Heavy Rain in Hawaii, September 15, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1027 AM HST SAT SEP 15 2007

.SYNOPSIS...
HIGH PRESSURE FAR NORTHEAST OF THE ISLANDS WILL MAINTAIN MODERATE
TO FRESH TRADE WINDS THROUGH NEXT WEEKEND. THROUGH TONIGHT
HOWEVER...WE CAN EXPECT LIGHTER WINDS AND MORE THAN THE USUAL AMOUNT
OF CLOUDS AND SHOWERS. WITH THE ATMOSPHERE MOIST AND SLIGHTLY
UNSTABLE...A FEW OF THE SHOWERS WILL BE BRIEFLY HEAVY. EXPECT A
RETURN TO MORE SETTLED TRADE WIND WEATHER BY SUNDAY...WHICH WILL
PERSIST INTO NEXT WEEK.

&&

.DISCUSSION...
A WEAK SURFACE TROUGH CONTINUES TO PASS THROUGH THE ISLANDS. QUIKSCAT
WINDS PLACE THE AXIS OF THE TROUGH OVER THE KAUAI CHANNEL WITH WINDS
OUT OF THE SOUTHEAST OVER MAUI AND OAHU WATERS AND TRANSITIONING
TO EAST NORTHEAST NEAR KAUAI. THE TROUGH HAS BROUGHT HIGHER
PRECIPITABLE WATER /PW/ VALUES TO THE BIG ISLAND AND MAUI AND OAHU
PW VALUES HAVE INCREASING STEADILY THIS MORNING. THE ADDED
MOISTURE...AND ALREADY PRESENT INSTABILITY FROM AN UPPER LEVEL LOW
TO THE NORTHWEST... HAS FORMED SOME HEAVIER SHOWERS OVER THE
KOOLAU MOUNTAINS. THE AHUIMANU GAGE ALONG THE KOOLAU RANGE
RECEIVED 0.91 INCHES IN 15 MINUTES...AND THE SAME CELL MOVED OVER
THE WAIHEE PUMP GAGE WHICH RECORDED 0.42 INCHES IN 15 MINUTES. THE
SHOWERS ARE MOVING ALONG AT A GOOD CLIP IN THE SOUTHEAST FLOW SO
SERIOUS FLOODING IS NOT EXPECTED AT THIS TIME...THOUGH MINOR
FLOODING IS POSSIBLE WITH THESE SHORT-LIVED HEAVY DOWNPOURS. HAVE
ADDED THE CHANCE OF HEAVY SHOWERS IN THE SHORT TERM FORECAST OVER
LOCATIONS ON OAHU.

THE UPPER LOW IS LOCATED NORTHWEST OF KAUAI IS PRODUCING
THUNDERSTORMS ALONG THE EASTERN QUADRANT OF THE CIRCULATION...OR
ABOUT 80 MILES NORTH OF KAUAI. THE LOW AND ITS ASSOCIATED
INSTABILITY WILL MOVE NORTHWEST AWAY FROM THE ISLANDS THROUGH THE
WEEKEND...AS THE MOISTURE ASSOCIATED WITH THE SURFACE TROUGH MOVES
WEST. EXPECT VERTICAL CLOUD GROWTH TO BE ENHANCED WHILE THE LOW/S
INFLUENCE IS BEING FELT...AND THEN DIMINISH BY SUNDAY. MID AND
UPPER LEVEL RIDGING WILL BUILD OVER THE ISLANDS IN ITS
WAKE...RETURNING OUR STABILIZING SUBSIDENCE INVERSION TO IT/S
NORMALLY LOWER HEIGHT.

THE SUBTROPICAL JET ASSOCIATED WITH THE UPPER LOW IS ADVECTING
CIRRUS BLOW OFF FROM THUNDERSTORMS FAR SOUTH OF THE ISLANDS
OVER THE BIG ISLAND AND MAUI COUNTY. THE CIRRUS IS APPROACHING THE
EAST COAST OF OAHU NOW AND WILL LIKELY COVER THE ISLAND BY THE
AFTERNOON. THE CIRRUS MAY PUT A CAP ON AFTERNOON CONVECTION IF IT
PROVIDES ENOUGH CLOUD COVER TO KEEP AFTERNOON TEMPERATURES DOWN.

THE MID AND LONG TERM FORECAST REMAINS THE SAME...
HIGH PRESSURE FAR TO OUR NORTHEAST WILL KEEP THE TRADES BLOWING
THROUGH NEXT WEEK. THE HIGH IS FORECAST TO STRENGTHEN AND MOVE
LITTLE AT THE START OF NEXT WEEK...RESULTING IN A SLIGHT BOOST IN
WIND SPEEDS...ESPECIALLY TUESDAY AND WEDNESDAY. BY LATE IN THE
WEEK...THE HIGH WILL WEAKEN AND TRADES WILL DECREASE.

&&

.MARINE...
THE HIGH SURF ADVISORY FOR SOUTH FACING SHORES HAS BEEN DROPPED
THIS MORNING...BUT SURF ALONG SOUTH FACING SHORES WILL REMAIN
ELEVATED THROUGH THE UPCOMING WEEK...FLIRTING WITH ADVISORY LEVELS
AGAIN ON SUNDAY AND WEDNESDAY. SMALL NORTHWEST SWELL IS EXPECTED
TO RUN THROUGH THE WEEKEND.

&&

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

&&

MORRISON