Heavy Rain in Hawaii, September 3, 2002

HAWAIIAN ISLANDS SYNOPTIC DISCUSSION AND GUIDANCE
NATIONAL WEATHER SERVICE HONOLULU HI
4 PM HST TUE SEP 3 2002

SUMMARY...A WEAK SURFACE TROUGH WILL MOVE WEST OF HAWAII ALLOWING
MODERATE TRADE WINDS TO GRADUALLY RETURN WEDNESDAY. ISOLATED SHOWERS
AND THUNDERSTORMS WILL END THIS EVENING...BUT BRIEF TRADE SHOWERS
WILL CONTINUE THROUGH THE REST OF THIS WEEK.

DISCUSSION...ISOLATED HEAVY SHOWERS FORMED OVER ALL ISLANDS WITH
AFTERNOON HEATING. A FEW THUNDERSTORMS ALSO FORMED OVER WEST OAHU
AND THE WEST MAUI MOUNTAINS. MOST OF THE PRECIPITATION FELL ON THE
WEST SIDES OF THE ISLANDS...AS THE TRADE WINDS WERE JUST STRONG
ENOUGH TO PUSH THE CONVECTION OTHER THERE. THE RAIN WILL END AFTER
SUNSET AND AS THE TEMPERATURES LOWER. THE POSSIBILITY OF FLOODING
HAS DIMINISHED ON THE BIG ISLAND...AND ALTHOUGH THERE WILL BE SOME
LATE AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS...THE FLASH
FLOOD WATCH IS NO LONGER NEEDED.

TYPHOON ELE...THOUGH WEST OF THE DATELINE...IS STILL INDIRECTLY
AFFECTING ISLAND WEATHER. AS IS USUALLY THE CASE IN FORECASTING
WEATHER...ONE HAS TO SEARCH THROUGH A NUMBER OF CAUSES AND EFFECTS
TO GET AT THE REASON FOR ONES WEATHER. IN THIS CASE...THE CYCLONIC
ELE PLACES AN ANTICYCLONIC RIDGE MORE OR LESS ALONG 170W...WHICH IN
TURN PLACES AN UPPER LEVEL TROUGH OVER THE STATE. THE TROUGH WITH
EMBEDDED LOW SEVERAL HUNDRED MILES SOUTHWEST OF KAUAI HAS WEAKENED
THE PRESSURE GRADIENT ACROSS THE STATE. THE FLATTENED GRADIENT SOUTH
OF A SURFACE HIGH FAR TO THE NORTH IS IN TURN THE REASON FOR THE
LIGHT WINDS.

A SURFACE HIGH FARTHER NORTH THAN NORMAL TRANSLATES TO A WEAKER AND
HIGHER THAN NORMAL SUBSIDENCE INVERSION. OR LOOKING AT IT ANOTHER
WAY...A DEEPER THAN NORMAL MARINE LAYER. COOL TEMPERATURES
ASSOCIATED WITH THE UPPER TROUGH HAVE STEEPENED THE LAPSE RATE
RESULTING IN AN UNSTABLE ATMOSPHERE. FINALLY...WE HAVE A SLUG OF
MOISTURE MOVING SLOWLY IN ON THE LIGHT TRADE WINDS.

ALL THE NECESSARY INGREDIENTS WILL BE PRESENT THIS EVENING...THOUGH
NOT TO A GREAT EXTENT. THE WINDS WILL NOT BE VERY LIGHT...AND
MOISTURE WILL BE LIMITED. THE TEMPERATURES ALOFT ARE NOT QUITE COLD
ENOUGH...AND THE ATMOSPHERE IS NOT AS UNSTABLE AS IT CAN GET. THE
INGREDIENTS ARE...NEVERTHELESS...SUFFICIENT TO WARRANT THE MENTION
OF HEAVY SHOWERS AND THUNDERSTORMS IN THE FORECAST FOR ALL ISLANDS
THIS EVENING.

PROGS SHARPEN THE UPPER TROUGH AND MOVE IT FAR ENOUGH TO THE WEST TO
ALLOW THE TIGHTENING OF THE PRESSURE GRADIENT AND THE GRADUAL RETURN
OF MODERATE TRADES WEDNESDAY. ALSO...THE HIGH FAR TO THE NORTH IS
FORECAST TO MOVE CLOSER...WHICH WILL AID IN THE STRENGTHENING OF THE
WINDS WEDNESDAY. CRAIG.

.HFO...WARNINGS...ADVISORIES...NONE
.      WATCHES...FLASH FLOOD BIG ISLAND CANCELLED AT 330 PM HST