High Surf in Hawaii, April 20 to 21, 2016

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
950 AM HST WED APR 20 2016

.SYNOPSIS...
MODERATE TO LOCALLY BREEZY TRADE WINDS WILL DOMINATE OUR WEATHER FOR
THE NEXT WEEK...AS HIGH PRESSURE PREVAILS NORTH AND NORTHEAST OF THE
ISLANDS. THE TRADE WINDS WILL DELIVER RANDOM CLUSTERS OF CLOUDS AND
SHOWERS TO WINDWARD AREAS...ESPECIALLY DURING NIGHTS AND MORNINGS.
WHILE LEEWARD AREAS MAY SEE SOME OCCASIONAL PERIODS OF CLOUDINESS...
LITTLE IN THE WAY OF RAINFALL IS EXPECTED.

&&

.DISCUSSION...
HIGH PRESSURE WILL PREVAIL N AND NE OF THE ISLANDS THROUGH THE
FORECAST PERIOD...DRIVING A MODERATE TO LOCALLY BREEZY TRADE WIND
FLOW OVER THE ISLANDS. RANDOM CLUSTERS OF STABLE AND SHOWERY LOW
CLOUDS WILL STREAM OVER THE ISLANDS FROM THE E...FOCUSING OVER
WINDWARD SLOPES AND COASTS. A MID-LEVEL RIDGE IS EXPECTED TO MOVE
OVER THE ISLANDS LATER TODAY AND REMAIN IN PLACE THROUGH FRIDAY
BEFORE GRADUALLY MOVING N EARLY NEXT WEEK. THIS FEATURE WILL ENSURE
THAT THE INCOMING LOW-LEVEL MOISTURE REMAINS CAPPED BELOW 8 THOUSAND
FEET OR SO...AND THUS HEAVY SHOWERS ARE NOT EXPECTED. HOWEVER...
STEEP LAPSE RATES BELOW THE SUBSIDENCE INVERSION WILL ALLOW TRADE
WIND CUMULI TO PRODUCE A FEW SHOWERS...AS WILL ISLAND TOPOGRAPHY.

A BIT OF A WEAKNESS IN THE RIDGE EXPECTED FRIDAY AND SATURDAY WILL
ALLOW TRADE WIND SPEEDS TO DIMINISH SLIGHTLY...BUT NOT ENOUGH TO
ALLOW WIDESPREAD LAND AND SEA BREEZE DEVELOPMENT. HOWEVER...SEA
BREEZES ALONG LEEWARD COASTS MAY DRIVE THE DEVELOPMENT OF AFTERNOON
CLOUDINESS THROUGH THE PERIOD...WHICH MAY BE ESPECIALLY NOTICEABLE
ON FRIDAY AND SATURDAY.

&&

.AVIATION...
HIGH PRESSURE NORTH OF THE AREA WILL MAINTAIN MODERATE EAST TO
NORTHEAST TRADE WINDS OVER THE STATE. THE EARLY MORNING SOUNDING
FROM PHLI SHOWED THE LOW LEVEL INVERSION TO BE NEAR 7 KFT. THE PHTO
BALLOON LIKELY WENT THROUGH A SHOWER THIS MORNING...SHOWING THE
INVERSION TO BE NEAR 10 KFT. HOWEVER...MORNING AMDARS IN AND OUT OF
HNL SHOW THE INVERSION TO BE CLOSER TO 5-6 KFT. THE STRONG LOW LEVEL
INVERSION COMBINED WITH THE MODERATE TRADE WINDS...MAY INTRODUCE
ISOLATED LIGHT LOW LEVEL TURB OVER AND SOUTH THROUGH WEST OF THE
MOUNTAINS. HOWEVER...WINDS OVERALL SHOULD REMAIN BELOW THE THRESHOLD
FOR ANY WIDESPREAD LOW LEVEL TURB.

THE PERSISTENT MVFR TO IFR CIGS/VIS SEEN OVER MAUI AND ESPECIALLY
THE BIG ISLAND LAST NIGHT AND THIS MORNING HAVE STARTED TO
LIFT...WITH AN OVERALL DRYING TREND EXPECTED TODAY. ALL AIRMET
SIERRAS FOR MOUNTAIN OBS HAVE BEEN CANCELLED. EXPECT ONLY ISOLATED
TO SCATTERED SHOWERS ALONG THE NORTH AND EAST SLOPES...AND ANY
SHOWERS SHOULD REMAIN LIGHT AND BRIEF.

A SHARP 250 MB TROUGH OVER THE EASTERN PORTION OF THE STATE WILL
CONTINUE TO MOVE EAST TODAY...AS WELL AS THE UPPER LEVEL JET. AIRMET
TANGO WILL REMAIN POSTED FOR UPPER LEVEL TURB BETWEEN FL320 AND
FL450 FOR AREAS EAST OF MOLOKAI. CONDITIONS SHOULD IMPROVE LATER
TODAY...AND THE AIRMET WILL LIKELY BE CANCELLED BY THIS AFTERNOON.

&&

.MARINE...
A LONG-PERIOD WNW SWELL CONTINUES TO GRADUALLY BUILD IN ISLAND
COASTAL WATERS...AND WILL LIKELY PEAK LATER TODAY. THIS IS PRIMARILY
BASED ON OBSERVATIONS FROM BUOY 51101 NW OF KAUAI...WHICH INDICATED
A GRADUAL RISE IN SWELL HEIGHTS OVERNIGHT...TO A PEAK OVER 8 FEET IN
THE LAST COUPLE OF HOURS. A HIGH SURF ADVISORY HAS BEEN POSTED FOR N
FACING SHORES OF MOST ISLANDS...AND FOR W FACING SHORES OF KAUAI AND
NIIHAU AS RESULTANT SURF IS EXPECTED TO PEAK BETWEEN 12 AND 16 FEET.
THE SWELL WILL THEN GRADUALLY DIMINISH FROM TONIGHT THROUGH THE
WEEKEND. A SMALL SOUTH SWELL IS POSSIBLE FRIDAY AND SATURDAY. A
SMALL TO MODERATE NW SWELL IS ALSO POSSIBLE AROUND TUESDAY.

A SMALL CRAFT ADVISORY /SCA/ IS IN EFFECT FOR WATERS MOST EXPOSED TO
THE SWELL...WHERE COMBINED SEAS WILL PEAK NEAR 10 FEET.
ADDITIONALLY...WINDS ACCELERATING BETWEEN MAUI AND THE BIG ISLAND
HAVE ALSO NECESSITATED A SCA FOR THE ALENUIHAHA CHANNEL.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST THURSDAY FOR NORTH AND WEST FACING
SHORES OF NIIHAU AND KAUAI...AND FOR NORTH FACING SHORES OF OAHU
MOLOKAI AND MAUI...

SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-OAHU
WINDWARD WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST THURSDAY FOR ALENUIHAHA
CHANNEL.

&&

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