Heavy Rain in Hawaii, April 29, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST FRI APR 29 2011

.SYNOPSIS...
LIGHT TO MODERATE TRADE WINDS WILL BRING WINDWARD SHOWERS...MAINLY
AT NIGHT AND IN THE MORNING. WITH THE LIGHT TRADE WINDS...LOCAL
AFTERNOON SEA BREEZES WILL DEVELOP...CAUSING CLOUDS AND A FEW
SHOWERS TO DEVELOP OVER LEEWARD AND INTERIOR AREAS. A NEARBY UPPER
LEVEL TROUGH WILL PROVIDE SOME INSTABILITY...WHICH MEANS THAT SOME
OF THE SHOWERS MAY BE BRIEFLY HEAVY...ESPECIALLY OVER THE BIG ISLAND
SLOPES. THE TRADES WILL INCREASE LATER NEXT WEEK AS HIGH PRESSURE
BUILDS TO THE NORTH.

&&

.DISCUSSION...
LOW LEVEL PLAYERS REMAIN UNCHANGED...WITH A 1035 MB HIGH FAR TO THE
NORTHEAST...AND A TROUGH SEVERAL HUNDRED MILES TO THE NORTH AND
NORTHWEST. THE TROUGH IS KEEPING THE PRESSURE GRADIENT NEAR THE
ISLANDS FLATTER THAN IT WOULD NORMALLY BE SOUTHWEST OF SUCH A LARGE
HIGH. WITH LIGHT TO MODERATE TRADE WINDS IN THE FLAT GRADIENT...
LOCAL LAND BREEZES SET UP OVER OVERNIGHT AND EARLY THIS MORNING...
AND AFTERNOON SEA BREEZES ARE EXPECTED THIS AFTERNOON.

ALOFT...A NORTHEAST TO SOUTHWEST TROUGH PERSISTS NEAR KAUAI. LAST
NIGHT/S BALLOON SOUNDING OFF OF LIHUE SHOWS A COLD POOL ALOFT
ASSOCIATED WITH THE TROUGH...WITH 500 MB TEMPERATURES MEASURED AT
-10C. THE COOL TEMPERATURES ALOFT ARE PROVIDING AN UNSTABLE LAYER
ABOVE 10 THOUSAND FEET...BUT THE LOW LEVEL HIGH IS PROVIDING A
TEMPERATURE INVERSION NEAR 8 THOUSAND FEET THAT IS PREVENTING MOST
SURFACE PARCELS FROM TAPPING INTO THE INSTABILITY ALOFT. IN OTHER
WORDS...THE SOMEWHAT UNSTABLE ATMOSPHERE MAY SET OFF A FEW HEAVY
SHOWERS DURING THE NEXT FEW AFTERNOONS...IF THEY CAN JUST PUNCH
THROUGH THE CAPPING INVERSION. THE MOST LIKELY PLACE WHERE THIS
COULD OCCUR IS ON THE BIG ISLAND SLOPES.

A NEW HIGH WILL MOVE QUICKLY FROM WEST TO EAST WELL NORTH OF THE
ISLANDS THIS WEEKEND...BUT A PERSISTENT LOW LEVEL TROUGH TO OUR
NORTH...THE LOW LEVEL REFLECTION OF THE UPPER TROUGH...WILL KEEP
TRADE WINDS ON THE LIGHT SIDE THROUGH THE WEEKEND. THE LIGHT
PREVAILING WINDS WILL BRING SHOWERS TO WINDWARD AND MAUKA AREAS
MAINLY AT NIGHT AND IN THE MORNING. ALSO...LOCAL SEA BREEZES WILL
LEAD TO SOME CLOUDS AND SHOWERS OVER INTERIOR AND LEEWARD AREAS
DURING AFTERNOON AND EARLY EVENING HOURS.

THE UPPER TROUGH AND THE COOL TEMPERATURES ALOFT ARE FORECAST TO
PERSIST WELL INTO NEXT WEEK. SO...THE FLAT LOW LEVEL PRESSURE
GRADIENT AND THE LIGHT PREVAILING TRADES...ALONG WITH LOCAL LAND AND
SEA BREEZES...WILL BE AROUND FOR AWHILE. ALSO...SOME OF THE SHOWERS
MAY BE HEAVY WHERE LOW LEVEL FORCING IS GREATEST. SUCH PLACES ARE
OVER LEEWARD WATERS WHERE MOISTURE CONVERGES IN ISLAND WAKE
PLUMES...AND OVER LEEWARD AND INTERIOR PORTIONS OF THE ISLANDS
WHERE AFTERNOON SEA BREEZES CONVERGE.

THE TROUGH ALOFT MAY NOT BE STRONG ENOUGH AND TEMPERATURES ALOFT MAY
NOT BE COLD ENOUGH TO SET OFF HEAVY SHOWERS THIS AFTERNOON AND
SATURDAY AFTERNOON...BUT A SLIGHT CHANCE OF AFTERNOON THUNDERSTORMS
IS FORECAST FOR BIG ISLAND LEEWARD SLOPES...INCLUDING THE INTERIOR
AND SUMMIT ZONES. A SHORTWAVE TROUGH ALOFT DIGGING INTO THE PRESENT
LONG WAVE TROUGH WILL LIKELY DROP 500 MB TEMPERATURE TO AROUND
-12C...WHICH IN TURN MAY PROVIDE ENOUGH INSTABILITY FOR CONVECTIVE
BUILDUPS TO GAIN SOME VIGOR SUNDAY AND MONDAY.

THE TROUGH ALOFT IS FORECAST TO STAY NEAR OR JUST EAST OF THE
ISLANDS THROUGH NEXT WEEK...AND HIGH PRESSURE IS FORECAST TO BUILD
NORTH OF THE ISLANDS BY THE MIDDLE OF THE WEEK. THIS WILL RESULT IN
STRONGER TRADES...BUT A CONTINUED POSSIBILITY OF HEAVY DOWNPOURS...
MOST NOTABLY OVER LEEWARD BIG ISLAND SLOPES.

&&

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

&&

CRAIG