Heavy Rain in Hawaii, September 12, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST FRI SEP 12 2008

.SYNOPSIS...
A WEAK RIDGE JUST NORTH OF THE MAIN HAWAIIAN ISLANDS WILL MOVE
SOUTHEAST OVER THE ISLANDS THROUGH THE WEEKEND. LIGHT EAST TO
SOUTHEAST WINDS TODAY WILL BECOME VARIABLE DURING THE WEEKEND.
SCATTERED SHOWERS WITH LOCAL DOWNPOURS ARE EXPECTED...MAINLY DURING
THE AFTERNOONS AND EVENINGS THROUGH THE WEEKEND. VOG MAY ALSO
BECOME MORE WIDESPREAD OVER THE WEEKEND DUE TO THE LIGHT AND
VARIABLE WINDS. HIGH PRESSURE WILL BUILD NORTH OF THE ISLANDS EARLY
NEXT WEEK BRINGING A GRADUAL RETURN OF TRADE-WIND WEATHER.

&&

.DISCUSSION...
SHOWERS AND CLOUDS HAVE DISSIPATED OVER THE ISLANDS THIS MORNING. A
VERY LIGHT SOUTHEAST FLOW AT LOW LEVELS IS PRESENT WITH A WEAKNESS
IN THE SURFACE RIDGE TO THE NORTH. MORNING SOUNDINGS SHOW EVIDENCE
OF A MID LEVEL LOW JUST SOUTH OF OAHU WITH MODERATE SOUTHWEST FLOW
AT HILO ABOVE 800 MB AND LIGHT EASTERLY FLOW AT LIHUE. THIS
TROUGHINESS CONTINUES ON UP TO THE UPPER LEVELS WITH THE 500MB
TROUGH AXIS NEAR KAUAI. EVEN THOUGH THE CLOUDS HAVE SHOWN A
DECREASING TREND...PLENTY OF MOISTURE IS PRESENT IN THE ISLANDS
TODAY. WITH LIGHT STEERING FLOW AND THE DEEPER MOIST MIXED LAYER
ADDS UP TO AN AFTERNOON WITH AN ELEVATED THREAT OF HEAVY...SLOW
MOVING SHOWERS. THE FOCUS OF THE SHOWERS WILL BE HIGHER TERRAIN
INITIALLY...THEN MOVING OFF INTO THE COASTAL ZONES LATER IN THE DAY
AS SEABREEZE CONVERGENCE INCREASES. ALSO WILL WATCH FOR PERSISTENT
SHOWERS IN DOWNSTREAM CONVERGENCE ZONES TO BE DIRECTED AT THE
DOWNSTREAM ISLANDS DUE TO THE SOUTHEAST FLOW.  OAHU AND KAUAI WILL
HAVE THE BIGGEST THREAT FOR HEAVIER SHOWERS LATER TODAY AS THEY ARE
ALSO CLOSER TO THE TROUGH ALOFT.

THE OTHER ISSUE FOR TODAY...AND INTO THE WEEKEND WILL BE VOG FROM
KILAUEA. MORNING VIS IMAGERY SHOWS SOME OF THE UPPER PART OF THE
PLUME CAUGHT UP IN THE SOUTHWEST FLOW ABOVE 7000 FEET. SURFACE FLOW
REMAINS NORTHERLY THIS MORNING AND PUSHING THE BULK OF THE EMISSIONS
OFFSHORE...BUT EXPECT THAT EASTERLY OR SOUTHEASTERLY SEABREEZES WILL
DEVELOP BY THIS AFTERNOON. EITHER WAY...THE LIGHT WINDS MAY LEAD TO
INCREASED HAZE AND VOG IN ALL OF THE ISLANDS. LIGHT DIURNAL WIND
CIRCULATIONS AT THE VOLCANO WILL CAUSE SO2 EMISSIONS TO BUILD UP
ALONG THE UPPER SLOPES...AND NIGHTTIME LAND BREEZES MAY BRING VOG
BACK DOWN TO COASTAL AREAS INCLUDING HILO TOWN.

THE 12Z GFS INDICATES NOT MUCH CHANGE TO THE OVERALL PATTERN THROUGH
THE WEEKEND AS A WEAK FRONT STALLS NORTH OF THE ISLANDS AND A WEAK
RIDGE ACROSS THE ISLANDS. IF ANYTHING...THE LARGE SCALE SURFACE FLOW
BECOMES EVEN WEAKER AND MORE CONFUSED...SO THAT SEA-LAND BREEZE
CIRCULATIONS DOMINATE THE LOCAL SCALE. THE MODEL STILL LIFTS THE
LINGERING TROUGH ALOFT TO THE NORTHEAST EARLY NEXT WEEK WITH RIDGING
ALOFT DEVELOPING IN ITS PLACE. THE SUBTROPICAL SURFACE HIGH BUILDS
BACK NORTH OF THE STATE NEXT WEEK BRINGING TRADES BACK TO THE STATE
STARTING MONDAY. THE LATEST FORECAST DATA NOW CUTS OFF THE NEXT
TROUGH IN THE WESTERLIES AROUND THE DATELINE AND 30N FOR THE END OF
NEXT WEEK...WHICH KEEPS THE RIDGE IN POSITION TO THE NORTH OF THE
STATE THROUGH THE END OF THE WEEK FOR LIGHT TO MODERATE TRADES. THE
12Z EC IS IN GOOD AGREEMENT WITH THIS GFS SOLUTION THROUGH NEXT WEEK.

&&

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

&&

BRENCHLEY