Heavy Rain in Hawaii, April 11, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
333 PM HST SAT APR 11 2015

.SYNOPSIS...
MODERATE TRADE WINDS WILL BECOME LOCALLY BREEZY SUNDAY INTO EARLY
NEXT WEEK...AS HIGH PRESSURE BUILDS FAR NORTH AND NORTHEAST OF
THE STATE. AREAS OF ENHANCED MOISTURE EMBEDDED IN THE TRADES WILL
CONTINUE TO GENERATE SHOWERS AND CLOUDS MAINLY OVER WINDWARD
AREAS...WITH CLOUDINESS AND SCATTERED SHOWERS AFFECTING SOME
LEEWARD AREAS AT TIMES. A DEVELOPING UPPER LEVEL TROUGH MAY BRING
INCREASED SHOWERS AND CLOUDS STATEWIDE DURING THE MIDDLE AND LATER
PART OF NEXT WEEK.

&&

.DISCUSSION...
THERE IS QUITE A BIT OF VARIATION IN THE CURRENT WEATHER ACROSS
THE ISLANDS THIS AFTERNOON. KAUAI HAS BEEN MOSTLY CLOUDY DURING
THE DAY WITH QUITE A BIT OF SHOWER ACTIVITY...EVEN SPILLING INTO
LEEWARD SECTIONS. OAHU AND MAUI COUNTY HAVE BEEN A GOOD BIT DRIER
WITH MORE SUNSHINE...EXCEPT FOR THE WINDWARD HALEAKALA SLOPES WHERE
SHOWERS AND CLOUDS PERSIST. ON THE BIG ISLAND...CLOUDS EXTEND FAR
UP THE SLOPES ALMOST TO THE SUMMITS...BUT RADAR AND RAIN GAUGE DATA
SHOW VERY LITTLE RAINFALL DURING THE PAST SEVERAL HOURS.

SURFACE ANALYSIS DEPICTS A 1029 MB HIGH ABOUT 900 MILES NORTH OF
OAHU...WITH MODERATE TRADES PREVAILING OVER THE ISLANDS TO THE
SOUTH OF THIS HIGH. ZONAL FLOW PREVAILS ALOFT...TO THE SOUTH OF AN
UPPER TROUGH NEAR THE DATELINE AND ANOTHER SMALL TROUGH A FEW
HUNDRED MILES NORTH OF THE STATE. MIMIC-TPW SATELLITE IMAGERY
SHOWS A POCKET OF ENHANCED MOISTURE OVER AND SW OF THE BIG
ISLAND AND MAUI...WITH ANOTHER POCKET OF HIGHER MOISTURE PASSING
JUST NORTH OF KAUAI...AND A SOMEWHAT DRIER AIRMASS OVER THE
CENTRAL ISLANDS. THE 00Z LIHUE SOUNDING SHOWED AN ELEVATED
INVERSION NEAR 9K FEET AND PWAT OF 1.14 INCHES. THE 00Z HILO
SOUNDING WAS DRIER THAN THIS MORNING WITH PWAT OF 1.21 INCHES...
BUT FOUND WEAK/HIGH DOUBLE INVERSIONS BASED NEAR 7K AND 11K FEET.

FOR TONIGHT...MODERATE TRADES WILL CONTINUE...WITH SHOWERS
REMAINING FOCUSED ON WINDWARD SLOPES. ALTHOUGH SOME WINDWARD AREAS
HAVE NOT SEEN MUCH RAIN DURING THE DAY...EXCEPT THAT SHOWERS WILL
LIKELY INCREASE WINDWARD OVERNIGHT IN THE TYPICAL NOCTURNAL MANNER.
SCATTERED SHOWERS MAY SPILL OVER INTO SOME LEEWARD AREAS. WOULD NOT
BE SURPRISED TO SEE AN ISOLATED HEAVIER DOWNPOUR SOMEWHERE ALONG
THE KONA COAST LATE THIS AFTERNOON OR THIS EVENING.

TRADES WILL BEGIN TO INCREASE ONCE AGAIN SUNDAY THROUGH MONDAY...
S A NEW HIGH BUILDS OVER THE NORTHEAST PACIFIC. PATCHES OF
SLIGHTLY ENHANCED MOISTURE EMBEDDED IN THE TRADES WILL PASS
THROUGH THE STATE AT TIMES...BUT NO MAJOR CHANGES IN AIRMASS ARE
EXPECTED. OVERALL EXPECT LITTLE CHANGE TO THE PREVAILING TRADE
WIND PATTERN...WITH SHOWERS AND CLOUDS REMAINING FOCUSED ON
WINDWARD AND MAUKA AREAS...AND MOST PREVALENT DURING THE NIGHT
AND EARLY MORNING HOURS. CLOUDS AND ISOLATED SHOWERS WILL DEVELOP
EACH AFTERNOON ON THE KONA COAST AS WELL.

