Heavy Rain in Hawaii, September 15, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST THU SEP 15 2011

.SYNOPSIS...
LIGHT TRADE WINDS WILL INCREASE BY THIS EVENING...AND THEN HOLD AT
MODERATE STRENGTH INTO EARLY NEXT WEEK. RAINFALL WILL REMAIN MAINLY
FOCUSED WINDWARD...BUT WILL INCREASE CONSIDERABLY FROM THIS
AFTERNOON INTO FRIDAY...BEFORE DIMINISHING GRADUALLY OVER THE
WEEKEND TO MORE TYPICAL LEVELS NEXT WEEK.

&&

.DISCUSSION...
HIGH PRESSURE TO THE NORTH MAINTAINED LIGHT TRADE WINDS OVER THE
MAIN HAWAIIAN ISLANDS...AND THE GENERALLY MODEST RAINFALL WAS
FOCUSED OVER WINDWARD SLOPES. HOWEVER...OVER THE NEXT 48 TO 60
HOURS...A LOW-LEVEL TO MID-LEVEL CIRCULATION AND ITS ASSOCIATED
MOISTURE WILL PROFOUNDLY AFFECT WEATHER ACROSS THE STATE.

THIS ANOMALOUS FEATURE...WHICH WAS NOT DEFINED CLEARLY ENOUGH TO BE
TERMED A LOW AS SUCH...SPRAWLED EAST FROM THE BIG ISLAND TO ABOUT
140W. A SUBSIDIARY SURFACE TROUGH WITHIN THE LARGER FEATURE...
EXTENDING ROUGHLY ALONG THE CURVE FROM 28N 159W TO 24N 155W TO 20N
154W...WAS MARKED BY A SHOWERY BAND OF CLOUDS. THIS CLOUD BAND
APPEARED TO CONSIST OF POCKETS OF SHALLOW MID-LEVEL CONVECTION
OCCURRING ABOVE A COMPLETELY SEPARATE LAYER OF RELATIVELY LOW
CLOUDS. EARLIER SATELLITE SCATTEROMETER DATA ALSO SHOWED SIGNIFICANT
SPEED CONVERGENCE ALONG THIS TROUGH...WITH THE PREVAILING WEAK
TRADES WEST OF IT...BUT SOLIDLY MODERATE TRADES TO THE EAST.

THE MOTION OF THIS CLOUD BAND AND TROUGH ARE IMPORTANT...SINCE THEY
CARRY WITH THEM SHARP INCREASES IN RAINFALL AND WIND SPEED...
RESPECTIVELY. OVER THE PAST SIX HOURS THIS MOTION HAS REMAINED
CONSISTENT WITH THE LOWER ESTIMATES CLOSER TO 10 KT...ON PAR WITH
THE WIND SPEED TO THE WEST OF IT...SO THE TIMING OF THE FORECAST
THROUGH THE SHORT TERM APPEARS TO BE ON TRACK.

ALTHOUGH SOUNDINGS AHEAD OF THE TROUGH WERE ONLY MODESTLY
UNSTABLE...ENOUGH MOISTURE WILL BE PRESENT FOR SHOWERS TO DEVELOP...
AND ENOUGH EASTERLY FLOW TO KEEP PUSHING THEM GRADUALLY ONTO
WINDWARD SLOPES. THUS...RAINFALL CAN BE EXPECTED TO PICK UP
GRADUALLY AS PRECIPITABLE WATER INCREASES EVEN BEFORE THE MAIN CLOUD
BAND ARRIVES.

IN ANY CASE...THE FEATURE TO THE EAST OF THE STATE INCLUDED MORE
THAN JUST THE SURFACE TROUGH AND ITS CLOUD BAND. SOUNDING DATA FROM
THE BIG ISLAND SUPPORTED MODEL PREDICTIONS OF ENHANCED MOISTURE AND
A CERTAIN AMOUNT OF CYCLONIC TURNING IN THE WIND FIELD ALOFT FROM
ABOUT 700 MB TO 550 MB...AND A WEAKNESS IN THE MID-LEVEL RIDGE NEAR
500 MB. TOGETHER WITH THE SURFACE FEATURE...THESE COMPONENTS SUGGEST
THAT AFTERNOON CONVECTION ON THE BIG ISLAND ONCE AGAIN COULD EXTEND
TO NEAR THE SUMMITS THIS AFTERNOON...AND PERHAPS ON FRIDAY AS WELL.

DESPITE THE DRY AIR SEPARATING THE LOW-LEVEL MOISTURE LAYER FROM THE
ONE ALOFT...CONDITIONS MAY BE UNSTABLE ENOUGH TO PRODUCE SOME HEAVY
RAIN IN AREAS OF GREATEST SHOWER DEVELOPMENT THIS AFTERNOON AND
TONIGHT. AREAS AFFECTED INCLUDE MOST WINDWARD ZONES...AND THE KONA
SLOPES.

POPS FOR THIS AFTERNOON HAVE BEEN RAISED OVER THE BIG ISLAND SUMMITS
AS WELL. DESPITE TEMPERATURES NEAR FREEZING THERE THIS MORNING...
HIGHER DEWPOINTS SHOULD PREVENT ANY SIGNIFICANT FROZEN
PRECIPITATION...AND KEEP LOWS WARMER INTO FRIDAY MORNING.

GUIDANCE AND CLIMATOLOGY AGREE THAT ZONAL RIDGING WILL REFORM BOTH
ALOFT AND NEAR THE SURFACE OVER A PERIOD OF 24 TO 48 HOURS AFTER THE
SURFACE TROUGH PASSES...BRINGING BACK MORE TYPICAL WET TRADES TO THE
STATE FROM SOUTHEAST TO NORTHWEST. IN THE LONGER TERM...TRADES ARE
EXPECTED TO PREVAIL INTO NEXT WEEK.

&&

.MARINE...
WITH TRADES ON THE RISE BY THIS EVENING...A SMALL CRAFT ADVISORY
/SCA/ LIKELY WILL BE NEEDED FOR THE TYPICALLY WINDY WATERS AROUND
THE BIG ISLAND AND MAUI. WINDS WILL REMAIN AROUND THE SCA THRESHOLD
FOR THESE WATERS INTO EARLY NEXT WEEK.

NO BIG SWELLS ARE EXPECTED THIS WEEK. THE SMALL SOUTH SWELL AND
SHORT-PERIOD NORTH SWELL WILL DECLINE SLOWLY THROUGH FRIDAY. A
SERIES OF SMALL SOUTH-SOUTHWEST SWELLS ARE FORECAST FOR THE WEEKEND
AND EARLY NEXT WEEK.

&&

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

&&

RYSHKO