Flash Flood and Heavy Rain in Hawaii, November 18, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
339 AM HST TUE NOV 18 2008

.SYNOPSIS...
SHOWERS...SOME HEAVY...WILL CONTINUE TO AFFECT THE ISLANDS TODAY
AS AN UPPER LEVEL DISTURBANCE MOVES OVER THE ISLANDS. TRADES WILL
GRADUALLY DECREASE WEDNESDAY WITH SOUTHEAST WINDS STARTING
THURSDAY...AND CONTINUING INTO THE WEEKEND. THE CHANCE OF RAIN
WILL INCREASE LATE IN THE WEEK AS TROPICAL MOISTURE MOVES UP FROM
THE SOUTH.

&&

.DISCUSSION...
MODERATE TO FRESH TRADE WINDS CONTINUE THIS MORNING WITH SHOWERS
RIDING ALONG WITH THE TRADES. SHOWERS ARE MAINLY AFFECTING WINDWARD
AREAS OF THE SMALLER ISLANDS BUT SOME ARE BREACHING LEEWARD
LOCATIONS AS WELL. BIG ISLAND WINDWARD SHOWERS HAVE INCREASED SLIGHTLY
OVERNIGHT BUT RADAR CONTINUES TO SHOW THE SMALLER ISLANDS GETTING
THE MAJORITY OF RAINFALL.

THE CULPRIT OF THE WET BLUSTERY TRADES IS A LINGERING SHEAR LINE
LEFT BEHIND BY A COLD FRONT EXITING TO THE NORTHEAST. ADDING TO THE MIX
IS AN UPPER LEVEL TROUGH...OR SHORTWAVE...THAT HAS MOVED OVER THE
WESTERN PORTION OF THE STATE. EVIDENCE OF THE SHORTWAVE CAN BE
SEEN IN THE LIHUE SOUNDING WHERE IS THE TEMPERATURE AT 500 MB
DECREASED BY 5 C OVER THE PAST 12 HOURS. THE SHORTWAVE...AND
EXPECTED UNSTABLE ATMOSPHERE...HAVE PROMPTED THE INCLUSION OF A
SLIGHT CHANCE OF THUNDERSTORMS AND HEAVY SHOWERS OVER MOST OF THE
ISLANDS THROUGH TODAY.

WEDNESDAY...TRADE WINDS AND SHOWERS WILL WEAKEN AS THE SURFACE
HIGH NORTH OF THE STATE...AND THE SHORTWAVE ALOFT...MOVE OFF TO
THE NORTHEAST.

THURSDAY AND INTO THE WEEKEND...THE MODELS ARE DEPICTING THE
TRADES TO DROP OFF AND SOUTHEAST WINDS TO PICK UP OVER ALL OF THE
ISLANDS. THE GFS...ECMWF...AND NOGAPS ALL SHOW AN UPPER LEVEL LOW
DROPPING DOWN FROM THE NORTH AND SETTLING NORTHWEST OF THE KAUAI
ON FRIDAY. THE UPPER LOW/S INFLUENCE REACHES THE SURFACE IN THE
FORM OF EITHER A TROUGH OR A SURFACE LOW ALSO LOCATED JUST
NORTHWEST OF THE STATE BY FRIDAY. THIS SCENARIO WILL BRING
INCREASED MOISTURE OVER THE ISLANDS FRIDAY AND THE ATMOSPHERE WILL
REMAIN MOIST THROUGH THE WEEKEND. THE LOCATION OF THE UPPER LOW
WILL BE KEY IN THE LIKELIHOOD OF HEAVY SHOWERS AND THUNDERSTORMS.
IF THE UPPER LOW IS CLOSE ENOUGH TO DESTABILIZE THE UPPER
ATMOSPHERE...AS THE GFS DEPICTS...THEN A WETTER SCENARIO WILL PLAY
OUT. HOWEVER...IF THE UPPER LOW MOVES FARTHER WEST OR EVEN TO THE
NORTHEAST...THEN A NOT SO WET SCENARIO WILL COME TO PASS. THE
PREVIOUS FORECAST HAD A CHANCE OF SHOWERS BROAD BRUSHED ACROSS THE
FORECAST AREA FROM FRIDAY THROUGH THE WEEKEND. HAVE ADDED IN THE
POSSIBILITY OF LOCALLY HEAVY RAINFALL OVER THE STATE FRIDAY AND
SATURDAY WITH MODELS COMING TO GENERAL CONSENSUS OF A WET END OF
THE WEEK FOR THE ALOHA STATE.

&&

.MARINE...
THE ROUGH SHORT PERIOD NORTHEAST SWELL CONTINUES TO BRING ADVISORY
LEVEL SURF TO THE EAST SHORES OF KAUAI AND OAHU...AND LOOKS TO REMAIN
ELEVATED THROUGH MUCH OF THE DAY TOMORROW.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST WEDNESDAY FOR KAUAI WINDWARD-
OAHU KOOLAU-OLOMANA.

SMALL CRAFT ADVISORY UNTIL 4 PM HST THIS AFTERNOON FOR KAUAI
CHANNEL-KAIWI CHANNEL-MAALAEA BAY-PAILOLO CHANNEL-ALENUIHAHA
CHANNEL.

SMALL CRAFT ADVISORY UNTIL 6 AM HST EARLY THIS MORNING FOR KAUAI
NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-OAHU
WINDWARD WATERS-OAHU LEEWARD WATERS-MAUI COUNTY WINDWARD WATERS-
MAUI COUNTY LEEWARD WATERS.

&&

MORRISON