Flash Flooding and Heavy Rain in Hawaii, August 16 to 20, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
345 PM HST SUN AUG 16 2015

.SYNOPSIS...
A BIT OF UNSETTLED WEATHER IS ON TAP FOR THE ISLANDS FOR THE FIRST
HALF OF THE WORK WEEK AS A SURFACE TROUGH INTERACTS WITH AN UPPER
LEVEL LOW NORTH AND NORTHWEST OF THE ISLANDS. THE SURFACE TROUGH
WILL BE STRONG ENOUGH TO DISRUPT THE NORMAL TRADE WIND FLOW BY
REPLACING IT WITH LIGHT SOUTHEAST TO SOUTH WIND FLOW OVER THE
SMALLER ISLANDS. THE LIGHT WINDS COMBINED WITH PLENTY OF LOW LEVEL
MOISTURE WILL LEAD TO SCATTERED SHOWERS IN GENERAL OVER ALL OF THE
SMALLER ISLANDS...AND SOME SHOWERS COULD BE LOCALLY HEAVY. LEEWARD
BIG ISLAND WILL HAVE ACTIVE AFTERNOON SHOWERS EACH DAY THIS WEEK. A
MORE STABLE TRADE WIND REGIME RETURNS TO THE AREA THURSDAY.

&&

.DISCUSSION...
TONIGHT...
SPOTTY SHOWERS ARE PRESENT OVER INTERIOR KAUAI...CENTRAL OAHU...THE
LEE SIDE OF HALEAKALA MAUI AS OF 3 PM. THERE IS STILL QUITE A BIT OF
DAYLIGHT LEFT SO DO NOT BE SURPRISED IF MORE CONVECTIVE SHOWERS POP
UP IN THE NEXT COUPLE OF HOURS OVER INTERIOR AND LEEWARD AREAS.
SHOWERS AND THUNDERSTORMS HAVE BEEN IN ACTION OVER THE HIGHER SLOPES
AND THE 2 SUMMITS OF THE BIG ISLAND...STARTING UP JUST BEFORE NOON.
THIS ACTIVITY SHOULD START TO DIMINISH STATEWIDE AS THE SUN SETS.
OVERNIGHT...THE TRADES WILL BE STRONG ENOUGH TO CARRY A FEW SHOWERS
ONTO THE WINDWARD DISTRICTS OF THE SMALLER ISLANDS. AS FOR WINDWARD
BIG ISLAND...A NORTH TO SOUTH LINE OF SHOWERS CURRENTLY JUST OFF
CAPE KUMUKAHI...WILL MOVE ASHORE AND GIVE THE WINDWARD DISTRICTS
SOME SHOWERY WEATHER OVERNIGHT. ALONG THE KONA COAST THERE WILL BE A
FEW LINGERING SHOWERS WELL INTO OVERNIGHT HOURS BEFORE DISSIPATING
PRIOR TO DAYBREAK MONDAY.

SATELLITE IMAGERY SHOWS AN AREA OF LIGHT AND VARIABLE WINDS SOME 100
MILES NORTH OF OAHU...WHICH ALSO SUGGESTS THE LOCATION OF THE
SURFACE TROUGH. THIS TROUGH EXTENDED SOUTHWARD THROUGH THE MOLOKAI
CHANNEL.

MONDAY THROUGH TUESDAY...
SOME IMPROVEMENT IS EXPECTED BY SUNRISE MONDAY FOR WINDWARD BIG
ISLAND AS THE BAND OF SHOWERS CURRENTLY APPROACHING CAPE KUMUKAHI
DISSIPATES. AFTERNOON CLOUDS WILL LIKELY SET IN BY LATE
MORNING/EARLY AFTERNOON...WITH UPSLOPE SHOWERS DEVELOPING. THE KONA
SIDE WILL GO THROUGH ANOTHER ROUND SHOWERS AS WELL...WITH
THUNDERSTORMS OVER THE HIGHER TERRAIN AND SUMMITS.

THE LATEST MODELS...PARTICULARLY THE GFS AND ECMWF /EC/...HAVE THE
SURFACE TROUGH OVER OAHU MONDAY...THEN EASE IT WESTWARD TO JUST WEST
OF KAUAI BY LATE TUESDAY. THIS WILL BRING A CHANGE IN THE WINDS FROM
LIGHT BACKGROUND EASTERLIES TO SOUTHEAST AND SOUTH WINDS BY LATE
TUESDAY AFTERNOON. THIS SETUP TENDS TO FAVOR AFTERNOON CONVECTION
OVER INTERIOR AND LEEWARD AREAS OF THE SMALLER ISLANDS. ON MONDAY
EVENING...THE KOOLAU MOUNTAINS ON OAHU AND ADJACENT WINDWARD
COMMUNITIES MAY HAVE SOME ENHANCED SHOWERS AS A RESULT OF
CONVERGENCE OVER THE KOOLAU/S FROM A SLIGHT SOUTH OF EAST WIND FLOW.

