Flash Flood and Heavy Rain in Hawaii, September 19 to 21, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
345 PM HST FRI SEP 19 2014

.SYNOPSIS...
LIGHT WINDS THROUGH THE WEEKEND WILL ALLOW LAND AND SEA BREEZES TO
DRIVE ISLAND WEATHER. IN THIS REGIME...SKIES WILL BE MOSTLY CLEAR
NIGHTS AND MORNINGS...WITH CLOUDS AND SHOWERS DEVELOPING OVER
INTERIOR AND MAUKA AREAS IN THE AFTERNOONS. LIGHT TO MODERATE TRADE
WINDS WILL RETURN EARLY NEXT WEEK...DELIVERING BRIEF PASSING SHOWERS
TO WINDWARD AREAS THROUGH WEDNESDAY. THE TRADE WINDS MAY BECOME
WETTER THURSDAY INTO NEXT WEEKEND...AS MOISTURE ASSOCIATED WITH A
WEAKENING FRONT BECOMES EMBEDDED IN THE TRADE WIND FLOW.

&&

.DISCUSSION...
CURRENTLY...A WEAK PRESSURE GRADIENT IS OVER THE AREA...WITH LIGHT
LARGE-SCALE WINDS ALLOWING ISLAND-SCALE WINDS TO DRIVE THE SENSIBLE
WEATHER PATTERN. RELATIVELY WEAK SURFACE HIGHS ARE FAR NW AND NE OF
THE ISLANDS...WHILE A SE-WARD DIGGING FRONT IS ABOUT 700 MILES N OF
KAUAI. LATEST SURFACE ANALYSIS ALSO FEATURES A WEAK N-S ORIENTED
TROUGH AXIS NEAR KAUAI AND OAHU...WITH WEAK NE WINDS W OF THE TROUGH
AND WEAK SE WINDS E OF THE TROUGH AXIS. A PAIR OF TROPICAL
DISTURBANCES S AND SE OF THE BIG ISLAND CONTAIN DEEP MOISTURE...WITH
THIS INCREASED MOISTURE EXTENDING AS FAR N AS THE BIG ISLAND.
AFTERNOON SOUNDINGS SHOW A MOIST AND RELATIVELY UNSTABLE AIR MASS IN
PLACE...WITH TODAY/S AFTERNOON CONVECTION CONFIRMING THAT THE MOST
UNSTABLE AIR PARCELS ARE ABLE TO EXTEND AS HIGH AS 20 TO 25 KFT...
SUPPORTING THE DEVELOPMENT OF ISOLATED HEAVY SHOWERS.

THROUGH SUNDAY...THE PRESSURE GRADIENT OVER THE ISLANDS WILL REMAIN
SUFFICIENTLY LOOSE FOR LAND AND SEA BREEZES TO BE THE DOMINANT
FACTOR DRIVING ISLAND WEATHER...WITH GENERALLY CLEAR SKIES AT NIGHT
AND IN THE MORNING...WITH CLOUDS AND SHOWERS DEVELOPING BY LATE
MORNING AND LINGERING THROUGH THE EARLY EVENING. ONE EXCEPTION WOULD
BE THE SE AND WINDWARD PORTIONS OF THE BIG ISLAND AND ADJACENT
WATERS...WHERE SHOWERS MAY BE A LITTLE MORE ACTIVE THROUGH SATURDAY
NIGHT DUE TO THE PASSAGE OF THE TROPICAL DISTURBANCES TO THE S. A
WEAK MID-LEVEL TROUGH PASSING N OF THE ISLANDS WILL EVENTUALLY GIVE
WAY TO MID-LEVEL RIDGING...WITH THE RIDGE STARTING TO BUILD ON
SUNDAY. THUS THIS WEEKEND/S AFTERNOON CONVECTION COULD ALSO FEATURE
SOME ISOLATED HEAVY DOWNPOURS...BUT MAY BE A LITTLE LESS LIKELY ON
SUNDAY.

THE LIGHT WINDS WILL ALLOW VOG AND SMOKE FROM THE BIG ISLAND/S
ACTIVE VOLCANO/LAVA FLOWS TO POOL NEAR THE SURFACE ON THE BIG ISLAND
OVERNIGHT AND EARLY MORNING THIS WEEKEND...BEFORE CLEARING IN THE
MID-MORNING. STRONGER WINDS NEXT WEEK WILL LIKELY PREVENT A STRONG
GROUND-BASED INVERSION FROM DEVELOPING...AND THE POOLING OF
PARTICULATES NEAR THE SURFACE SHOULD BE LESS PRONOUNCED.

