Flash Flooding and Hail in Hawaii, March 2 to 6, 2012

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE HONOLULU HI
1225 PM HST FRI MAR 2 2012

.SYNOPSIS...
LIGHT TO MODERATE TRADE WINDS WILL CONTINUE INTO SUNDAY...FAVORING
AN EAST-SOUTHEAST DIRECTION. SHOWERS EMBEDDED IN THIS FLOW WILL
FAVOR WINDWARD AND MAUKA AREAS...WITH INCREASED SHOWERS OVER
WINDWARD PORTIONS OF THE BIG ISLAND AND MAUI EXPECTED THROUGH EARLY
SATURDAY. A SHEAR LINE MOVING TOWARD THE ISLANDS LATE THIS WEEKEND
AND EARLY NEXT WEEK HAS THE POTENTIAL TO BRING UNSETTLED WEATHER
AND STRONGER TRADE WINDS...ESPECIALLY NEAR KAUAI.

&&

.DISCUSSION...
LATEST SURFACE ANALYSIS SHOWS A PAIR OF FAIRLY STRONG HIGH PRESSURE
SYSTEMS FAR NE AND NW OF THE ISLANDS...WITH A STALLED FRONT BETWEEN
THE TWO LOCATED FAR NNW OF THE ISLANDS...AND A PERSISTENT N-S
ORIENTED TROUGH ABOUT 400 MILES W OF KAUAI. LOW LEVEL WIND FLOW OVER
THE ISLANDS IS CURRENTLY UNDER THE INFLUENCE OF THE HIGH TO THE
NE...WITH THE TRADE WIND FLOW VEERED TO THE ESE AND IN A WEAKENED
STATE...AS THE FRONT HAS RESULTED IN A WEAKER PRESSURE GRADIENT OVER
THE ISLANDS. A WEAK TROUGH ALOFT CONTINUES TO SUPPORT DEEP
CONVECTION OVER WATERS W OF KAUAI...WITH LIGHTNING DETECTION
REVEALING QUITE A FEW STROKES OVER OFFSHORE WATERS...JUST W OF
KAUAI LEEWARD COASTAL WATERS. SKIES OVER LEEWARD AREAS OF THE
ISLANDS ARE MOSTLY SUNNY THIS MORNING...WITH SHOWERS MOST PREVALENT
OVER WINDWARD BIG ISLAND AND WINDWARD WATERS OF MAUI AND THE BIG
ISLAND. HAVE UPDATED FORECASTS FOR TODAY AND TONIGHT...PRIMARILY TO
ADD ISOLATED THUNDERSTORMS TO WATERS W OF NIIHAU TODAY...AND TO
INCREASE RAIN CHANCES OVER WINDWARD BIG ISLAND AND ADJACENT WATERS
THROUGH TONIGHT.

THE SHORT TERM FORECAST ALLOWS FOR SOME SHOWERS TO DEVELOP OVER
LEEWARD KAUAI THIS AFTERNOON AS THE LIGHT TRADE WINDS ALLOW SEA
BREEZES TO CONVERGE IN A MODERATELY UNSTABLE ENVIRONMENT.
OTHERWISE...CLOUDY AND DAMP CONDITIONS WILL PREVAIL OVER WINDWARD
BIG ISLAND...AND PARTS OF WINDWARD MAUI...AS AN E-W ORIENTED BAND OF
SHOWERY CUMULUS EXTENDS FOR SEVERAL HUNDRED MILES TO THE ESE.
TYPICAL TRADE WIND WEATHER IS EXPECTED ELSEWHERE.

LITTLE SIGNIFICANT CHANGE TO THE OVERALL PATTERN IS EXPECTED
THROUGH SATURDAY...WITH SHOWERS ESE OF THE BIG ISLAND SPREADING INTO
WINDWARD BIG ISLAND AND MAUI...WITH OTHER PORTIONS OF MAUI COUNTY
AND OAHU SOMEWHAT DRIER AS THEY WILL BE IN THE LEE OF THE BIG
ISLAND...WITH THE ESE FLOW PREVAILING. THE UPPER TROUGH W OF KAUAI
IS FORECAST TO FINALLY EJECT TO THE E AND FLATTEN...AS A STRONGER
UPPER TROUGH MOVES TOWARD THE ISLANDS FROM THE NW. THE LIGHTER ESE
WINDS MAY AGAIN ALLOW SOME CONVECTION TO DEVELOP OVER THE LEEWARD
SIDE OF KAUAI TOMORROW...WITH LINGERING MOISTURE AND INSTABILITY IN
PLACE.

BEYOND SATURDAY...A DIFFICULT FORECAST REGIME PREVAILS...AS THE
FLOW PATTERN STILL BECOMES INCREASINGLY UNCERTAIN BY SUN/MON.
FORECAST MODELS CONTINUE TO SHOW RUN TO RUN INCONSISTENCIES...WHICH
LEADS TO A REDUCTION IN CONFIDENCE...ESPECIALLY IN LIGHT OF THE
VARIETY OF SOLUTIONS/TIMINGS OFFERED.

