Flash Flooding and Heavy Rain in Hawaii, November 10 to 15, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
422 PM HST TUE NOV 10 2009

.SYNOPSIS...
ISLAND WEATHER WILL BE UNSETTLED THE NEXT COUPLE OF DAYS...AS A
STRONG LOW ALOFT DEVELOPS NEAR THE ISLAND CHAIN. A SURFACE TROUGH
WILL DEVELOP NORTHEAST OF THE ISLANDS TONIGHT IN RESPONSE TO THE
DEVELOPING LOW ALOFT...WITH NORTHEAST WINDS BECOMING RATHER STRONG
NEAR KAUAI. THE SURFACE TROUGH WILL TRACK FROM EAST TO WEST ACROSS
THE ISLAND CHAIN WEDNESDAY NIGHT AND THURSDAY...AND THEN LINGER WEST
OF THE ISLANDS INTO THE WEEKEND. THIS COMBINATION WILL KEEP WEATHER
UNSETTLED...WITH HEAVY SHOWERS AND THUNDERSTORMS POSSIBLE OVER ALL
ISLANDS TONIGHT THROUGH THURSDAY...AND POSSIBLY INTO THE WEEKEND.

&&

.DISCUSSION...
A DYNAMIC WEATHER PATTERN CONTINUES TO EVOLVE IN THE VICINITY OF THE
HAWAIIAN ISLANDS THIS EVENING...AS A DEEP LOW ALOFT IS IN THE
PROCESS OF CARVING ITSELF OUT IN THE MIDDLE AND UPPER ATMOSPHERE.
WATER VAPOR IMAGERY HIGHLIGHTS THE ENE-WSW ORIENTED...DEEPENING AND
DEVELOPING TROUGH DIGGING SOUTHWARD ALONG 160W NEAR 25N.

THIS DEVELOPING LOW ALOFT AND IT/S SURFACE REFLECTION WILL BE THE
MAIN PLAYERS IN ISLAND WEATHER OVER THE NEXT COUPLE OF DAYS...AND
WILL BRING TOGETHER THE INGREDIENTS FOR HEAVY RAINFALL. THE LOW
ALOFT WILL BRING A DESTABILIZING POOL OF COLD AIR TO THE MID- AND
UPPER-LEVELS OF THE ATMOSPHERE...WHILE ALSO INDUCING A SURFACE
TROUGH/WEAK SURFACE LOW.

OVERALL FORECAST PHILOSOPHY IS RELATIVELY UNCHANGED FROM THE EARLIER
DISCUSSION...AS THE APPROACHING TROUGH ALOFT IS FORECAST TO DEVELOP
INTO A CUT-OFF LOW A COUPLE HUNDRED MILES TO THE NE OF THE ISLANDS
TONIGHT...TRACK STEADILY WSW TO A POSITION JUST W OF THE ISLANDS
THROUGH THURSDAY...THEN REMAIN NEARLY STATIONARY INTO THE WEEKEND
WHILE GRADUALLY FILLING. THE ASSOCIATED SURFACE TROUGH/WEAK
SURFACE LOW IS EXPECTED TO DEVELOP NE OF THE ISLANDS TONIGHT. WITH
STRONG HIGH PRESSURE FAR N OF THE ISLANDS...THE PRESSURE GRADIENT
BETWEEN THE TROUGH AND THE HIGH WILL RESULT IN BRISK NE WINDS...
ESPECIALLY OVER KAUAI COUNTY AND ADJACENT WATERS...WHERE A WIND
ADVISORY AND SMALL CRAFT ADVISORY ARE IN PLACE.

