Heavy Rain in Hawaii, September 21, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
330 PM HST SUN SEP 21 2008

.SYNOPSIS...
A SURFACE RIDGE NORTH OF THE ISLANDS WILL MAINTAIN GENTLE TO
MODERATE TRADE WINDS THROUGH MIDWEEK. AN UPPER LOW WILL COMBINE WITH
A WEAK TROUGH AT THE SURFACE TO PRODUCE INCREASING SHOWERS TONIGHT
THROUGH MONDAY...WITH LOCALLY HEAVY RAIN POSSIBLE. THE UPPER LOW
WILL LINGER OVER THE STATE THROUGH MIDWEEK WITH WETTER THAN NORMAL
TRADES EXPECTED.

&&

.DISCUSSION...
LIGHT TO MODERATE TRADES CONTINUE ACROSS THE HAWAIIAN ISLANDS THIS
AFTERNOON. A FEW SHOWERS HAVE DEVELOPED THIS AFTERNOON...MOST
NOTABLY IN THE KONA DISTRICT. THIS AFTERNOONS SOUNDINGS SHOW A WEAK
STABLE LAYER REMAINS AT ABOUT THE 12 K FOOT LEVEL...BUT UNSTABLE
WITH SOME CONVECTIVE POTENTIAL BOTH ABOVE AND BELOW THAT LAYER.
THE UPPER LEVEL WINDS ARE SOUTHWESTERLY AT BIG ISLAND AND NORTHERLY
AT LIHUE INDICATING THE UPPER LOW IS CENTERED ABOUT THE MIDDLE OF
THE ISLANDS THIS AFTERNOON. THE WEAK SURFACE TROUGH EAST OF THE BIG
ISLAND CONTINUES TO DRIFT TOWARDS THE STATE THIS AFTERNOON. THE
SATELLITE PRESENTATION HAS NOT BEEN IMPRESSIVE WITH A LACK OF COLD
CLOUD TOPS...STILL WOULD EXPECT SOME DEVELOPMENT AS THE SURFACE
TROUGH REACHES DEEPER INSTABILITY OVER THE ISLANDS LATE TONIGHT AND
MONDAY.

EXPECT SHOWERS WILL INCREASE ACROSS BIG ISLAND AND WINDWARD MAUI
ISLAND THIS EVENING...THEN THE REST OF THE ISLANDS EARLY MONDAY.
TEMPERATURES WILL DROP NEAR OR SLIGHTLY BELOW FREEZING TONIGHT OVER
THE TOP OF THE BIG ISLAND SUMMITS. WITH MOISTURE INCREASING THERE
WILL BE A LITTLE THREAT OF FREEZING RAIN TONIGHT AND EARLY TOMORROW
AND HAVE ADDED THIS TO THE GRIDDED FORECAST FOR ELEVATIONS ABOVE
12500 FEET. WITH DECENT INSTABILITY IN PLACE LATE TONIGHT AND MONDAY
CENTERED ACROSS OAHU AND MAUI...WITH LIFTED INDEX -2 TO -4. THIS
WILL GIVE US AT LEAST A SLIGHT THREAT OF THUNDERSTORMS OVER THE
WINDWARD WATERS STRETCHING FROM MAUI TO OAHU. AS UPPER LEVEL
DYNAMICS REMAIN WEAK...WILL KEEP FROM MENTIONING THUNDER OVER ANY
ISLAND ZONES...HOWEVER IT APPEARS AT LEAST LOCALLY HEAVY RAINFALL IS
POSSIBLE ANYWHERE ACROSS THE STATE MONDAY. THE CONTINUED LIGHT TO
MODERATE TRADE WINDS WILL KEEP THE SHOWERS FROM SITTING ANYWHERE TOO
LONG...AND A FLOOD WATCH IS NOT NECESSARY AT THIS TIME.

THE VARIOUS MODEL GUIDANCE KEEPS THE UPPER LOW IN THE AREA THROUGH
MUCH OF THE COMING WEEK...WITH SOME DIFFERENCES TOWARDS THE END OF
THE WEEK. THE EC MOVES THE UPPER LOW WEST OF THE STATE WITH RIDGING
BUILDING IN BY FRIDAY AND SATURDAY.  THE GFS MOVES THE UPPER LOW
NORTH OF THE ISLANDS WITH A TROUGH AXIS REMAINING OVER THE STATE
INTO SATURDAY.  IN ADDITION...THE GFS TAPS INTO DEEPER TROPICAL
MOISTURE AND DRAWS IT NORTH INTO THE UPPER LOW CIRCULATION OVER THE
STATE BY LATE IN THE WEEK. THE GFS SOLUTION WOULD BE CONSIDERABLY
WETTER THAN THE EC AND WETTER THAN THE CURRENT FORECAST POPS. WILL
WAIT FOR BETTER CONSISTENCY IN THE MODEL SOLUTIONS BEFORE ADJUSTING
THE END OF THE WEEK FORECAST.

&&

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

&&

BRENCHLEY