Heavy Rain in Hawaii, December 18 to 22, 2013

AREA FORECAST DISCUSSION...CORRECTED MARINE
NATIONAL WEATHER SERVICE HONOLULU HI
924 AM HST WED DEC 18 2013

.SYNOPSIS...
LOCALLY HEAVY SHOWERS AND POSSIBLY THUNDERSTORMS WILL REPLACE THE
CURRENT MILD WEATHER FROM EAST TO WEST TONIGHT THROUGH FRIDAY.
CONDITIONS WILL BECOME LESS UNSETTLED AGAIN LATE IN THE WEEKEND.
WINDS ARE EXPECTED TO REMAIN LIGHT TO MODERATE FROM THE EAST TO
SOUTHEAST INTO NEXT WEEK.

&&

.DISCUSSION...
FAIR CONDITIONS WILL DETERIORATE OVER 24-36 HOURS AS A POTENT
MID/UPPER LOW MOVES OVER THE STATE...ESPECIALLY OVER THE EASTERN
PORTION OF THE STATE. RESULTING UNSTABLE CONDITIONS WILL SUPPORT
LOCALLY HEAVY RAIN AND THUNDERSTORMS THIS AFTERNOON OVER HAWAII
COUNTY SPREADING WESTWARD THROUGH THURSDAY NIGHT. A FLASH FLOOD
WATCH IS IN EFFECT FOR MAUI/HAWAII COUNTY AND MAY BE EXPANDED
FURTHER EASTWARD. FREEZING LEVELS WILL DROP CONSIDERABLY RESULTING
IN PERIODS OF HEAVY SNOW OVER THE HIGHEST ELEVATIONS OF THE BIG
ISLAND. SNOW FLURRIES MAY REACH AS LOW AS 10 KFT...HOWEVER NO
ACCUMULATION IS EXPECTED AT THIS ELEVATION. A WINTER STORM WATCH
REMAINS IN EFFECT FROM LATE TONIGHT THROUGH THURSDAY NIGHT. GOES
SATELLITE DERIVED WINDS AND GUIDANCE DO NOT SUGGEST A HIGH WIND
THREAT FOR THE SUMMITS. RAINFALL COVERAGE AND INTENSITY OVER THE
EASTERN ISLANDS WILL INCREASE SIGNIFICANTLY LATE TONIGHT AND DURING
THE DAY THURSDAY COINCIDENT WITH A TRADE WIND SURGE.

THE LOW ALOFT WILL MEANDER AROUND OVER THE NEXT SEVERAL DAYS AS COLD
POOL SHIFTS EAST ALLOWING FOR IMPROVING GRADUALLY CONDITIONS
FRIDAY/SATURDAY. ISOLATED HEAVY SHOWERS MAY REMAIN POSSIBLE. ANOTHER
SHORT WAVE TROUGH WILL DIVE INTO THE AREA SATURDAY NIGHT. SHOWER
INTENSITY SHOULD INCREASE AGAIN LATE IN THE WEEKEND INTO
MONDAY...AGAIN MAINLY FOR WINDWARD ZONES OF THE EASTERN MAUI/HAWAII
COUNTIES.

&&

.PREVIOUS DISCUSSION.../ISSUED 400 AM HST WEDNESDAY MORNING/
OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED LITTLE WIND
AT THE SURFACE...THOUGH SOUNDING DATA STILL SHOWED LIMITED
NORTHEASTERLY PREVAILING FLOW. WINDWARD RAINFALL INCREASED FROM
VIRTUALLY NOTHING EARLY IN THE NIGHT...TO LIGHT BUT NOTICEABLE
ACCUMULATION...MAINLY ALONG WINDWARD SLOPES...AS GRADUALLY
INCREASING SHOWERS DRIFTED OVER THE ISLANDS FROM THE NORTHEAST.

MORE DRAMATIC WAS THE LOW ALOFT AND ITS DEEP CONVECTION APPROACHING
THE STATE FROM THE NORTHEAST. THIS FEATURE WILL BRING SIGNIFICANT
CHANGES TO THE CURRENTLY MILD LOCAL CONDITIONS THROUGH THE SECOND
HALF OF THE WEEK. ALTHOUGH SUCH LOWS VARY CONSIDERABLY IN THEIR
CHARACTERISTICS...THIS ONE APPEARED TO POSSESS BOTH SUBSTANTIAL
DYNAMICS AND A RESPECTABLE COLD POOL. SATELLITE ESTIMATES OF
PRECIPITABLE WATER ALSO SHOWED AN IMPRESSIVE SLUG OF MOISTURE
CONVERGING UNDER THE LOW. THUS...THE THUNDERSTORMS BUBBLING
AROUND THIS FEATURE CAME AS NO SURPRISE.

