Heavy Rain in Hawaii, August 25, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1005 AM HST SAT AUG 25 2007

.SYNOPSIS...
AN UPPER LOW ABOUT 250 MILES NORTH OF HONOLULU WILL CONTINUE MOVING
WESTWARD TODAY AND TONIGHT. UNSTABLE AIR AND LIGHT WINDS WILL GIVE
RISE TO LOCALLY HEAVY AFTERNOON SHOWERS OR THUNDERSHOWERS OVER THE
INTERIOR OF THE ISLANDS. THE AIRMASS WILL START TO BECOME MORE
STABLE ON SUNDAY AS THE UPPER LOW MOVES AWAY...AND MORE SETTLED
WEATHER IS EXPECTED NEXT WEEK AS LIGHT TO MODERATE TRADE WINDS
RETURN.

&&

.DISCUSSION...
AN UPPER LEVEL LOW ABOUT 225 NM NNE OF PHNL IS THE DOMINANT PLAYER
IN OUR WEATHER TODAY. THE LOW IS MOVING TOWARD THE W AT ABOUT 15 KT
AND THUS WILL CONTINUE TO PASS NORTH OF KAUAI AND OAHU TODAY. THE
PROXIMITY OF THE UPPER LOW IS HAVING THREE SIGNIFICANT EFFECTS. THE
FIRST IS TO DESTABILIZE THE AIRMASS. THE 12Z SOUNDINGS FROM PHLI
AND PHTO SHOWED 500 MB TEMPS OF -6.9C AND -9.3C...RESPECTIVELY...AND
THE MORNING ACARS SOUNDINGS FROM PHNL SHOW 500 MB TEMPS OF -8 TO
-9C. THIS IS A LITTLE COOLER AND MORE UNSTABLE THAN THE MODELS HAD
BEEN EXPECTING. THE OTHER EFFECT IS TO WEAKEN THE SURFACE PRESSURE
GRADIENT OVER THE ISLANDS. THE END RESULT IS THAT LOW LEVEL WINDS
HAVE SHIFTED FROM NE TRADES TO SE...WITH LOW CLOUD MOTION FROM THE
SE AT ABOUT 8 KT. THE THIRD IS A LARGE AREA OF DEEPER MOISTURE
WHICH IS MOVING ALONG WITH THE UPPER LOW.

THE COMBINATION OF INCREASED INSTABILITY...MOISTURE...AND OROGRAPHIC
LIFTING OF SEA BREEZES WILL AGAIN RESULT IN TOWERING CUMULUS
BILLOWING UP INTO LOCALLY HEAVY SHOWERS OVER THE INTERIOR OF THE
ISLANDS THIS AFTERNOON. WITH THE AIRMASS EVEN A BIT MORE MOIST AND
UNSTABLE THAN 24 HOURS AGO...THERE COULD BE ISOLATED THUNDERSHOWERS
THIS AFTERNOON AS WELL...AND HAVE ADDED THESE TO THE GRIDDED
FORECAST. THE HEAVIER SHOWERS AND THUNDERSHOWERS WILL FADE AS WE GET
INTO THE EARLY EVENING HOURS WITH THE LOSS OF DAYTIME HEATING...
THOUGH THERE COULD STILL BE SOME LINGERING DOWNPOURS OVER THE
COASTAL WATERS INTO TONIGHT.

WITH THE UPPER LOW EXITING THE ISLANDS SCENE SUNDAY...LOOK FOR THE
RETURN OF LIGHT TO MODERATE TRADE WINDS AND TRADE WIND SHOWERS BY
MONDAY. LOOK FOR THE TRADES TO HANG AROUND FOR THE REST OF THE NEW
WEEK AS THE RIDGE OF HIGH PRESSURE REMAINS WELL ENTRENCHED A
THOUSAND MILES NORTH OF HERE. THE TRADES MAY EVEN TURN UP A NOTCH
THE SECOND HALF OF THE WEEK AS THE RIDGE STRENGTHENS.

&&

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

R BALLARD