GLOBAL MODELS HAVE BEEN CONSISTENT IN MOVING A VERY STRONG SURFACE
HIGH /1044 MB/ INTO A POSITION FAR N OF THE ISLANDS ON SUNDAY...
BRINGING GALE FORCE TRADE WINDS TO WATERS N OF THE ISLANDS...
INCLUDING THE NORTHERN OFFSHORE WATERS. DESPITE THIS CONSISTENCY...
MODELS HAVE BEEN QUITE INCONSISTENT ON THE EFFECTS THIS HIGH WILL
HAVE ON ISLAND WEATHER...AND THE TIMING OF THESE POSSIBLE EFFECTS.
OF COURSE...THESE INCONSISTENCIES LINE UP WITH THE LATE WEEKEND
WEATHER FORECAST...AND BECOME EVEN GREATER EARLY NEXT WEEK.

THE STRONG HIGH WILL PUSH AN ACTIVE SHEAR LINE TOWARD THE ISLANDS
FROM THE N ON SUNDAY...WHILE A CLOSED MID LEVEL LOW DEVELOPS NW OF
KAUAI. GFS AND ECMWF BOTH GENERALLY AGREE ON THIS SCENARIO...BUT THE
STRENGTH AND TIMING OF THE CUT OFF DEVELOPMENT HAS PROFOUND EFFECTS
ON THE MOVEMENT AND PROGRESSION OF THE SHEAR LINE...ITS ASSOCIATED
WET WEATHER...AND THE STRONG TO NEAR GALE E/NE WINDS LOCATED JUST N
OF THE BOUNDARY. GFS DEVELOPS THE CUT OFF MORE QUICKLY...WHICH LEADS
TO A SLOWER MOVING SHEAR LINE WHICH BARELY SPREADS TO KAUAI SUNDAY
NIGHT...BEFORE STALLING JUST N OF KAUAI THROUGH TUESDAY. THE ECMWF
SOLUTION IS SLOWER AND WEAKER WITH THE CUT OFF DEVELOPMENT...LEADING
TO A MORE PROGRESSIVE SHEAR LINE THAT REACHES KAUAI SUNDAY...AND
POSSIBLY OAHU SUNDAY NIGHT. FORECAST GRIDS LEAN TOWARD AN ECMWF
SOLUTION...BUT WE WILL BE REVIEWING LATEST GUIDANCE TO SEE WHERE
TWEAKS TO THE FORECAST WILL BE NEEDED. IT APPEARS THAT THE POTENTIAL
FOR A PERIOD OF WET...AND POSSIBLY WINDY...WEATHER ON KAUAI IS
INCREASING...STARTING ON SUNDAY AND PERSISTING INTO NEXT WEEK.

&&

.AVIATION.../UPDATE ISSUED 1225 PM HST FRI/
SE FLOW IS PRODUCING ENHANCED LLVL CONVERGENCE LEE OF MOLOKAI. THIS
IS INTERACTING WITH THE KOOLAUS PRODUCING MVFR-IFR VIS ALONG N OF
PALI PASS. AIRMET SIERRA WILL POSSIBLY CONTINUE BEYOND 03/04Z.

ELSEWHERE WINDWARD AREAS WILL SEE THE BULK OF THE SHRA AND ANY MVFR
CONDITIONS. TS CONTINUE TO PULSE W OF KAUAI JUST OUTSIDE THE FA. NO
SIGMET IS NEEDED AT THIS TIME.

&&

.MARINE...
NO MARINE ADVISORIES ARE CURRENTLY POSTED...AS DECREASED WINDS NEAR
AND UPSTREAM OF THE ISLANDS HAVE ALLOWED SEAS TO DIMINISH. THE
STRONG HIGH BUILDING FAR N OF THE ISLANDS LATE THIS WEEKEND MAY
BRING STRONG TRADES THAT WILL LEAD TO WINDS/SEAS ABOVE SMALL CRAFT
ADVISORY...ESPECIALLY NEAR KAUAI.

GFS BASED WAVE WATCH III BRINGS A LARGE...RELATIVELY SHORT-PERIOD...
NE SWELL TOWARD THE ISLANDS LATE THIS WEEKEND INTO EARLY NEXT
WEEK...AS A SURFACE LOW DEVELOPS ON THE SE SIDE OF A VERY STRONG
HIGH. HOWEVER...ONE OF THE DISCREPANCIES IN THE MID/LONG TERM MODELS
IS THE POSITION AND STRENGTH OF THE DEEP SURFACE LOW WHICH WOULD
INFLUENCE THE IMPACT OF THE SWELL DIRECTION AND WIND WAVES OVER
HAWAII. THOSE WITH MARINE INTERESTS SHOULD REMAIN VIGILANT TO THE
LATEST FORECASTS...AS DRAMATIC SHIFTS IN THE FORECAST ARE POSSIBLE.

&&

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

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...DEJESUS