Heavy Rain in Hawaii, March 31, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST MON MAR 30 2015

.SYNOPSIS...
EXPECT THE CURRENT MODERATE EASTERLY WINDS TO WEAKEN AND SHIFT TO A
MORE SOUTHEASTERLY FLOW STARTING TONIGHT AND THROUGH TUESDAY. THIS
WILL ALLOW FOR PERIODS OF DAYTIME SEA BREEZES AND SCATTERED
SHOWERS ACROSS LEEWARD AND INTERIOR SECTIONS TUESDAY AFTERNOON AND
EARLY EVENING. BEST CHANCES OF RAIN WILL BE OVER KAUAI AND OAHU.
THE EASTERLY TRADES WILL RETURN ON WEDNESDAY...WITH SHOWERS AGAIN
FAVORING WINDWARD AND MAUKA AREAS DURING THE LATER PART OF THE WEEK.

&&

.DISCUSSION...
BROAD SFC HIGH PRESSURE N OF THE MAIN HAWAIIAN ISLANDS IS KEEPING A
MODERATE TRADE WIND REGIME ACROSS THE AREA. AT THE UPPER LVLS...A
SHORT WAVE TROUGH RIDING ALONG THE SUBTROPICAL JET FAR N OF THE
ISLANDS WILL MOVE CLOSER TO THE STATE AND SUPPORT THE DEVELOPMENT
OF A LOW LEVEL TROUGH...WHICH WILL GRADUALLY MOVE TOWARDS THE
ISLAND CHAIN TONIGHT. THIS SYNOPTIC SCENARIO WILL WEAKEN AND VEER
THE TRADES TO A MORE SE FLOW STARTING TONIGHT...AS WELL AS WEAKEN
THE INVERSIONS CURRENTLY DEPICTED BY THE 00Z SOUNDINGS AT AROUND 8K
FT.

EXPECT BEST CHANCES FOR RAIN TO REMAIN OVER WINDWARD AND MAUKA
AREAS OF THE SMALLER ISLANDS THROUGH TUE MORNING. AT 8 PM HST DATA
FROM THE DOPPLER RADAR NETWORK WAS SHOWING THE MORE ACTIVE SHOWERS
FOCUSING OVER THE S AND CENTRAL HALF OF THE KOOLAU RANGE IN
OAHU...THE E MOLOKAI MOUNTAINS AND THE W MAUI MOUNTAINS. SHOWERS
ALSO LINGER OVER INTERIOR KAUAI. HOWEVER...THE E/SE FLOW WILL REMAIN
STRONG ENOUGH TO PUSH SOME OF THE LARGER SHOWERS INTO LEEWARD
LOCATIONS AT TIMES...INCLUDING THE HONOLULU METRO AREA TONIGHT.

BY TUE LATE MORNING AND INTO THE AFTERNOON...THE INFLUENCE OF THE
AFOREMENTIONED SHORTWAVE FEATURE AND THE ASSOCIATED LOW LVL TROUGH
WILL CAUSE A DISRUPTION OF THE SE FLOW ESPECIALLY ACROSS LEEWARD
AREAS OF THE SMALLER ISLANDS. THUS...DAYTIME SEA BREEZE CIRCULATIONS
ARE LIKELY TO DEVELOP TUE AFTERNOON AND EARLY EVENING...WHICH WILL
BRING CLOUD BUILD-UPS AND LOCALIZED SHOWERS OVER LEEWARD AND
INTERIOR AREAS. ENHANCED MOISTURE ACCOMPANYING THE TROUGH WILL ALSO
INCREASE CHANCES FOR LOCALIZED HEAVY RAIN...ALTHOUGH ANY DOWNPOURS
THAT MAY DEVELOP SHOULD BE BRIEF.

