Heavy Rain in Hawaii, May 22 to 23, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1015 AM HST FRI MAY 22 2015

.SYNOPSIS...
A WEAK COLD FRONT WILL MOVE OVER KAUAI THIS AFTERNOON...OAHU THIS
EVENING...AND MAUI COUNTY OVERNIGHT AND SATURDAY. THE FRONT WILL
REACH WINDWARD BIG ISLAND SATURDAY AND SATURDAY NIGHT BEFORE
DISSIPATING SOUTH OF THE BIG ISLAND ON SUNDAY. A BRIEF PERIOD OF
BREEZY NORTHEAST WINDS WILL ACCOMPANY THE FRONT...THEREBY FOCUSING
MOST OF ITS ASSOCIATED MOISTURE ALONG WINDWARD SLOPES AND COASTS.
AFTER THE FRONT PASSES...LIGHT TO MODERATE NORTHEAST TO NORTH WINDS
WILL PREVAIL INTO NEXT WEEK...DELIVERING BRIEF WINDWARD SHOWERS...
MAINLY DURING NIGHTS AND MORNINGS.

&&

.DISCUSSION...
A 100 TO 150 MILE WIDE ENE-WSW ORIENTED BAND OF BROKEN TO OVERCAST
SHOWERY LOW CLOUDS MARKS A COLD FRONT THAT IS EASING SOUTHWARD
TOWARD THE ISLANDS THIS MORNING. THE LEADING EDGE OF THE FRAGMENTED
CLOUD BAND IS NEAR KAUAI AT 10 AM...WITH THE BOUNDARY MOVING SOUTH
AT SPEEDS BETWEEN 10 AND 15 MPH. THE FRONT IS FORECAST TO MOVE OVER
KAUAI TODAY...OAHU/MAUI COUNTY TONIGHT...AND THE BIG ISLAND ON
SATURDAY BEFORE FINALLY BREAKING APART SE OF THE BIG ISLAND ON
SUNDAY. WINDS ARE LIGHT AND VARIABLE AT THE SURFACE AHEAD OF THE
FRONT...AND WILL SWITCH TO THE NNE AFTER THE FRONT PASSES...AND THUS
MOST OF THE SHOWERS ASSOCIATED WITH THE BOUNDARY WILL FOCUS ALONG N
AND E FACING SLOPES AND COASTS. WITH MINIMAL SUPPORT FROM ALOFT...
EXCESSIVE RAINFALL IS NOT EXPECTED WITH THE FRONTAL PASSAGE.

AFTER THE FRONT PASSES...NE WINDS WILL EASE AND GENERALLY REMAIN
LIGHT TO MODERATE INTO THE MIDDLE OF NEXT WEEK. THE FRONT...AND
FOLLOWING NE WINDS...WILL USHER IN A SLIGHTLY DRIER AIR MASS...WITH
DEW POINTS DROPPING ABOUT 5 DEGREES F.

THE LOW WITH WITH WHICH THE FRONT IS ASSOCIATED IS CURRENTLY ABOUT
700 MILES NNE OF THE ISLANDS...AND MOVING NE...BUT THIS MOTION IS
EXPECTED TO HALT LATER TODAY. THE LOW IS THEN EXPECTED TO MOVE S AND
WEAKEN THROUGH THE WEEKEND...AS HIGH PRESSURE BUILDS FAR NW OF THE
AREA. THIS WILL KEEP A RELATIVELY COOL NE WIND FLOW OVER THE AREA...
BUT THE FLOW WILL BECOME QUITE LIGHT AT TIMES...ESPECIALLY AS WE
APPROACH THE MIDDLE PART OF NEXT WEEK. SHALLOW BANDS OF MOISTURE
ROTATING AROUND THE LOW WILL FAVOR THE N AND E FACING SLOPES...BUT
THE ATMOSPHERE WILL REMAIN RELATIVELY STABLE THROUGH THE PERIOD. GFS
CONTINUES TO INDICATE THAT ONE SUCH WEAK BAND WILL MOVE OVER THE
ISLANDS SUNDAY AND SUNDAY EVENING. OTHERWISE...LAND AND SEA BREEZES
MAY DOMINATE THE ISLAND WIND/WEATHER PATTERN EARLY NEXT WEEK WHEN
THE SYNOPTIC SCALE FLOW BECOMES QUITE WEAK. THE LOW WILL EXIT TO THE
NE BY THE SECOND HALF OF NEXT THE WEEK ALLOWING THE RETURN OF LIGHT
TO MODERATE TRADE WINDS BY THE END OF THE WEEK.

