Heavy Rain in Hawaii, March 25, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST WED MAR 25 2015

.SYNOPSIS...
A FRONT NEAR KAUAI THIS MORNING WILL MOVE DOWN ACROSS THE SMALLER
ISLANDS TODAY...BEFORE REACHING THE BIG ISLAND TONIGHT. GUSTY
NORTHEASTERLY TRADE WINDS AND WET WEATHER WILL ACCOMPANY THIS FRONT.
THESE BREEZY NORTHEAST WINDS WILL CARRY LOW CLOUDS AND SHOWERS
MAINLY TO WINDWARD AND MAUKA SECTIONS OF THE STATE. THE GUSTY TRADE
WINDS AND WET WEATHER WILL CONTINUE THROUGH THURSDAY NIGHT. LIGHT TO
MODERATE TRADES WITH SLIGHTLY DRIER WEATHER IS EXPECTED ACROSS MOST
OF THE ALOHA STATE FROM FRIDAY INTO THIS WEEKEND.

&&

.DISCUSSION...
A 150 MILE WIDE BAND OF BROKEN LOW CLOUDS AND EMBEDDED SHOWERS
ASSOCIATED WITH A WEAK SURFACE FRONT IS LOCATED OVER KAUAI THIS
MORNING. THIS FEATURE IS ORIENTED NORTHEAST TO SOUTHWEST AND
TRAVELING TO THE SOUTHEAST AT 10 TO 15 MPH. IT IS EXPECTED TO REACH
OAHU BY MIDDAY...MAUI COUNTY BY LATE AFTERNOON AND THE BIG ISLAND
TONIGHT. THIS BOUNDARY WILL PRODUCE CLOUDY SKIES AND SHOWERS WHICH
WILL FOCUS OVER WINDWARD AND MAUKA AREAS. THE FLOW ALOFT OVER THE
STATE IS FROM THE NORTHWEST AND SIGNIFICANT SUBSIDENCE IS EVIDENT
ALOFT IN WATER VAPOR IMAGERY. THIS MEANS THE ATMOSPHERE OVER THE
REGION IS RELATIVELY STABLE AND NO DEEP CONVECTION IS EXPECTED.
EARLY MORNING SOUNDINGS ALSO AGREE WITH STABLE MIDLEVEL INVERSION IN
PLACE AT LIHUE AND HILO.

WINDS ARE EXPECTED TO STRENGTHEN FROM THE NORTH TO NORTHEAST NEAR
THE FRONT. THE STRONG WINDS HAS ALREADY REACHED KAUAI THIS MORNING
AND WILL SPREAD SOUTHEASTWARD TODAY AND TONIGHT. THE STRONG WINDS
ARE THE RESULT OF THE TIGHT PRESSURE GRADIENT BETWEEN THE FRONT AND
A BUILDING HIGH PRESSURE SYSTEM BEHIND THE FRONT TO THE NORTHWEST.
THE WINDS WILL FOCUS RAINFALL OVER WINDWARD AREAS...BUT MAY CARRY A
FEW SHOWERS OVER TO LEEWARD SECTIONS OF SOME OF THE SMALLER ISLANDS.
THESE BREEZY NORTHEAST WINDS WILL LIKELY CONTINUE THROUGH THURSDAY
NIGHT.

THE SURFACE WINDS ARE FORECAST TO SHIFT TO A MORE EASTERLY DIRECTION
LATE THURSDAY AS THE HIGH PRESSURE SYSTEM SLIDES TO THE NORTH OF
HAWAII AND THE FRONT DISSIPATES NEAR THE BIG ISLAND. THESE EASTERLY
TRADES WILL CONTINUE TO CARRY MOISTURE FROM THE FRONT BACK OVER
WINDWARD AND MAUKA SECTIONS OF THE STATE INTO FRIDAY. WINDWARD
SECTIONS OF MAUI AND THE BIG ISLAND WILL REMAIN PARTICULARLY WET DUE
TO THE LOCATION OF THE FRONTAL MOISTURE.

THE TRADE WINDS ARE EXPECTED TO START WEAKENING FRIDAY AS THE
SURFACE HIGH MOVES OFF TO THE NORTHEAST OF THE AREA. LIGHT TO
MODERATE TRADES ARE FORECAST SATURDAY AS A FRONT FAR TO THE NORTH OF
THE REGION WEAKENS THE SURFACE RIDGE NORTH OF THE STATE.

