Heavy Rain in Hawaii, February 9 to 10, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
800 PM HST MON FEB 9 2009

.SYNOPSIS...
LIGHT TO MODERATE SOUTHEASTERLY FLOW WILL CONTINUE ACROSS THE AREA
THROUGH TUESDAY AS A WEAK TROUGH PASSES TO THE NORTH. TRADES ARE
EXPECTED TO INCREASE AND BECOME QUITE STRONG DURING THE SECOND HALF
OF THE WORK WEEK AND INTO THE WEEKEND AS A HIGH PRESSURE RIDGE TO
OUR NORTH STRENGTHENS.

&&

.DISCUSSION...
SOUTHEASTERLY FLOW ACROSS THE ISLANDS THIS EVENING IS BEING BLOCKED
BY THE BIG ISLAND RESULTING GENERALLY LIGHT WINDS FOR THE ISLANDS.
RADAR LOOPS SHOW A LARGE EDDY HAS DEVELOPED OVER THE CENTRAL
ISLANDS RESULTING IN WESTERLY FLOW ACROSS SOUTHERN OAHU AND
NORTHWEST FLOW SOUTH OF MOLOKAI AND LANAI. THE MOST NOTABLE SHOWER
ACTIVITY IS OVER THE WINDWARD COAST OF OAHU WHERE A LINE OF SHOWERS
DEVELOPED JUST OFFSHORE AND PUSHED SLOWLY ONSHORE DROPPING LOCALLY
HEAVY RAINFALL. ALSO SHOWING UP ON THE EDGE OF RADAR DATA IS THE
STRONGER CONVECTION ASSOCIATED WITH THE WEAK SURFACE TROUGH NORTH OF
THE ISLANDS. EARLIER CONVECTION OVER THE BIG ISLAND SUMMITS HAS
DIMINISHED WITH THE SUNSET. THE WEATHER PATTERN WILL CHANGE LITTLE
OVERNIGHT WITH LIGHT WINDS EXPECTED.  THE MAIN THREAT OF SHOWERS
WILL BE IN THE CONVERGENCE AREAS DOWNSTREAM OF THE ISLANDS THAT
THEN COULD MOVE OVER THE SMALLER ISLANDS.

THE WEAK TROUGH IS PROJECTED TO HANG JUST NORTH OF THE STATE THROUGH
TUESDAY AND THIS WILL MEAN SIMILAR LIGHT WINDS FOR THE
ISLANDS...WITH AFTERNOON SEABREEZES AND INTERIOR SHOWERS.
INSTABILITY THROUGH THE MID LEVELS WILL BE MARGINAL...SO
THUNDERSTORMS ARE NOT EXPECTED.

ON THE WEDNESDAY...THE GFS WEAKENS THE TROUGH AND PUSHES IT AWAY
FROM THE ISLANDS ALLOWING TRADES TO RETURN. AT THE SAME TIME...AN
UPPER LEVEL TROUGH IS EXPECTED TO BREAK OFF FROM THE WESTERLIES AND
PUSH SOUTH ACROSS THE ISLANDS BRINGING COLDER AIR ALOFT AND
INCREASED INSTABILITY. ON THURSDAY THE SURFACE RIDGE TO THE NORTH
STRENGTHENS AND THIS TREND CONTINUES INTO FRIDAY AND SATURDAY. THE
HIGH WILL ALSO BE PUSHING SOUTH...WHICH INCREASES THE PRESSURE
GRADIENT AND BRINGS STRONG TRADE WINDS TO THE ISLANDS. THE VARIOUS
MODELS HAVE BEEN CONSISTENT WITH STRENGTH OF THE HIGH FOR THE END OF
THE WEEK...STRONG ENOUGH FOR 25 KNOT SURFACE WINDS ACROSS THE
ISLANDS...WHICH MEANS MUCH STRONGER WINDS IN FAVORED LOCATIONS SUCH
AS CHANNELS AND HEADLANDS. THESE STRONG WINDS CONTINUE THROUGH THE
WEEKEND.

INITIALLY ON WEDNESDAY AND THURSDAY THE TRADES WILL BE WET WITH THE
UPPER TROUGH OVERHEAD AND PLENTY OF MOISTURE. THIS WILL BRING
SHOWERY CONDITIONS TO WINDWARD AND MAUKA LOCATIONS WEDNESDAY AND
THURSDAY. UPPER RIDGING AND DRYING IS EXPECTED BY THE END OF THE
WEEK WILL BRING LESS THAN NORMAL TRADE SHOWERS FOR THE WEEKEND.

&&

.MARINE...
STRONG TRADE WINDS EXPECTED ACROSS THE AREA TOWARDS THE END OF THE
WEEK WILL BRING LARGE SWELL AND ROUGH SURF...POTENTIALLY WELL ABOVE
THE 8-FOOT ADVISORY LEVEL...TO EAST-FACING SHORES. WINDS MAY REACH
GALE FORCE IN THE WINDIER AREAS ON FRIDAY AND SATURDAY.

&&

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

BRENCHLEY