&&

.AVIATION...
A WEAKENING COLD FRONT WILL MOVE THROUGH KAUAI THIS AFTERNOON AND
MAUI COUNTY TONIGHT. WINDS AHEAD OF THE FRONT REMAIN LIGHT AND SEA
BREEZES WILL TAKE HOLD BY THIS AFTERNOON. MODERATE NORTH TO
NORTHEAST WINDS WILL FILL IN BEHIND THE FRONT.

SEA BREEZES WILL INTRODUCE THE CHANCE FOR INTERIOR CLOUDS AND
SHOWERS THIS AFTERNOON...ESPECIALLY OVER LEEWARD BIG ISLAND.
TOWERING CUMULUS WITH TOPS REACHING 20 KFT CAN NOT BE RULED OUT. AS
THE FRONT PASSES INDIVIDUAL ISLANDS...CLOUDS AND SHOWERS WILL BEGIN
TO FOCUS ALONG WINDWARD AND MOUNTAIN AREAS. MOUNTAIN OBSCURATION
COULD BECOME A PROBLEM THIS AFTERNOON AS THE FRONT MOVES DOWN THE
ISLAND CHAIN.

GUIDANCE IS SUGGESTING UPPER LEVEL LIGHT TO MODERATE TURBULENCE
PASSING WEST TO EAST OVER THE ISLANDS AS THE FRONT MOVED DOWN THE
ISLAND CHAIN. A FEW PILOT REPORTS FROM THIS MORNING PLACE THE
TURBULENCE BETWEEN FL300 AND FL390. AIRMET TANGO FOR UPPER LEVEL
TURB IS POSTED.

&&

.MARINE...
THE EXPECTATION HAS BEEN THAT WINDS AND SEAS WILL REMAIN BELOW SMALL
CRAFT ADVISORY /SCA/ CRITERIA FOR ALL WATERS INTO NEXT WEEK...BUT
NOW THERE IS A SLIGHT CAVEAT. LATEST HI-RES GUIDANCE HOWEVER
INDICATES THAT A BRIEF SURGE IN NE WINDS IN THE WAKE OF THE FRONTAL
PASSAGE COULD BRING A PERIOD OF NEAR 25 KT WINDS TO KAUAI LEEWARD
WATERS TONIGHT AND SATURDAY...AND A SHORT-LIVED SCA MAY BE POSTED
LATER TODAY FOR THAT AREA.

AN EXTENDED RUN OF S SWELL...GENERATED BY SEVERAL LOWS IN THE DEEP S
PACIFIC OVER THE LAST WEEK...WILL BRING SOMEWHAT ELEVATED SURF TO S
FACING SHORES INTO THE END OF NEXT WEEK. SURF HEIGHTS ARE EXPECTED
TO REMAIN BELOW ADVISORY LEVELS FOR THE BULK OF THE TIME...BUT
OVERLAPPING SWELLS FROM SUNDAY INTO TUESDAY COULD BRING SURF TO NEAR
ADVISORY-LEVELS ALONG EXPOSED S FACING SHORES. A COUPLE OF SMALL
SWELLS FROM THE NNW AND WNW ARE EXPECTED OVER THE WEEKEND AND EARLY
NEXT WEEK...WITH RESULTANT SURF WELL BELOW ADVISORY LEVELS.

&&

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

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...EATON