Flash Flood and Heavy Rain in Hawaii, May 11 to 13, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST SAT MAY 11 2013

.SYNOPSIS...
NORTHEASTERLY TRADE WINDS WILL CONTINUE TO GRADUALLY STRENGTHEN
TODAY...WITH MODERATE TO FRESH TRADE WINDS EXPECTED TO PERSIST
THROUGH MOST OF NEXT WEEK. THE RETURNING TRADE WINDS ARE EXPECTED TO
BE WETTER THAN NORMAL THROUGH MONDAY...AS MOISTURE NEAR THE ISLANDS
BECOMES ENHANCED...AND IS PUSHED OVER WINDWARD FACING COASTS AND
SLOPES. A MORE TYPICAL EASTERLY TRADE WIND WEATHER PATTERN IS
EXPECTED BY TUESDAY...WITH WINDWARD LOW CLOUDS AND SHOWERS FAVORING
NIGHTS AND MORNINGS.

&&

.DISCUSSION...
A DISSIPATING NORTHEAST TO SOUTHWEST ORIENTED FRONTAL BOUNDARY
APPEARS TO BE NEARLY STATIONARY JUST NORTHEAST OF THE BIG ISLAND
EARLY THIS MORNING. AS A RESULT...BANDS OF BROKEN TO OVERCAST LOW
CLOUDS WITH EMBEDDED SHOWERS LIE ACROSS THE ISLANDS AND ADJACENT
COASTAL WATERS ACCORDING TO SATELLITE IMAGERY AND RADAR REFLECTIVITY
DATA. THE GREATEST COVERAGE OF LOW CLOUDS AND PRECIPITATION IS
CURRENTLY OVER WINDWARD MAUI...MOLOKAI AND THE BIG ISLAND AND THE
ADJACENT COASTAL WATERS. ELSEWHERE...A 1028 MB SURFACE HIGH IS NEAR
34N 171W...OR MORE THAN 1000 MILES NORTHWEST OF LIHUE. A SURFACE
RIDGE EXTENDS SOUTHEAST FROM THIS HIGH...WHICH IS MOVING EAST AT
ABOUT 10 MPH...THROUGH A POINT MORE THAN 500 MILES NORTH OF HILO. AS
THE PRESSURE GRADIENT INCREASES NORTH OF THE ISLANDS...MOST SURFACE
OBSERVATIONS ARE INDICATING A TREND TOWARD STRENGTHENING
NORTHEASTERLY WINDS ACROSS THE MOST OF THE SMALLER HAWAIIAN ISLANDS
AND THE NORTHERN BIG ISLAND EARLY THIS MORNING.

ALOFT...A MID-TROPOSPHERIC TROUGH IS IN THE VICINITY OF THE EASTERN
ISLANDS. THIS SYSTEM IS SUPPORTING THE WEAK FRONT NORTHEAST OF THE
BIG ISLAND. WATER VAPOR IMAGERY INDICATES AN UPPER LEVEL HIGH IS
CENTERED ABOUT 475 MILES WEST OF LIHUE. AN UPPER LEVEL RIDGE EXTENDS
EAST FROM THIS HIGH ACROSS THE THE MAIN HAWAIIAN ISLANDS. THE UPPER
LEVEL HIGH APPEARS TO BE MOVING SLOWLY EASTWARD. THE WATER VAPOR
IMAGERY ALSO SHOWS UPPER LEVEL SUBSIDENCE ABOVE MOST OF THE HAWAIIAN
ISLANDS...WHICH IS CONTRIBUTING TO THE RELATIVELY STABLE ATMOSPHERIC
CONDITIONS BEING EXPERIENCED EARLY THIS MORNING ACROSS MOST OF THE
ALOHA STATE.

THE FORECAST MODELS INDICATE THE SURFACE HIGH NORTHWEST OF KAUAI
WILL CONTINUE TO MOVE EASTWARD THIS WEEKEND. AS A RESULT...THE
PRESSURE GRADIENT WILL CONTINUE TO INCREASE ACROSS THE ISLANDS. THIS
WILL CAUSE THE NORTHEASTERLY TRADE WINDS TO GRADUALLY STRENGTHEN
FROM WEST TO EAST ACROSS THE SMALLER ISLANDS...WITH THE INCREASING
NORTHEASTERLY TRADES SPREADING TO THE BIG ISLAND LATER TODAY AND
TONIGHT. THE TRADE WINDS WILL LIKELY VEER TO A MORE EAST
NORTHEASTERLY DIRECTION BY LATE SUNDAY AS THE HIGH CONTINUES TO MOVE
EAST.

