Flash Flooding and Heavy Rain in Hawaii, November 4 to 5, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT NOV 3 2007

.SYNOPSIS...
A LOW 500 MILES NORTH NORTHWEST OF KAUAI IS DROPPING SOUTH. THE
APPROACHING LOW WILL PRODUCE UNSETTLED WEATHER WITH A CHANCE OF
HEAVY RAINFALL THROUGH EARLY NEXT WEEK. DRIER...TRADE-WIND WEATHER
WILL RETURN LATER NEXT WEEK.

&&

.DISCUSSION...
RADAR AND SATELLITE IMAGERY SHOW A NORTHEAST TO SOUTHWEST LINE OF
ACTIVE THUNDERSTORMS IN THE KAUAI CHANNEL MOVING EAST SLOWLY. IN THE
PAST HOUR A NEW EAST TO WEST LINE OF SHOWERS HAS FORMED SOUTH OF
OAHU AND IS MOVING NORTH SLOWLY. ELSEWHERE AROUND THE ISLANDS IT IS
CLOUDY...BUT MOSTLY DRY.

AT 800 PM A LOW WAS CENTERED 500 MILES NORTH NORTHWEST OF KAUAI.
OVER THE PAST SEVERAL HOURS THE LOW HAS BEEN MOVING SOUTH AT 17 MPH.
A FRONTAL CLOUD BAND CURVES AROUND THE SOUTHEAST SIDE OF THE LOW TO
A POINT 100 MILES NORTHWEST OF KAUAI AND EXTENDS WEST SOUTHWEST FROM
THAT POINT. OVER THE PAST SEVERAL HOURS THE LOW HAS MOVED SOUTHEAST
AT AROUND 5 MPH. THE LOW LEVEL FLOW NEAR THE ISLANDS HAS SHIFTED
RAPIDLY AHEAD OF THE APPROACHING LOW AND FRONT. EAST SOUTHEAST FLOW
NEAR THE BIG ISLAND VEERS TO SOUTH NEAR OAHU AND KAUAI.

RECENT RUNS OF THE GFS MODEL SHOW THE LOW CONTINUING TO DROP
SOUTH...REACHING A POINT ABOUT 180 MILES WEST OF KAUAI TOMORROW
NIGHT. MEANWHILE...THE FRONTAL BAND IS FORECAST TO WEAKEN AND A
BROAD NORTHEAST TO SOUTHWEST BAND OF CLOUDS AND SHOWERS WILL DEVELOP
OVER KAUAI AND OAHU TONIGHT AND MOVE SOUTHEAST ACROSS THE ISLANDS
REACHING THE BIG ISLAND BY MONDAY. THIS SCENARIO LOOKS PRETTY
REASONABLE. A THREAT OF HEAVY RAIN CONTINUES FOR ALL ISLANDS AND A
FLASH FLOOD WATCH WILL REMAIN IN EFFECT. THE MOST ACTIVE WEATHER IS
LIKELY TO BE CONCENTRATED IN NARROW LINES WITHIN THE BROADER CLOUD
BAND. FLASH FLOODING REMAINS THE GREATEST THREAT...BUT WE WILL ALSO
BE LOOKING AT THE POSSIBILITY OF SEVERE THUNDERSTORMS AND FOR SNOW
OR FREEZING PRECIPITATION AND STRONG WINDS ON THE SUMMITS OF MAUNA
KEA AND MAUNA LOA.

THE COMPUTER MODELS OFTEN HAVE TROUBLE WITH CUT-OFF LOWS LIKE THE
ONE AFFECTING THE ISLANDS. THE MODELS ARE IN PRETTY GOOD AGREEMENT
OVER THE NEXT 48 HOURS BUT BEGIN TO DIVERGE AFTER THAT. FOR NOW...
WE WILL FOLLOW THE GFS MODEL SHOWING THE LOW WEAKENING AND MOVING
AWAY TO THE WEST SOUTHWEST EARLY NEXT WEEK. THE LOW MAY REMAIN CLOSE
ENOUGH TO MAINTAIN A THREAT OF HEAVY RAIN THROUGH TUESDAY NIGHT. BY
WEDNESDAY THE ATMOSPHERE WILL BE GETTING MORE STABLE AND TRADE WINDS
WILL BE RETURNING.

&&

.MARINE...
WE HAVE ISSUED A MARINE WEATHER STATEMENT FOR THE THUNDERSTORMS IN
THE KAUAI CHANNEL. WE WILL BE WATCHING THESE STORMS IN CASE SPECIAL
MARINE WARNINGS ARE NEEDED. A LARGE SOUTH SWELL HAS BEEN GENERATED
BY A PERSISTENT FETCH NEAR NEW ZEALAND. WE WILL BE WATCHING THIS
SWELL CLOSELY TO SEE IF THE HIGH SURF ADVISORY NEEDS TO BE UPGRADED
TO A WARNING. IF THE SWELL IS BIG ENOUGH TO REQUIRE A WARNING...WE
MAY ALSO NEED TO ISSUE A MARINE WEATHER STATEMENT FOR HARBOR SURGES
AND WAVES BREAKING IN CHANNEL ENTRANCES.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

FLASH FLOOD WATCH THROUGH LATE TUESDAY NIGHT FOR ALL ISLANDS.

HIGH SURF ADVISORY FOR SOUTH-FACING SHORES.

&&

DONALDSON