MODELS SUGGEST THAT A SIGNIFICANTLY WETTER AND CLOUDIER TRADE WIND
PATTERN MAY DEVELOP ACROSS THE STATE DURING THE MIDDLE AND LATER
PART OF NEXT WEEK. THE LATEST GFS AND ECMWF DEVELOP A MID/UPPER
LEVEL TROUGH JUST TO OUR WEST DURING THIS TIME...DESTABILIZING THE
ATMOSPHERE AND DRAWING UP DEEPER TROPICAL MOISTURE OVER THE STATE.
THIS TROUGH ALSO BRINGS EXTENSIVE HIGH LEVEL MOISTURE OVERHEAD
THROUGH THE SAME PERIOD...AND WE COULD SEE SOME DAYS WITH LITTLE
SUNSHINE EVEN ACROSS LEEWARD AREAS DURING THE SECOND HALF OF THE
WEEK. IT SHOULD BE NOTED THAT THE PREVIOUS 00Z EC RUN HAD THE
UPPER TROUGH DEVELOPING A FEW HUNDRED MILES FURTHER TO THE
EAST...LEAVING THE STATE ON THE DRY SIDE OF THE TROUGH WHICH WOULD
MAKE A HUGE DIFFERENCE IN THE SENSIBLE WEATHER. FOR NOW...HAVE
NUDGED THE FORECAST SOMEWHAT WETTER FOR THE MIDDLE AND LATER PART
OF NEXT WEEK...ESPECIALLY FOR WINDWARD AREAS. WILL CONTINUE TO
MONITOR LATER MODEL RUNS DURING THE NEXT FEW DAYS.

&&

.AVIATION...
VFR CONDITIONS ARE EXPECTED TO PREVAIL FOR MOST LOCATIONS OVERNIGHT
WITH SOME MVFR CONDITIONS POSSIBLE OVER WINDWARD SLOPES DUE TO LOW
CLOUDS AND SHRA.

&&

.MARINE...
SIGNIFICANT WAVE HEIGHTS HAVE HELD AROUND 7 FEET WITH 8 SECOND
PERIODS AT THE HILO...PAUWELA AND MOKAPU POINT CDIP BUOYS DURING
THE DAY. THIS WAVE ENERGY IS SPREAD OUT IN A BROAD NE/E
DIRECTIONAL BAND HOWEVER...AND WHILE SURF ON EAST FACING SHORES
WILL BE ROUGH/CONFUSED...EXPECT SURF HEIGHTS TO REMAIN BELOW
ADVISORY LEVELS. REPORTED SURF HEIGHTS ALONG THOSE SHORES
GENERALLY RANGED FROM 3 TO 6 FEET TODAY. SURF SHOULD DECREASE
SLIGHTLY ON SUNDAY...THEN INCREASE AGAIN EARLY NEXT WEEK AS THE
LONG FETCH OF TRADE WINDS BUILDS AGAIN TO THE EAST-NORTHEAST OF
THE STATE.

OTHERWISE...SMALL SURF WILL CONTINUE ON NORTH AND WEST FACING
SHORES THROUGH TUESDAY. A LONG PERIOD NW SWELL MAY BRING ADVISORY
LEVEL SURF TO THESE SHORES FROM LATE WEDNESDAY INTO THURSDAY. ON
SOUTH FACING SHORES...SMALL SWELL AND SURF WILL CONTINUE THROUGH
THE NEXT SEVERAL DAYS.

THE LATE MORNING ASCAT PASS MISSED MOST OF THE HAWAIIAN COASTAL
WATERS...BUT DID FIND SOME 20 KT WINDS SOUTH OF THE BIG ISLAND...
AS DID AN EARLIER RAPIDSCAT PASS. WINDS ARE PROBABLY JUST BELOW
ADVISORY LEVELS IN THE TYPICALLY WINDY WATERS AROUND MAUI COUNTY
AND THE BIG ISLAND AT PRESENT...BUT HIGH-RESOLUTION MODELS
INDICATE THAT WINDS WILL INCREASE ACROSS THESE AREAS ONCE AGAIN
SUNDAY AND SUNDAY NIGHT AS HIGH PRESSURE BUILDS FAR TO OUR NORTH.
A SMALL CRAFT ADVISORY HAS BEEN POSTED FOR SOME OF THE ZONES
AROUND MAUI COUNTY EFFECTIVE 6 AM SUNDAY MORNING. THE ADVISORY
WILL LIKELY NEED TO BE EXPANDED TO COVER THE WATERS SOUTH OF THE
BIG ISLAND DURING LATER SHIFTS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY FROM 6 AM SUNDAY TO 6 AM HST TUESDAY FOR
MAALAEA BAY-PAILOLO CHANNEL-ALENUIHAHA CHANNEL.

&&

DISCUSSION/MARINE...JACOBSON
AVIATION...BURKE