Drought in Hawaii, September 1 to 30, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST TUE AUG 31 2010

.SYNOPSIS...
LIGHT TO MODERATE TRADE WINDS WILL WEAKEN FURTHER THROUGH FRIDAY.
SHOWERS WILL INCREASE SLIGHTLY...ESPECIALLY LEEWARD IN THE
AFTERNOON...AND WINDWARD AT NIGHT. CONDITIONS WILL BECOME MORE
TYPICAL OVER THE WEEKEND.

&&

.DISCUSSION...
SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED
LIGHT TO MODERATE TRADE WINDS BLOWING...THOUGH PRESSURE TRENDS
SUGGESTED NEGLIGIBLE CHANGE FROM 24 HOURS AGO. HOWEVER...SOUNDINGS
SHOWED FAIRLY LIGHT WINDS THROUGH MOST OF THE SURFACE MIXED
LAYER...SO LITTLE HORIZONTAL MOMENTUM WAS AVAILABLE TO MIX DOWN.

SUBSIDENCE WAS THE THEME OF THE NIGHT...WITH STACKED RIDGING
PROMINENT THROUGH MOST OF THE ATMOSPHERE OVER THE DISTRICT. NOT
SURPRISINGLY...RAINFALL ACROSS THE STATE AMOUNTED TO ALMOST
NOTHING...AND PROBABLY WOULD HAVE BEEN ONLY SLIGHTLY GREATER EVEN IF
ATMOSPHERIC MOISTURE WERE INCREASED FROM ITS CURRENT BELOW-AVERAGE
CONCENTRATION. THE MAIN AREA OF RAIN WAS LIMITED TO THE KAUAI LEE
PLUME...WHERE CLOUDS FROM UPWIND PROVIDED AT LEAST A LITTLE
MOISTURE...AND THE CIRCULATION OF THE WAKE ALLOWED SOME DYNAMICAL
UPLIFT.

GUIDANCE AGREES FAIRLY WELL THAT CONDITIONS SHOULD CHANGE NOTICEABLY
OVER THE NEXT 24 TO 36 HOURS. ALOFT...TROUGHING FROM THE NORTHEAST
IS EXPECTED TO SPREAD TOWARD THE ISLANDS...GRADUALLY WEAKENING AND
DISPLACING THE HIGH OVER THE STATE. REDUCED SUBSIDENCE WILL PERMIT
MORE SHOWER DEVELOPMENT...WHILE AT THE SAME TIME WEAKENING THE
SURFACE RIDGE...AND THUS THE TRADES. GUIDANCE ALSO PREDICTS MOISTER
AIR TO PASS OVER THE DISTRICT WEDNESDAY NIGHT AND THURSDAY NIGHT.
TYPICALLY...GREATER MOISTURE AND REDUCED SUBSIDENCE WOULD LEAD TO
CLEAR INCREASES IN SHOWER ACTIVITY...BUT THE VERY WEAK TRADES MAY
PRODUCE INSUFFICIENT OROGRAPHIC FORCING...ESPECIALLY WHEN CONTENDING
WITH LAND BREEZES AT NIGHT. THE FINAL OUTCOME WILL DEPEND ON A
DELICATE BALANCE OF FORCES...BUT ON THE WHOLE...IT LOOKS LIKE SOME
INCREASE IN SHOWERS CERTAINLY WILL OCCUR. SOME OF THESE LIKELY WILL
DEVELOP EVEN LEEWARD IN THE AFTERNOON...AS SLACK TRADES LEAD TO
HUMID AND RELATIVELY STILL CONDITIONS. WIND SPEED WILL PICK UP A BIT
AGAIN BY THE WEEKEND AS THE SURFACE HIGH SHIFTS...THOUGH THIS CHANGE
MAY BE QUITE SUBTLE.

BY LATE IN THE WEEKEND...MODEL SOLUTIONS START TO DIVERGE MORE
SIGNIFICANTLY...MAINLY IN THEIR TREATMENT OF THE UPPER TROUGH. SOME
DEEPEN IT INTO A CUT-OFF LOW...AND THEN PARK THIS FEATURE NORTH OF
THE STATE...WHILE OTHERS EXTEND THE TROUGH MORE TOWARD THE SOUTH...
WHILE KEEPING IT EAST OF THE ISLANDS. ALTHOUGH THIS DIFFERENCE
SOUNDS IMPRESSIVE...AND INDEED IT REPRESENTS NOTICEABLE CHANGES IN
ISLAND WEATHER...WEAK TRADES WILL REMAIN IN THE FORECAST. THE UPPER
TROUGH ONLY WOULD MAKE THE DIFFERENCE BETWEEN WET TRADES OR DRY...OR
BETWEEN WEAK TO MODERATE TRADES ON THE ONE HAND...AND VERY WEAK
TRADES WITH SEA BREEZES ON THE OTHER. FOR NOW...IT SEEMS PRUDENT TO
KEEP CLOSE TO CLIMATOLOGY.

&&

.MARINE...
SURF WILL REMAIN SMALL ON ALL SHORES THROUGH THE FORECAST PERIOD AS
NO SIGNIFICANT SWELLS ARE EXPECTED.

WITH THE TRADES STRENGTHENING THIS WEEKEND...A SMALL CRAFT ADVISORY
MAY BE NEEDED FOR THE TYPICALLY WINDIER COASTAL WATERS AROUND MAUI
COUNTY AND THE BIG ISLAND.

&&

.FIRE WEATHER...
ALTHOUGH THE KBDI IS WELL ABOVE THE CRITICAL THRESHOLD OF 600...AND
RELATIVE HUMIDITY AT THE HONOLULU INTERNATIONAL AIRPORT IS EXPECTED
TO DROP TO 45 PERCENT TOMORROW...TRADES ARE NOT EXPECTED TO BE
STRONG ENOUGH TO SATISFY RED FLAG WARNING CONDITIONS. STRENGTHENING
TRADES FROM FRIDAY THROUGH THE WEEKEND LIKELY WILL RAISE FIRE
WEATHER CONCERNS ONCE AGAIN.

&&

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

&&

RYSHKO