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Hawaiian Islands Synoptic Discussion and Guidance

126
FXHW60 PHFO 260149
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST MON MAY 25 2015

.SYNOPSIS...
COOL AND RELATIVELY DRY NORTHEASTERLY WINDS WILL CONTINUE THROUGH
TUESDAY...WITH WINDS GRADUALLY WEAKENING. THESE WINDS WILL CARRY
LOW CLOUDS AND A FEW LIGHT SHOWERS OVER THE ISLANDS...WITH THE
SHOWERS FOCUSING ALONG NORTHEAST FACING SLOPES. FROM TUESDAY INTO
NEXT WEEKEND...WINDS ARE EXPECTED TO BE SUFFICIENTLY LIGHT TO ALLOW
LAND AND SEA BREEZES TO PREVAIL...WITH A SOMEWHAT CLOUDY AND
UNSETTLED WEATHER PATTERN EXPECTED.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS A BROAD TROUGH OVER THE STATE...WITH A
RIDGE NOSING IN FROM THE NORTHWEST. AT THE SURFACE...COMPLEX LOW
PRESSURE IS LOCATED NORTHEAST OF THE STATE...WITH REMNANTS OF THE
PREVIOUS FRONT EAST AND SOUTHEAST OF THE BIG ISLAND. ANOTHER WEAK
SURFACE BOUNDARY IS NORTHEAST OF THE STATE. HIGH PRESSURE IS LOCATED
TO THE NORTHWEST...WITH A RIDGE EXTENDING SOUTHEAST ACROSS THE
ISLANDS. 00Z SOUNDINGS SHOW INVERSION HEIGHTS RANGING FROM 5KFT AT
LIHUE TO NEAR 8KFT AT HILO. PRECIPITABLE WATERS REMAIN DRIER THAN
NORMAL AROUND 1 INCH. EARLY AFTERNOON MIMIC TOTAL PRECIPITABLE WATER
IMAGERY SHOWS AN AREA OF MOISTURE POOLED NEAR THE BIG ISLAND...WITH
DRIER AIR TO THE NORTH AND NORTHEAST. POCKETS OF SLIGHTLY HIGHER
MOISTURE ARE EMBEDDED WITHIN THE DRIER AIR NORTH OF THE STATE.

AN UNSEASONABLE POST-FRONTAL AIRMASS REMAINS IN PLACE ACROSS THE
STATE. LIGHT TO LOCALLY MODERATE WINDS ARE OUT OF THE
NORTH-NORTHEAST...AND ARE STRONGEST NEAR KAUAI. PATCHES OF OVERCAST
LOW CLOUDS ARE AFFECTING PORTIONS OF THE STATE...MAINLY MOST AREAS
EXCEPT MOLOKAI /THOUGH ADDITIONAL CLOUDS ARE LOCATED OFFSHORE TO THE
NORTH/. THE LOW CLOUDS ACROSS THE ISLANDS ARE RELATIVELY STABLE...
AND RAINFALL AMOUNTS HAVE BEEN LIGHT WITH LESS THAN A TENTH OF AN
INCH FALLING OVER THE PAST 12 HOURS. THE GRADIENT IS WEAK ENOUGH FOR
OFFSHORE LAND BREEZES TO DEVELOP ONCE AGAIN...BUT WITH THE AREA OF
STABLE CLOUDS UPSTREAM IT WILL BE DIFFICULT FOR THE ISLANDS TO CLEAR
OUT COMPLETELY. HOWEVER WITH THE STABLE AND RELATIVELY DRY
AIRMASS...ANY SHOWERS TONIGHT ARE EXPECTED TO REMAIN LIGHT.