VOLCANIC HAZE...OR VOG...IS LIKELY TO SHOW UP OVER THE WESTERN
ISLANDS ON TUESDAY AS WELL...LASTING THROUGH WEDNESDAY.

TUESDAY NIGHT AND WEDNESDAY...
THERE ARE SOME MINOR DIFFERENCES BETWEEN THE EC AND GFS. THE EC HAS
THE UPPER LOW DEPARTING THE AREA NORTH OF KAUAI TUESDAY
NIGHT...WHILE THE GFS HAS IT LEAVING 12 HOURS LATER. THE MODELS HAVE
THE SURFACE TROUGH CONTINUING TO MOVE WESTWARD...SO ALL OF THE
SMALLER ISLANDS REMAINS UNDER A SOUTHEAST TO SOUTH WIND FLOW. UNDER
THIS REGIME...SHOWERS ARE EXPECTED TO PICK UP OVER LAND DURING THE
DAY THEN BECOME ISOLATED OVERNIGHT. OVER THE WATERS HOWEVER...THIS
CONVERGENT SOUTHERLY FLOW MAY BRING A RISE IN SHOWERS OVER THE
WATERS SOUTH OF THE ISLANDS. IF THIS FLOW IS STRONG ENOUGH TO
OVERCOME THE OFFSHORE LAND BREEZE...A FEW OF SHOWERS WILL MAKE
TO LAND AND AFFECT THE LEEWARD DISTRICTS. THIS IS WHAT I AM BANKING
ON SHOULD THIS PATTERN HOLD. WEDNESDAY SHOULD BE NEARLY A REPEAT OF
TUESDAY FOR THE WESTERN ISLANDS.

WEDNESDAY NIGHT ON...
THE TRADES ARE SLATED TO START RETURNING AS EARLY AS WEDNESDAY
AFTERNOON BEGINNING WITH MAUI EASTWARD. ALL ISLANDS SHOULD BE UNDER
A LIGHT TO MODERATE TRADE WIND FLOW BY SUNRISE THURSDAY...AND THIS
FLOW WILL STRENGTHEN TO MODERATE SPEEDS BY THURSDAY EVENING...AND
PERSIST INTO THE WEEKEND.

&&

.AVIATION...
A WEAK SURFACE TROUGH OVER AND TO THE N OF THE ISLANDS WILL KEEP A
LIGHT WIND REGIME IN PLACE THROUGH TUESDAY...WITH LOCAL SEA BREEZES
AND LAND BREEZES EXPECTED IN MOST AREAS. AFTERNOON SHOWERS ARE
EXPECTED TO DEVELOP OVER THE INTERIOR AND LEEWARD LOCALES...AND
CAN/T RULE OUT A SHOWER IMPACTING A FEW OF THE TERMINALS. OVERALL
HOWEVER...PREVAILING VFR CONDITIONS ARE EXPECTED AT ALL TAF SITES.

NO AIRMETS ARE CURRENTLY IN EFFECT. AIRMET SIERRA FOR MTN OBSC MAY
BE NEEDED FOR PORTIONS OF THE ISLANDS LATER THIS AFTERNOON AS
SHOWERS BEGIN TO MOVE DOWNSLOPE.

&&

.MARINE...
NO SMALL CRAFT ADVISORIES ARE EXPECTED FOR MOST IF NOT ALL OF THE
WEEK. NO SIGNIFICANT SWELLS ARE EXPECTED THROUGH THE MIDDLE OF NEXT
WEEK AS WELL WITH SURF HEIGHTS WELL BELOW ADVISORY LEVELS.

WE DECIDED TO INCLUDE A SLIGHT CHANCE OF A THUNDERSTORM OVER THE
SURROUNDING OFFSHORE WATERS FOR THE TIME BEING...WITH THE POSSIBILITY
OF INCLUDING THE COASTAL WATERS IN THE NEXT 24 HOURS.

&&

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

&&

DISCUSSION/MARINE...LAU
AVIATION...JELSEMA