Drought in Hawaii, May 1 to 31, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
840 PM HST FRI APR 30 2010

.SYNOPSIS...
AN APPROACHING COLD FRONT WILL CAUSE WINDS TO SWING TOWARD THE SOUTH
AND SOUTHEAST THROUGH SUNDAY. THE FRONT IS EXPECTED TO REACH KAUAI
EARLY SUNDAY...THEN MOVE TO OAHU DURING THE DAY ON SUNDAY...BEFORE
STALLING AND GRADUALLY WEAKENING THROUGH MONDAY. THE FRONT IS
EXPECTED TO BRING HEAVY RAIN AND THE THREAT OF THUNDERSTORMS TO
KAUAI AND OAHU...WHERE A FLASH FLOOD WATCH IS IN EFFECT FOR SATURDAY
NIGHT AND SUNDAY. VOG MAY INCREASE ACROSS THE SMALLER ISLANDS UNTIL
RETURNING TRADES PUSH IT AWAY EARLY NEXT WEEK.

&&

.DISCUSSION...
INITIAL LOOK AT 00Z MODEL GUIDANCE OFFERS LITTLE REASON TO CHANGE
THE WEEKEND FORECAST PHILOSOPHY. THE SHORT TERM FORECAST LOOKS TO BE
IN LINE WITH THE LATEST RADAR AND SATELLITE OBSERVATIONS...WHICH
SHOWS THE SE FLOW AT CLOUD LEVEL CARRYING CLOUDS AND SHOWERS TOWARD
THE NW. RADAR SHOWS SHOWERS ARE MOST ACTIVE OVER WINDWARD OAHU...
LANAI...THE WEST MAUI MOUNTAINS AND LEEWARD MOLOKAI...WITH THESE
SHOWERS PEELING OFF THE ISLANDS TOWARD THE NW. SHOWERS EMANATING
FROM MOLOKAI THEREFORE ARE MOVING OVER SE PORTIONS OF OAHU...WHERE
CONDITIONS WILL BE A LITTLE DAMPER THAN USUAL THIS EVENING. LEEWARD
SHOWERS ARE EXPECTED TO DIMINISH AS THE EVENING PROGRESSES...BUT
MOISTURE ASSOCIATED WITH THE PERSISTENT SHOWER BAND OVER WINDWARD
WATERS IS FORECAST TO MOVE ASHORE LATER TONIGHT AND EARLY SATURDAY.
THIS MOISTURE IS FORECAST TO LINGER IN THE AREA THROUGH THE
WEEKEND...AND MAY HELP TO FUEL THE HEAVY RAINFALL ANTICIPATED LATER
THIS WEEKEND. ONE THOUGHT IS THAT THE FLASH FLOOD WATCH MAY NEED TO
BE EXPANDED EASTWARD TO INCLUDE MAUI COUNTY...BUT WILL WAIT UNTIL
THE MORNING BEFORE MAKING THAT DETERMINATION.

&&

.PREV DISCUSSION ISSUED 400 PM...
A 1035 MB HIGH NEAR 36N143W...FAR NORTHEAST OF THE MAIN HAWAIIAN
ISLANDS...CONTINUES TO INDUCE WINDS AT OR NEAR SMALL CRAFT ADVISORY
THRESHOLD ACROSS WATERS SOUTH OF THE BIG ISLAND THIS AFTERNOON. AT
THE SAME TIME...LOW LEVEL WINDS ARE VEERING SLOWLY TO
SOUTHEASTERLIES IN THE FACE OF A COLD FRONT SLOWLY APPROACHING FROM
THE NORTHWEST. THESE SOUTHEASTERLY WINDS ARE BLOCKED BY THE BIG
ISLAND...CREATING AN INTERESTING SITUATION WHERE ISLAND WINDS ARE
LIGHT BUT WINDS OFFSHORE CONTINUE TO BE STRONG IN LOCALIZED AREAS.
WINDS HAVE VEERED ENOUGH THIS AFTERNOON TO ALLOW US TO DROP THE
SMALL CRAFT ADVISORY FOR THE ALENUIHAHA CHANNEL....BUT THIS ADVISORY
WILL REMAIN IN EFFECT FOR WATERS SOUTH OF THE BIG ISLAND THROUGH
TONIGHT. WHILE UNSTABLE WEATHER IS IN STORE FOR THE ISLANDS THIS
WEEKEND...ALL REMAINS PEACEFUL THIS AFTERNOON. LATEST SOUNDINGS SHOW
A LINGERING TRADE INVERSION NEAR 7100 FEET...WITH 1.1 TO 1.2 INCHES
OF PRECIPITABLE WATER AVAILABLE FOR SHOWERS. THE HILO SOUNDING SHOWS
THE AIRMASS REMAINS MORE STABLE TOWARDS THE EASTERN END OF THE CHAIN
WITH POSITIVE LIFTED INDEX VALUES. SATELLITE LOOP SHOWS A BAND OF
BROKEN TO OVERCAST SHOWERY LOW CLOUDS LIES JUST NORTHEAST OF AND
PARALLEL TO THE ISLAND CHAIN...WITH CLOUD ELEMENTS ALONG SOUTHERN
PORTIONS OF THE BAND BREAKING AWAY AND AFFECTING WINDWARD AREAS OF
THE ISLANDS THIS AFTERNOON. RADAR SHOWS THESE CLOUDS ARE PRODUCING
ONLY ISOLATED TO SCATTERED LIGHT SHOWERS.