A DIGGING SHORTWAVE WILL HELP TO CARVE OUT A BROAD LONGWAVE TROUGH
OVER THE ISLANDS BY THE SECOND HALF OF THE WEEK. AN AREA OF MORE
SHOWERY CLOUDS MAY ACCOMPANY THIS FEATURE FROM THE NORTH. CURRENT
TIMING WOULD BRING THIS PATCH OF MOISTURE TO KAUAI TUESDAY
NIGHT/WEDNESDAY. AT THE SAME TIME...THE FALLING HEIGHTS MAY INDUCE A
WEAK TROUGH NEAR THE ISLANDS. THE MOISTURE SOUTHEAST OF THE STATE
LEFT BEHIND BY THE OLD FRONT WILL LIKELY BE PULLED NORTHWESTWARD
TOWARD THE BIG ISLAND. THE COMBINATION OF FALLING HEIGHTS AND
INCREASING LOW-LEVEL MOISTURE WILL HELP TO WEAKEN AND LIFT THE
INVERSION...ALLOWING FOR GREATER MOISTURE DEPTH. CROSS SECTIONS SHOW
MOISTURE UP TO 700MB DURING THE SECOND HALF OF THE WEEK...AND
POTENTIALLY HIGHER OVER THE WEEKEND. AT THE SAME TIME...WEAKENING
WINDS WILL ALLOW FOR A MORE WIDESPREAD LAND AND SEA BREEZE PATTERN.
BETWEEN THE COMPLEX LOW TO THE NORTHEAST OF THE STATE AND THE
POTENTIAL FOR A WEAK TROUGH TO DEVELOP FROM THE FORMER FRONT...THE
GRADIENT WILL WEAKEN FIRST ACROSS THE EASTERLY HALF OF THE STATE.
THE CURRENT FORECAST REFLECTS THE ISLAND-SCALE LAND AND SEA BREEZE
PATTERN...WITH INCREASING RAINFALL CHANCES AND AMOUNTS THROUGH THE
WEEK DUE TO THE INCREASE IN MOISTURE AND INVERSION HEIGHT.

OVER THE WEEKEND...A WEAK SURFACE HIGH BUILDS NORTH OF THE STATE.
THE SURFACE PATTERN REMAINS RATHER COMPLEX TO OUR NORTH...BUT
FEATURES A SLIGHT STRENGTHENING OF THE GRADIENT OVER THE ISLANDS.
WHILE WE GET BACK TO A LIGHT TRADE WIND BACKGROUND FLOW...WINDS
APPEAR TO REMAIN LIGHT ENOUGH FOR LOCALIZED SEA BREEZES ACROSS
SHELTERED LEEWARD AREAS. THEREFORE WE TRANSITION TO HYBRID RAINFALL
PATTERN...WITH CLOUDS AND SHOWERS FOCUSED ACROSS WINDWARD AREAS AT
NIGHT AND MORNING AND ACROSS LEEWARD AREAS DURING THE AFTERNOON AND
EVENING.

&&

.AVIATION...
A LIGHT TO MODERATE N TO NE FLOW WILL CONTINUE TO PILE UP MAINLY
STABLE STRATOCUMULUS WITH ISOLATED LIGHT SHOWERS ACROSS N AND
WINDWARD SLOPES. WITH CLOUD BASES A BIT HIGHER THAN NORMAL...VFR
CONDITIONS SHOULD PREVAIL...AND SOME CEILINGS WILL SPILL OVER TO
LEEWARD SECTIONS OF THE SMALLER ISLANDS...ESPECIALLY ON KAUAI AND
OAHU. NO AIRMETS ARE EXPECTED.

&&

.MARINE...
SMALL CRAFT ADVISORY LEVEL WINDS OR SEAS ARE NOT EXPECTED THROUGH
THE UPCOMING WEEK...EVEN THOUGH SWELLS FROM THE S...SSW...NNE...AND
WNW ARE EXPECTED. OPEN OCEAN SWELL HEIGHTS ARE GENERALLY EXPECTED TO
BE 3 FEET OR LESS...AND WITH LOCAL WINDS REMAINING LIGHT TO
MODERATE...COMBINED SEAS WILL REMAIN BELOW THE 10 FOOT THRESHOLD.

SURF WILL REMAIN SOMEWHAT ELEVATED ALONG SOUTH FACING SHORES THROUGH
THE WEEKEND...WITH THE FIRST PEAK TONIGHT/TUESDAY. LATEST
OBSERVATIONS FROM THE BARBERS POINT BUOY SHOW SEAS REMAINING NEARLY
STEADY...WITH DECREASING LONG-PERIOD ENERGY BUT SOME CLIMB IN THE
MODERATE PERIODS. THIS MATCHES WITH THE GRADUALLY DIMINISHING PERIOD
FROM WAVEWATCH III GUIDANCE...AND ADDS CONFIDENCE TO THE DECISION TO
KEEP SURF BELOW ADVISORY LEVELS. WEBCAMS/LIFEGUARD OBSERVATIONS
INDICATE SURF JUST BELOW ADVISORY LEVELS...AND WE WILL STICK WITH
SIMILAR VALUES FOR TOMORROW AS WELL BEFORE DIMINISHING SLIGHTLY.
LATEST SURFACE ANALYSES AND WAVE WATCH MODEL OUTPUT INDICATE THAT A
NEAR-STORM FORCE LOW NOW BREWING EAST OF NEW ZEALAND IS PRODUCING A
SWELL THAT WILL ARRIVE NEXT WEEKEND...WITH INITIAL INDICATIONS THAT
THIS SWELL WILL LEAD TO SURF NEAR ADVISORY LEVELS.

&&

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

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...WROE



Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office