THE SHORTWAVE AND LOW LVL TROUGH QUICKLY MIGRATE TO THE NE TUE
NIGHT AND INTO WED...ALLOWING FOR THE TRADES TO RE-ESTABLISH ACROSS
THE AREA. THIS WILL BRING BACK THE MORE TYPICAL WINDWARD/MAUKA
SHOWERS FOR THE REST OF THE WORK WEEK. HOWEVER...THE GLOBAL MODELS
SHOW DIFFERENCES IN THE MID/LONG RANGE SOLUTIONS...WITH GFS BEING
MORE AGGRESSIVE IN PUSHING ANOTHER UPPER LVL TROUGH TO THE N OF THE
STATE ON THE THU/FRI TIMEFRAME. THIS SOLUTION BRINGS THE TROUGH
CLOSE ENOUGH TO KAUAI TO AGAIN DISRUPT THE TRADES. THE REST OF
GUIDANCE...ESPECIALLY ECMWF...DOWNPLAY THIS SCENARIO AND KEEP THE
TRADES IN PLACE THROUGH FRI. ATTM...THE OFFICIAL FORECAST WILL
CONTINUE TO FOLLOW THE ECMWF SOLUTION WITH A TRADE WIND WEATHER
REGIME PREVAILING THROUGH THE REST OF THE WORK WEEK. BUT THE
FORECAST PHILOSOPHY COULD CHANGE IF FUTURE MODEL RUNS BEGIN
TRENDING TOWARDS GFS. MODELS ALSO SUGGEST THE POTENTIAL FOR AN AREA
OF ENHANCED MOISTURE APPROACHING THE STATE FROM THE SE FOR THE
UPCOMING WEEKEND...WHICH COULD BRING AN INCREASE IN SHOWERS
ESPECIALLY OVER THE BIG ISLAND.

&&

.AVIATION...
A SURFACE TROUGH DEVELOPING NW OF KAUAI WILL BE NEAR THE ISLANDS
TUESDAY...WITH TRADE WINDS DIMINISHING ACROSS THE AREA. DEEPENING
BOUNDARY LAYER MOISTURE AND INCREASED INSTABILITY WILL PRODUCE TEMPO
MVFR VIS/CIG SHRA AND POTENTIALLY ISOL IFR VIS/CIG IN +SHRA.
ALTHOUGH CLOUDS AND SHRA WILL REMAIN FOCUSED ACROSS WINDWARD AND
MAUKA AREAS FOR MUCH OF THE PERIOD...AFTERNOON CLOUDS AND SHRA ARE
EXPECTED OVER LEEWARD AND INTERIOR PORTIONS OF KAUAI AND OAHU
TUESDAY. AIRMET SIERRA FOR MTN OBSC IS IN PLACE FOR MOST AREAS...AND
IS EXPECTED TO REMAIN IN PLACE INTO TUESDAY.

WHILE VFR WILL PREVAIL AT THE TERMINALS...PERIODS OF MVFR VIS/CIG
IN SHRA CAN BE EXPECTED AT PHTO AND PHLI.

&&

.MARINE...
SMALL CRAFT ADVISORY CONDITIONS WILL CONTINUE OVER THE ALENUIHAHA
CHANNEL AND THE WATERS SOUTH OF THE BIG ISLAND THROUGH TONIGHT.
AFTER A BRIEF DISRUPTION OF THE TRADES ON TUESDAY...WINDS WILL
LIKELY INCREASE INTO ADVISORY LEVELS OVER THE TYPICALLY WINDER
WATERS AROUND THE BIG ISLAND AND MAUI COUNTY ON WEDNESDAY.

THE CURRENT NORTHWEST SWELL WILL CONTINUE TO DECREASE ON TUESDAY AND
FADE BY WEDNESDAY. A SMALL TO MODERATE SOUTH-SOUTHEAST SWELL WILL
ALSO BE ON A GRADUAL DECLINE OVER THE NEXT COUPLE OF DAYS...FOLLOWED
BY A SMALL SOUTH-SOUTHWEST SWELL FOR THE SECOND HALF OF THE WORK
WEEK. ELEVATED WIND SWELL ALONG EAST FACING SHORES WILL DROP A NOTCH
ON TUESDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR THE ALENUIHAHA
CHANNEL AND BIG ISLAND LEEWARD WATERS.

&&

DISCUSSION...REYNES
AVIATION...BIRCHARD