THE SURFACE TROUGH IS STILL FORECAST TO TRACK W WITH THE SUPPORTING
LOW ALOFT...AND WINDS WILL BECOME LIGHT AND VARIABLE...AND THEN
SWITCH TO THE SE...AS THE TROUGH PASSES W OVER ANY ONE ISLAND. THE
TROUGH IS EXPECTED TO BE W OF ALL ISLANDS BY THURSDAY...WITH A MOIST
AND CONVERGENT SOUTHERLY FLOW IN PLACE OVER ALL ISLANDS. LATEST
MODEL RUNS INDICATE THE POSSIBILITY OF A PERIOD OF DISTURBED WEATHER
OVER THE ISLANDS TONIGHT AS THE SHORT WAVE TO THE N DIGS S...BUT
THEN FOCUS THE BULK OF THE HEAVY PRECIP WELL N OF THE ISLANDS ON
WEDS/WEDS NIGHT...WHERE CONVERGENCE IS MAXIMIZED ON THE N SIDE OF
THE DEVELOPING SURFACE TROUGH. BY LATE THURSDAY...THE SURFACE TROUGH
WILL BE W OF THE ISLANDS...AND DEEP S/SE FLOW ON THE E SIDE OF THE
LOW ALOFT/SURFACE TROUGH IS EXPECTED TO BRING BOUTS OF WIDESPREAD
HEAVY RAIN TO MOST AREAS ACROSS THE STATE. THE COLD POOL OF AIR
ASSOCIATED WITH THE LOW ALOFT WILL BRING THE THREAT OF WINTER
WEATHER TO THE SUMMITS AND UPPER ELEVATIONS OF THE BIG ISLAND...
WHERE A WINTER STORM WATCH REMAINS IN PLACE.

THE MOVEMENT AND PLACEMENT OF THE LOW ALOFT/LOW-LEVEL TROUGH WILL
HAVE MAJOR IMPACTS ON WINDS AND WEATHER EXPERIENCED OVER THE
ISLANDS...AND WE WILL LIKELY BE MODIFYING FORECASTS...POSSIBLY
DRAMATICALLY AT TIMES...AS THE EVENT EVOLVES OVER THE NEXT COUPLE OF
DAYS.

THE WEAKENING OF THE LOW INTO A TROUGH AND THE WARMING OF THE UPPER
ATMOSPHERE WILL BE SLOW...SO THE CHANCE OF HEAVY RAIN/FLOODING AND
UPPER ELEVATION WINTER WEATHER COULD EASILY EXTEND INTO THE WEEKEND.
MODELS ARE NOT BEING CONSISTENT WITH THEIR REPRESENTATION WITH
WEEKEND WEATHER OVER THE ISLANDS...AND CONFIDENCE IS A LOT LOWER
THAN NORMAL...GIVEN FORECAST MODELS INABILITY TO CORRECTLY
PARAMETRIZE ALL THE CONVECTION THAT IS SLATED TO OCCUR BETWEEN NOW
AND THEN.

&&

.MARINE...
SURF WILL REMAIN BELOW ADVISORY LEVELS INTO WEDNESDAY. A
ROUGH...SHORT-PERIOD NE SWELL IS EXPECTED LATE WEDNESDAY THROUGH
FRIDAY...AND A HIGH SURF ADVISORY MAY BE REQUIRED FOR E FACING
SHORES. SMALL CRAFT ADVISORIES ARE IN PLACE FOR WATERS AROUND KAUAI
DUE TO THE EXPECTED EVOLUTION OF STRONG NE WINDS TONIGHT THROUGH
WEDNESDAY NIGHT.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH LATE THURSDAY NIGHT FOR NIIHAU-KAUAI-
OAHU-MOLOKAI-LANAI-KAHOOLAWE-MAUI-BIG ISLAND.

WIND ADVISORY UNTIL 6 PM HST WEDNESDAY FOR NIIHAU-KAUAI.

HIGH WIND WATCH FROM WEDNESDAY MORNING THROUGH THURSDAY AFTERNOON
FOR BIG ISLAND SUMMITS.

WINTER STORM WATCH FROM WEDNESDAY MORNING THROUGH THURSDAY
MORNING FOR BIG ISLAND SUMMITS.

SMALL CRAFT ADVISORY UNTIL 4 AM HST THURSDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL.

&&

BIRCHARD