Heavy Rain in Hawaii, September 7 to 8, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
840 PM HST THU SEP 6 2012

.SYNOPSIS...
TRADE WINDS WILL CONTINUE TO DIMINISH...AND WILL BE RATHER LIGHT
FRIDAY AND SATURDAY...AS THE WINDS TURN TO THE EAST-SOUTHEAST. WHILE
BRIEFLY HEAVY SHOWERS WILL FAVOR WINDWARD AND MAUKA AREAS
OVERNIGHT...AN UPPER LEVEL LOW PASSING OVER KAUAI WILL BRING AN
INCREASING THREAT OF LOCALLY HEAVY DOWNPOURS ON FRIDAY...MAINLY TO
KAUAI AND OAHU. MORE TYPICAL...AND DRIER...TRADE WIND WEATHER IS
EXPECTED TO REDEVELOP SUNDAY AND MONDAY.

&&

.DISCUSSION...
THE ISLAND ATMOSPHERE APPEARS TO BE GETTING INCREASINGLY UNSTABLE
THIS EVENING...WITH RADAR RETURNS DEPICTING LOCALLY HEAVY SHOWERS
MOVING OVER THE ISLANDS FROM THE E IN A WEAKENING TRADE WIND
FLOW. SOME OF THE SHOWERS ARE DROPPING RAINFALL AT A RATE IN EXCESS
OF 1 INCH/HOUR...WITH AN ALARM FROM THE RAIN GAGE IN LANAI CITY
RECENTLY RECEIVED...DUE TO 0.29 INCHES FALLING IN 15 MINUTES. 24
HOUR SURFACE PRESSURE TRENDS CONTINUE TO THE NEGATIVE...DOWN ABOUT 2
MB...SIGNALING THE WEAKENING OF THE TRADE WIND SUPPORTING HIGH TO
THE NE. THE HIGH WILL WEAKEN FURTHER OVERNIGHT...AND THE LOCAL
PRESSURE GRADIENT WILL BE ERODED BY AN APPROACHING UPPER LEVEL LOW
CURRENTLY CENTERED ABOUT 400 MILES NW OF KAUAI. WILL BE UPDATING
PORTIONS OF THE OVERNIGHT FORECAST...MAINLY TO INCREASE POPS
WINDWARD...AND TO SOME DEGREE LEEWARD...TO ACCOUNT FOR LATEST RADAR
AND SAT TRENDS.

LATEST GUIDANCE DEPICTS THE UPPER LEVEL LOW WILL PASS OVER KAUAI
LATE FRIDAY...WITH INCREASED INSTABILITY NEAR ITS CORE PRIMARILY
AFFECTING KAUAI...AND TO A LESSER DEGREE ON OAHU. THIS FEATURE WILL
FURTHER WEAKEN THE TRADES AND CAUSE WINDS TO SHIFT OUT OF THE ESE
ACROSS THE SMALLER ISLANDS...ALLOWING AFTERNOON SEA BREEZES TO DRIVE
THE DEVELOPMENT OF CONVECTIVE BUILDUPS OVER INTERIOR AND LEEWARD
AREAS OF KAUAI AND OAHU. EVEN THOUGH LOW LEVEL MOISTURE DOES NOT
LOOK TO INCREASE SIGNIFICANTLY...THE INSTABILITY SHOULD BE
SUFFICIENT TO PRODUCE SOME LOCALLY HEAVY SHOWERS AND A SLIGHT CHANCE
OF THUNDERSTORMS NEAR KAUAI FRIDAY AFTERNOON AND NIGHT...AND
POSSIBLY ON SATURDAY. UNCERTAINTY IS GREATER NEAR OAHU...BUT WITH
INSTABILITY AND LOW LEVEL ESE FLOW DEVELOPING...THE CONCERN
INCREASES FOR TERRAIN ANCHORED HEAVY SHOWERS OVER THE KOOLAUS.