THE FRONT N OF THE ISLANDS NOW WILL MOVE S THROUGH SUNDAY...BUT WILL
STALL N OF THE ISLANDS...CONTRIBUTING TO THE CONTINUED LIGHT WIND
REGIME. A SECOND FRONT WILL DIG TOWARD THE ISLANDS EARLY NEXT
WEEK...WITH A MODERATELY STRONG HIGH PASSING N OF THE ISLANDS...AND
THE FRONTAL REMNANTS...TUESDAY THROUGH FRIDAY OF NEXT WEEK. LATEST
GUIDANCE SHOWS TRADE WINDS INCREASING MONDAY IN RESPONSE TO THE
HIGH/S PASSAGE...WITH LIGHT TO MODERATE TRADE WINDS PERSISTING FOR
THE REMAINDER OF THE WEEK. MODELS SHOWS THE SECOND FRONT BECOMING
EMBEDDED WITHIN THE TRADE FLOW AS AN E-W ORIENTED BAND OF CLOUDS AND
SHOWERS THAT POTENTIALLY AFFECTS ISLAND WEATHER AROUND THURSDAY AND
FRIDAY. WITH TRADE WINDS BLOWING...THE BULK OF THIS MOISTURE WILL
FOCUS ON WINDWARD SLOPES AND COASTS...POSSIBLY LINGERING INTO THE
WEEKEND. DEEP EASTERLY FLOW WILL LIKELY ADVECT SOME OF THESE SHOWERS
OVER TO LEEWARD AREAS ON THE SMALLER ISLANDS.

LOCAL SEA SURFACE TEMPERATURES /SSTS/ ARE ANOMALOUSLY WARM...
CONTRIBUTING TO THE ONGOING MUGGY CONDITIONS...WITH SURFACE DEW
POINTS IN THE LOWER 70/S. SSTS WILL NOT CHANGE MUCH OVER THE NEXT
WEEK...AND ALTHOUGH THE INCREASING TRADE WINDS MAY MAKE IT FEEL A
LITTLE COOLER NEXT WEEK...THEY ARE NOT EXPECTED TO HAVE A DRAMATIC
EFFECT ON THE RELATIVELY HIGH DEW POINTS.

&&

.AVIATION...
CONVECTION FLARED UP OVER THE ISLANDS THIS AFTERNOON...ESPECIALLY
FOR INTERIOR KAUAI...OAHU...AND THE KAU AND SADDLE AREAS ON THE BIG
ISLAND...WITH TCU TOPS BETWEEN 15 AND 22 KFT. EXPECT THIS
ACTIVITY...TEMPO MVFR WITH ISOL IFR CIGS AND VIS AND ELEVATED
TOPS...TO CONTINUE INTO THE EARLY EVENING. CONDITIONS SHOULD START
TO IMPROVE WITH SUNSET AND CLEAR OUT OVERNIGHT...AS LAND BREEZES
ARE EXPECTED TO FORM AGAIN TONIGHT...WITH THE EXCEPTION OF SOUTH BIG
ISLAND. ENHANCED MOISTURE SKIRTING JUST SOUTH OF THE BIG ISLAND
TONIGHT WILL HELP TO FUEL SCATTERED SHOWERS WITH TEMPO MVFR AND
POSSIBLE ISOL IFR OVER SE BIG ISLAND...INCLUDING S HILO...PUNA...AND
KAU DISTRICTS.

&&

.MARINE...
LITTLE TO NO CHANGE WITH RESPECT TO THE MARINE FORECASTS...WITH
WINDS AND SEAS REMAINING BELOW SMALL CRAFT ADVISORY CRITERIA /SCA/
INTO EARLY NEXT WEEK. INCREASING TRADE WINDS MAY GET STRONG ENOUGH
TO WARRANT SCA ISSUANCE FOR THE WINDIER MARINE ZONES AROUND MAUI AND
THE BIG ISLAND AROUND THE MIDDLE OF NEXT WEEK.

SURF WILL REMAIN BELOW ADVISORY LEVELS ALONG ALL SHORES OVER THE
NEXT WEEK...BUT SMALL SWELLS WILL PROVIDE SOME SURF. SMALL SWELLS
FROM THE NW ARE EXPECTED OVER THE WEEKEND AND AGAIN ON WEDNESDAY/
THURSDAY...WITH A NNE SWELL POSSIBLE THURSDAY/FRIDAY. TINY
LONG-PERIOD SWELLS FROM THE S WILL MAINTAIN SURF NEAR OR BELOW
SEASONAL AVERAGES ALONG S FACING SHORES THROUGH THE PERIOD.

&&

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

&&

PUBLIC/MARINE...BIRCHARD
AVIATION...EATON