AS THE TRADES INCREASE AND SLOWLY VEER TO THE EAST NORTHEAST...
EXPECT THE REMNANT MOISTURE ASSOCIATED WITH THE DISSIPATING FRONT
NORTHEAST OF THE BIG ISLAND TO BE PUSHED BACK WESTWARD ACROSS THE
ISLANDS. IN ADDITION...PERSISTENT LOWER TROPOSPHERIC TROUGHING IN
THE 850-700 MB LAYER WILL ENHANCE THE LOW CLOUDS AND SHOWERS...
WHICH IS EXPECTED TO RESULT IN A RATHER WET TRADE WIND PATTERN
THROUGH MONDAY. THE FORECAST MODELS CONTINUE TO INDICATE THE WET
WEATHER WILL MAINLY AFFECT WINDWARD OAHU AND MAUI COUNTY AND THE
NORTHERN SECTIONS OF THE WINDWARD BIG ISLAND TODAY AND SUNDAY. THE
AREA OF ENHANCED LOW CLOUDS AND SHOWERS THEN APPEARS TO SHIFT
WESTWARD OVER WINDWARD OAHU AND KAUAI ON MONDAY...BEFORE DEPARTING
TO THE NORTHWEST OF THE AREA ON TUESDAY.

THE FORECAST MODELS APPEAR TO DEPICT A WEAK SURFACE TROUGH
DEVELOPING NEAR AND NORTH OF THE ISLANDS. THE CURRENT FORECAST TAKES
THIS FEATURE INTO ACCOUNT...SO THERE MAY BE A BRIEF PERIOD OF WEAKER
TRADES VEERING TO AN EAST SOUTHEAST DIRECTION ACROSS PARTS OF THE
STATE EARLY NEXT WEEK. AS THE EASTERLY TRADE WINDS BECOME
REESTABLISHED AROUND TUESDAY...EXPECT LOW CLOUDS AND SHOWERS TO
MAINLY FAVOR WINDWARD FACING COAST AREAS AND SLOPES...ESPECIALLY
DURING THE NIGHT TIME AND MORNING HOURS.

THE FORECAST MODELS ALSO CONTINUE TO SUGGEST AN UPPER LEVEL TROUGH
MAY DEVELOP IN THE VICINITY OF THE ISLANDS AROUND MID-WEEK.
HOWEVER...THE GUIDANCE DOES NOT APPEAR TO SHOW A REFLECTION OF THIS
UPPER LEVEL FEATURE IN THE MID-TROPOSPHERE. THEREFORE...THE TROUGH
ALOFT IS NOT EXPECTED TO CAUSE A SIGNIFICANT ENHANCEMENT OF TRADE
WIND SHOWER COVERAGE OR INTENSITY IN THE MIDDLE OF NEXT WEEK.

&&

.AVIATION...
VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS MOST THE STATE
TODAY...BUT BRIEF MVFR CIGS AND MTN OBSC ARE EXPECTED AT TIMES DUE
TO THE ABUNDANT LOW LEVEL MOISTURE LEFT BEHIND BY AN OLD FRONTAL
CLOUD BAND. THESE ISOLATED MVFR PERIODS MOST LIKELY OVER THE
WINDWARD AND MAUKA AREAS DUE TO THE RECENT RETURN OF THE TRADE WINDS.

&&

.MARINE...
MODERATE NORTHEASTERLY TRADE WINDS AND RELATIVELY SMALL SWELLS ARE
EXPECTED TO KEEP CONDITIONS ACROSS THE HAWAIIAN COASTAL WATERS BELOW
THE SMALL CRAFT ADVISORY /SCA/ CRITERIA THROUGH THIS AFTERNOON.
HOWEVER...INCREASING WIND SPEEDS MAY BRING SCA CONDITIONS TO THE
TYPICAL WINDY WATERS ADJACENT TO THE BIG ISLAND AND THE ISLANDS OF
MAUI COUNTY STARTING TONIGHT. THIS SCA IS EXPECTED TO REMAIN IN
EFFECT THROUGH SUNDAY NIGHT. WINDS AND SEAS MAY DIMINISH SLIGHTLY
EARLY NEXT WEEK...BUT WILL LIKELY INCREASE AGAIN OVER THESE
TYPICALLY WINDIER WATERS BY MID-WEEK.

A SMALL TO MODERATE...RELATIVELY SHORT-PERIOD...NORTH NORTHWEST
SWELL WILL REMAIN IN PLACE THIS WEEKEND BEFORE DIMINISHING MONDAY.
THE WAIMEA BAY BUOY IS CURRENTLY SHOWING A SIGNIFICANT WAVE HEIGHT
OF 6 TO 7 FEET WITH A WAVE PERIOD OF 11 SECONDS. A SMALL SOUTH SWELL
WITH A LONG WAVE PERIOD IS ALSO EXPECTED TO CONTINUE THIS
WEEKEND...WITH A LARGER AND LONGER PERIOD SOUTH SWELL FORECAST TO
BUILD FROM LATE WEDNESDAY NIGHT INTO THURSDAY. THIS SECOND SWELL
ARRIVING FROM THE SOUTHERN HEMISPHERE WILL LIKELY CAUSE SURF TO
REACH THE HIGH SURF ADVISORY CRITERIA ALONG SOUTH FACING SHORES OF
THE ENTIRE ISLAND CHAIN FROM EARLY THURSDAY MORNING THROUGH FRIDAY.

&&

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

&&

DISCUSSION/MARINE...HOUSTON
AVIATION...FUJII