER UPPER LEVEL TROUGH TO THE WEST OF THE ISLANDS BY THE NEW
YEAR. FORECAST CHARTS NOW INDICATE THIS SYSTEM MAY BE SLIGHTLY
STRONGER THAN PREVIOUS INDICATIONS...WITH A STRONGER SURFACE PRESENT
IN THE FORM OF A TROUGH WEST OF THE ISLANDS. THUS THE LIGHTER WINDS
MAY LAST SLIGHTLY LONGER OVER THE ISLANDS INTO THE NEW YEAR...WITH
THE RETURN OF THE STRONGER TRADE WINDS BY LATER THURSDAY. FORECAST
CHARTS STILL HINT LINGERING INSTABILITY OVER THE AREA IN THE MEAN
TIME. THUS THE CURRENT WET WEATHER REGIME MAY CONTINUE...AND EVEN
THE THREAT OF THUNDERSTORMS REMAINS IN OUR AREA. SHALL LEAVE CURRENT
FORECAST PACKAGE AS IS FOR NOW...PENDING ANOTHER MODEL RUN BEFORE
MAKING MUCH CHANGE TO THE FORECAST FOR THE NEXT COUPLE OF DAYS.

THE TRADES ARE TO INCREASE BY THE WEEKEND AS THE RIDGE BUILDS BACK
NORTH OF THE STATE. FORECAST CHARTS INDICATE THE AIR MASS WILL
REMAIN MOIST...AND THUS THE WET TRADE WEATHER WILL PROBABLY PREVAIL
THROUGH THE REST OF WEEK.

ANOTHER LOW PRESSURE TROUGH IS STILL EXPECTED TO APPROACH THE
ISLANDS FROM THE WEST BY EARLY NEXT WEEK...WITH WINDS BECOMING
LIGHT SOUTHERLY AGAIN. LOOK FOR A MORE CONVECTIVE WEATHER REGIME TO
RETURN THEN...WITH LOCAL LAND AND SEA BREEZES BECOMING MORE
PROMINENT.

&&

.MARINE...
THE NEW WEST NORTHWEST SWELL HAS REACHED THE NORTH SHORES OF OAHU
THIS AFTERNOON AND WILL CONTINUE TO BUILD THROUGH THE EVENING. SURF
HEIGHTS WILL REACH ADVISORY LEVELS ON NORTH AND WEST FACING SHORES
OF KAUAI AND NIIHAU...WHILE THE REST OF THE SMALLER ISLANDS WILL BE
SOMEWHAT SHELTERED...AND THUS SEE LOWER HEIGHTS. THIS SWELL WILL
REACH THE BIG ISLAND THOUGH AND WILL BRING ADVISORY LEVEL SURF TO
THE WEST FACING SHORES OF THAT ISLAND TOO.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FOR BIG ISLAND SUMMITS THROUGH 6 AM WEDNESDAY.

HIGH SURF ADVISORY FOR NORTH AND WEST FACING SHORES OF KAUAI AND
NIIHAU AND FOR WEST FACING SHORES OF BIG ISLAND LATE TUESDAY NIGHT
THROUGH THURSDAY.

&&

HUI