OUTSIDE CHANCE OF A BRIEF FUNNEL CLOUD/WATERSPOUT SIGHTING NEAR
DEVELOPING CLOUD LINES ON FRIDAY...BUT WILL WAIT TO SEE WHAT EARLY
MORNING SOUNDINGS SHOW BEFORE POSSIBLY ISSUING A MARINE WEATHER
STATEMENT OR SPECIAL WEATHER STATEMENT TO HIGHLIGHT THIS POTENTIAL
THREAT.

THE EXTENT OF DESTABILIZATION THAT THE LOW PROVIDES TO THE
ATMOSPHERE OVER THE E END OF THE STATE IS STILL IN QUESTION...BUT
MODERATE TO HEAVY SHOWERS CURRENTLY AFFECTING MANY AREAS OF MAUI
COUNTY HIGHLIGHTS THE FACT THAT THE TRADE WIND INVERSION IS ALREADY
WEAKENED AND ELEVATED. AFTERNOON CONVECTION OVER THE BIG ISLAND
SLOPES MAY BE ENHANCED BY THE PRESENCE OF THE LOW...BUT AT PRESENT
LITTLE IN THE WAY OF LOW LEVEL MOISTURE LIES UPSTREAM OF THE
WINDWARD COAST...ESPECIALLY WHEN COMPARED TO THE INCREASED MOISTURE
CURRENTLY STREAMING TOWARD THE OTHER ISLANDS FROM THE E.

TRADE WINDS WILL SHIFT BACK TO A MORE TYPICAL E DIRECTION AND
STABILITY WILL SLOWLY BUILD BACK AS THE UPPER LOW FADES AND LIFTS
NE ON SUNDAY. FORECAST MODELS HAVE BEEN HINTING AT AN INCREASE IN
LOW LEVEL MOISTURE WITHIN THE TRADE WIND FLOW ON SUNDAY...FOLLOWED
BY A DRIER SHOWER PATTERN AND INCREASING TRADE WINDS ON MONDAY THAT
WILL PERSIST INTO THE MIDDLE PART OF NEXT WEEK.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL AT THE TERMINALS OVERNIGHT...WITH BRIEF
MVFR CONDITIONS...DUE TO CLD AND SHRA...PRIMARILY LIMITED TO
WINDWARD TERRAIN. AN UPPER LEVEL LOW NEARING THE STATE FROM THE NW
WILL BRING INCREASING CHANCES OF OCCASIONAL MVFR/ISOL IFR CONDITIONS
NEAR KAUAI LATER FRIDAY...WITH A SLIGHT CHANCE OF TS. THE LOW ALOFT
WILL ALSO BRING AN INCREASING CHANCE OF MODERATE TURB TO THE MID AND
UPPER LEVELS OF THE ISLAND ATMOSPHERE THROUGH FRIDAY NIGHT...WITH
DIMINISHING TURB AS THE LOW WEAKENS AND KICKS OUT TO THE NE ON
SATURDAY.

&&

.MARINE...
WITH WINDS ON THE DECLINE...THE SMALL CRAFT ADVISORY /SCA/ WAS
CANCELLED EARLIER TODAY. ON FRIDAY TRADES WILL EASE DOWN TO GENTLE
TO MODERATE STRENGTH OVER ALL WATERS EXCEPT FOR THE ALENUIHAHA
CHANNEL AND WATERS NEAR SOUTH POINT...AND THE FLOW WILL VEER TO A
DIRECTION SLIGHTLY S OF DUE E NEAR THE SMALLER ISLANDS. TRADES WILL
GRADUALLY STRENGTHEN AND SHIFT BACK TOWARD A MORE TYPICAL DIRECTION
ON SUNDAY...AND THE SCA FLAG SHOULD BE FLYING AGAIN SUNDAY NIGHT OR
MONDAY.

NO BIG SWELLS ARE IN STORE. A SMALL NW SWELL IS EXPECTED TO FILL IN
ON FRIDAY AND DIMINISH SATURDAY...WITH LONG PERIOD FORERUNNERS IN
THE 16-19 SECOND RANGE ALREADY STARTING TO ARRIVE AT BUOY 51101 NW
OF KAUAI. DETAILS ON THE ISLAND SURF FORECAST CAN BE FOUND IN THE
SURF DISCUSSION /SRDHFO/.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

BIRCHARD