SATURDAY NIGHT THROUGH WEDNESDAY...MEDIUM RANGE GUIDANCE IS IN GOOD
AGREEMENT THROUGH EARLY NEXT WEEK...WITH SOLUTIONS DIVERGING A BIT
BY THE MIDDLE OF THE UPCOMING WORK WEEK. THE GFS AND ECMWF BOTH SHOW
A WEAKENING COLD FRONT APPROACHING THE ISLANDS FROM THE NORTHWEST
SATURDAY NIGHT THEN STALLING OUT ROUGHLY 100 TO 150 MILES NORTH OF
KAUAI ON SUNDAY. THIS WILL TEMPORARILY WEAKEN THE TRADE WIND FLOW
ACROSS THE ISLANDS...BUT THE DEEPEST MOISTURE AND CHANCES FOR
ENHANCED RAINFALL ASSOCIATED WITH THE FRONT SHOULD REMAIN NORTH OF
THE STATE. HIGH PRESSURE WILL THEN BUILD EASTWARD NORTH OF THE
ISLANDS BETWEEN 30N AND 40N SUNDAY NIGHT AND MONDAY...RESULTING IN A
BRIEF STRENGTHENING OF THE TRADES AND KEEPING THE FRONTAL BOUNDARY
NORTH OF THE STATE. BY THE MIDDLE OF THE NEXT WEEK...BOTH THE GFS
AND ECMWF AGREE THAT AN UPPER LEVEL TROUGH WILL APPROACH FROM THE
WEST...WEAKENING THE SURFACE RIDGING OVER THE STATE...BUT DIFFER
WITH RESPECT TO THE STRENGTH AND LOCATION OF THE SURFACE
REFLECTION/SURFACE TROUGH WHICH DEVELOPS AS A RESULT. FOR NOW WILL
FOLLOW A GENERAL MODEL COMPROMISE WHERE DETAILS ARE A BIT MORE
UNCERTAIN. NOT EXPECTING ANY SIGNIFICANT WEATHER DURING THE LONGER
RANGE FORECAST PERIODS AT THIS TIME...WITH THE FLOW BEING DOMINATED
BY TRADES OF VARYING STRENGTHS...AND SHOWERS FAVORING WINDWARD AND
MAUKA AREAS.

&&

.AVIATION...
A BAND OF SHOWERY OVERCAST LOW CLOUDS OVER KAUAI IS MOVING SOUTHEAST
AROUND 10 KT. THERE ARE WIDESPREAD MVFR CEILINGS AND VSBY WITHIN
THE BAND ON KAUAI. STRONG N TO NE WINDS ALONG AND BEHIND THE BAND
WILL PRODUCE MOD TURB BLW 080. THE CLOUD BAND WILL MOVE OVER OAHU
BEFORE NOON AND REACH MAUI COUNTY DURING AFTERNOON. AIRMET SIERRA
FOR MOUNTAIN OBSCURATION HAS ALREADY BEEN POSTED FOR KAUAI AND
OAHU. MOUNTAIN OBSCURATION WILL LIKELY DEVELOP OVER MAUI COUNTY AND
REQUIRE AN AIRMET LATER TODAY.

AIRMET TANGO FOR LOW LEVEL TURBULENCE DUE TO THE STRONG N TO NE
WINDS IS IN EFFECT FOR KAUAI AND WILL LIKELY BE NEEDED FOR OAHU AND
MAUI COUNTY STARTING THIS AFTERNOON.

&&

.MARINE...
STRONG HIGH PRESSURE BEHIND A FRONT NEAR KAUAI THIS MORNING WILL
BRING GUSTY NORTHEAST WINDS. AS THE FRONT MOVES DOWN THE ISLAND
CHAIN TODAY...WIND SPEEDS WILL REACH THE THRESHOLD FOR A SMALL CRAFT
ADVISORY /SCA/ ACROSS NEARLY ALL OF THE COASTAL WATERS. THESE
STRONG WINDS WILL LIKELY CONTINUE THROUGH TONIGHT...AND THROUGH
THURSDAY NIGHT FOR SELECT WINDY AREAS. THE WINDS ARE EXPECTED TO
DIMINISH BELOW THE SCA CRITERIA FOR MOST AREAS FROM FRIDAY AND
SATURDAY. INCREASING TRADE WINDS OVER THE TYPICALLY WINDY WATERS
ADJACENT TO THE BIG ISLAND AND MAUI COUNTY MAY ONCE AGAIN REQUIRE
A SCA STARTING SUNDAY AND MONDAY.

A SMALL TO MODERATE LONG-PERIOD SOUTH SWELL WILL MAINTAIN ELEVATED
SURF ALONG SOUTH FACING SHORES INTO TONIGHT. SURF HEIGHTS ARE
FORECAST TO REMAIN JUST BELOW THE HIGH SURF ADVISORY CRITERIA FOR
SOUTH FACING SHORES TODAY. THIS SOUTH SWELL WILL GRADUALLY SUBSIDE
FROM THURSDAY THROUGH FRIDAY.

A MODERATE NORTH NORTHWEST SWELL WILL BUILD RAPIDLY TONIGHT PEAK
EARLY THURSDAY...THEN SUBSIDE THROUGH SATURDAY. ANOTHER MODERATE
NORTHWEST SWELL WILL ARRIVE EARLY SUNDAY MORNING...PEAK SUNDAY
NIGHT...THEN GRADUALLY SUBSIDE EARLY NEXT WEEK. BASED ON THE LATEST
GUIDANCE FROM THE WAVEWATCH III MODEL...SURF HEIGHTS WILL LIKELY
REMAIN BELOW THE HIGH SURF ADVISORY CRITERIA ALONG NORTH AND WEST
FACING SHORES OF THE ISLANDS THROUGH BOTH NORTHWEST SWELL EVENTS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI CHANNEL-MAUI COUNTY
WINDWARD WATERS-MAUI COUNTY LEEWARD WATERS-MAALAEA BAY- PAILOLO
CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS.

SMALL CRAFT ADVISORY FROM 6 PM THIS EVENING TO 6 AM HST THURSDAY
FOR BIG ISLAND SOUTHEAST WATERS.

&&

DISCUSSION...FOSTER/JELSEMA
MARINE...FOSTER
AVIATION...DONALDSON