THANKS TO THE DECREASING AND VEERING WINDS...WE WILL SEE A MIX OF A
TRADE WIND DISTRIBUTION OF CLOUDS AND SHOWERS OVERNIGHT AND SATURDAY
MORNING...FOLLOWED BY A MILDLY CONVECTIVE DISTRIBUTION OF CLOUDS AND
SHOWERS SHIFTED LEEWARD SATURDAY AFTERNOON. OUR TRADE INVERSION WILL
ACT TO CAP CONVECTION AND LIMIT RAINFALL TONIGHT THROUGH SATURDAY
AFTERNOON.

A COLD FRONT ABOUT 630 MILES NORTHWEST OF THE MAIN HAWAIIAN
ISLANDS...ALONG WITH ITS SUPPORTING UPPER TROUGH...WILL BE OUR MAJOR
WEATHER DRIVER THIS WEEKEND. MODELS ARE IN GENERAL AGREEMENT ABOUT
SUNDAY AS THE DAY OF PEAK INSTABILITY...WITH MINOR DIFFERENCES IN
THE LOCATION OF THE UPPER TROUGH AND EVENTUAL CUTOFF LOW SUPPORTING
THE APPROACHING FRONT. WITH MODELS SHOWING THE UPPER TROUGH
APPROACHING FROM THE NORTHWEST...AND EXITING TOWARD THE NORTH TO
NORTHEAST...WE FEEL THE THREAT OF HEAVY RAINFALL WILL BE GREATEST
OVER KAUAI AND OAHU...AND LESS SO FOR MAUI COUNTY AND THE BIG
ISLAND. LOCALIZED HEAVY SHOWERS ARE POSSIBLE ON KAUAI AND OAHU LATE
SATURDAY NIGHT THROUGH SUNDAY EVENING. FURTHERMORE...WE HAVE NOW
INCLUDED A SLIGHT CHANCE OF THUNDERSTORMS FOR KAUAI...NIIHAU...OAHU
AND ADJACENT WATERS FOR THE SAME TIME FRAME...BASED ON LOCAL
MODEL-DERIVED THUNDERSTORM TOOLS. WE ALSO HAVE ISSUED A FLASH FLOOD
WATCH FOR KAUAI AND OAHU...EFFECTIVE LATE SATURDAY NIGHT THROUGH
SUNDAY EVENING. MODELS VARY SLIGHTLY ON FRONTAL ARRIVAL...WITH GFS
SHOWING THE FRONT REACHING KAUAI SUNDAY MORNING. OTHER MODELS DELAY
ARRIVAL UNTIL EARLY AFTERNOON. ALL AGREE ON FRONTAL STALLING NEAR
OAHU...THEN WASH OUT BY MONDAY EVENING.

THE UPPER TROUGH WILL LIFT RAPIDLY TO THE NORTHEAST ON MONDAY. THIS
WILL HERALD A RETURN TO TRADE WIND WEATHER BY TUESDAY. BUILDING
TRADE WINDS ARE EXPECTED TO STICK AROUND THROUGH THE REMAINDER OF
THE FORECAST PERIOD AND...WITH AN UPPER RIDGE DEVELOPING DIRECTLY
OVERHEAD...THE TRADES WILL BRING LIMITED RAINFALL TO WINDWARD
SLOPES.

&&

.MARINE...
WINDS TO THE SOUTH OF THE BIG ISLAND CONTINUE TO BE STRONG ENOUGH TO
CONTINUE A SMALL CRAFT ADVISORY /SCA/ THROUGH TONIGHT. THE SCA WILL
BE REISSUED FOR THE ALENUIHAHA CHANNEL WITH THE EVENING MARINE
PACKAGE...BASED PRIMARILY ON OBSERVATIONS FROM KAHOOLAWE...WHICH
INDICATE E WINDS CONTINUE TO ACCELERATE AROUND UPOLU POINT AND
THROUGH THE CHANNEL. EXPECT WINDS TO DIMINISH ON SATURDAY AS THE
SURFACE RIDGE NEARS THE ISLANDS IN ADVANCE OF THE APPROACHING FRONT.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM LATE SATURDAY NIGHT THROUGH SUNDAY EVENING
FOR NIIHAU KAUAI AND OAHU.

SMALL CRAFT ADVISORY UNTIL 4 AM HST SATURDAY FOR BIG ISLAND LEEWARD
WATERS-BIG ISLAND SOUTHEAST WATERS-ALENUIHAHA CHANNEL.

&&

DISCUSSION/MARINE...BIRCHARD
PREV DISCUSSION...POWELL