GUIDANCE BROADLY AGREED THAT THE LOW WILL DRIFT TOWARD THE WEST
SOUTHWEST SLOWLY INTO FRIDAY...BRINGING ITS INSTABILITY...COLD
POOL...AND MOISTURE TO THE STATE. INCREASING SHOWERS AND
THUNDERSTORMS WILL SPREAD FROM EAST TO WEST...LIKELY REACHING THE
BIG ISLAND AND MAUI BY THIS EVENING...AND THEN CONTINUING ON TO THE
REST OF THE ISLANDS. THE DEEP CONVECTION WILL PRODUCE LOCALLY HEAVY
SHOWERS...WHILE THUNDERSTORMS WILL ADD ADDITIONAL RISKS. OVER HIGHER
ELEVATIONS...MOST OR ALL OF THE PRECIPITATION WILL BE FROZEN.

FROM FRIDAY ONWARD...SOLUTIONS SHOWED BETTER AGREEMENT THAN EARLIER
RUNS DID. CONSENSUS STILL PREDICTED THE LOW TO STALL IN PLACE...AND
THEN GRADUALLY OPEN OUT TO THE NORTHEAST OVER THE WEEKEND. STABILITY
WILL RETURN TO THE ISLANDS SLOWLY...BUT A PRECISE END TIME TO THE
HEAVY SHOWERS MAY REMAIN ELUSIVE UNTIL AFTER THE FACT. IF NOTHING
ELSE...REVERSION TO CLIMATOLOGY SUGGESTED A RETURN OF MORE TYPICAL
TRADE-WIND WEATHER BY EARLY NEXT WEEK...THOUGH GUIDANCE DIFFERED
SIGNIFICANTLY WITH RESPECT TO TRADE WIND SPEEDS.

&&

.AVIATION...
VFR CONDITIONS WILL BE THE GENERAL RULE ALTHOUGH POCKETS OF MVFR
CIGS AND CB ARE EXPECTED TO DEVELOP IMPACTING MAINLY WINDWARD
PORTIONS OF MAUI COUNTY AND THE BIG ISLAND AS THE DAY DEVELOPS.
CONDITIONS WILL DETERIORATE RAPIDLY LATE THIS EVENING AND OVERNIGHT
AS AN AREA OF MID/UPPER LOW PRESSURE MOVES OVER THE STATE. MOUNTAIN
OBSCURATION WILL BE A POSSIBILITY FOR MAUI AND THE BIG ISLAND ALONG
WITH DEEPENING CB/TCU WITH TOPS ABOVE 300. THE THREAT FOR MVFR/IFR
CIGS AND VIS WILL BE PRESENT WITH THE CB/TCU THROUGHOUT THE NIGHT
INTO THURSDAY.

AIRMET TANGO REMAINS IN EFFECT FOR MODERATE TURBULENCE FROM OAHU TO
THE BIG ISLAND DUE TO STRONG NORTHERLY WINDS AHEAD OF THE MID/UPPER
LOW.

&&

.MARINE...
EASTERLY FLOW IS EXPECTED THROUGH THE FORECAST PERIOD HOWEVER WILL
REMAIN BELOW SCA CRITERIA IN THE COASTAL WATERS. THUNDERSTORMS ARE
ONGOING OVER THE NORTHEAST OFFSHORE ZONE RIGHT NOW. THEY SPREAD
ACROSS THE AREA FROM EAST TO WEST OVER THE NEXT 24 TO 48 HOURS.
DANGEROUS LIGHTNING...GUSTY WINDS AND AREAS OF REDUCED VISIBILITY
WILL BECOME INCREASINGLY LIKELY TONIGHT THROUGH THURSDAY NIGHT.

SURF WILL BE ON THE RISE THURSDAY AS FRESH WEST-NORTHWEST SWELL
BUILDS ACROSS THE AREA. THIS SWELL HAS A STRONG WESTERLY COMPONENT
SO MANY WEST FACING BEACHES THAT ARE TYPICALLY SHADOWED WILL SEE
SIGNIFICANT SURF...AND CONVSERSELY...SOME OPEN NORTH FACING BEACHES
WILL SEE LESS CONSISTENT SMALLER SURF. WARNING LEVEL SURF IS
ANTICIPATED ON THE MOST EXPOSED WEST-NORTHWEST FACING BEACHES. THE
KONA COAST OF THE BIG ISLAND COULD SEE VERY LARGE SURF EXCEEDING 20
FEET IN SOME AREAS FRIDAY. HIGH SURF ADVISORY/WARNINGS WILL BE
ISSUED THIS AFTERNOON WITH GREATER DETAILS ON TIMING AND SURF
HEIGHTS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM LATE WEDNESDAY NIGHT THROUGH LATE
THURSDAY NIGHT FOR MAUI-BIG ISLAND.

WINTER STORM WATCH FROM LATE WEDNESDAY NIGHT THROUGH LATE
THURSDAY NIGHT FOR BIG ISLAND SUMMITS.

&&

SYNOPSIS...DEJESUS/RYSHKO
PUBLIC/MARINE...DEJESUS
AVIATION...CANTIN
PREVIOUS...RYSHKO