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Hawaiian Islands Synoptic Discussion and Guidance

713
FXHW60 PHFO 251949
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM 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. HIGH PRESSURE
IS LOCATED NORTHWEST OF THE STATE. 12Z SOUNDINGS FROM HILO AND LIHUE
SHOW INVERSION HEIGHTS JUST UNDER 7KFT...WITH PRECIPITABLE WATERS
AROUND 1.1 INCHES. EARLY MORNING MIMIC TOTAL PRECIPITABLE WATER
IMAGERY SHOWS A BAND OF MOISTURE ASSOCIATED WITH THE OLD FRONT
SOUTHEAST OF THE BIG ISLAND...WITH SLIGHTLY DRIER AIR UPSTREAM TO
THE NORTH AND NORTHEAST OF THE STATE.

AN UNSEASONABLE POST-FRONTAL AIRMASS REMAINS IN PLACE ACROSS THE
STATE...WITH LIGHT TO LOCALLY MODERATE NORTH-NORTHEAST WINDS.
PATCHES OF OVERCAST LOW CLOUDS ARE AFFECTING PORTIONS OF THE
STATE...MAINLY MOST AREAS EXCEPT OAHU /THOUGH ADDITIONAL CLOUDS ARE
LOCATED JUST OFFSHORE TO THE NORTH/. THE LOW CLOUDS ACROSS THE
ISLANDS ARE RELATIVELY STABLE. RAINFALL AMOUNTS HAVE BEEN GENERALLY
LESS THAN A TENTH OF AN INCH OVER THE PAST 12 HOURS...EXCEPT FOR
SLIGHTLY HIGHER VALUES ACROSS WINDWARD KAUAI. WITH THE STABLE AND
DRY AIRMASS...SHOWERS ARE EXPECTED TO REMAIN LIGHT TODAY. IN
ADDITION TO WINDWARD CLOUDS...SOME BUILDUPS ARE EXPECTED ACROSS
LEEWARD AREAS BY AFTERNOON AS WELL DUE TO THE WEAKENING WINDS AND
LOCALIZED SEA BREEZES.

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...
STABLE N TO NE FLOW WILL CONTINUE TO PILE UP MAINLY STABLE
STRATOCUMULUS ACROSS N AND N FACING SLOPES WITH ISOL LIGHT SHOWERS.
AFTERNOON SEA BREEZES WILL PRODUCE CLOUDS OVER SHELTERED LEEWARD
TERRAIN...ESPECIALLY ON THE BIG ISLAND AND MAUI. WITH CLOUD BASES A
BIT HIGHER THAN NORMAL...VFR CONDITIONS ARE EXPECTED TO PREVAIL...
EVEN ACROSS N AND NE FACING SLOPES.

&&

.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 S FACING SHORES INTO THE
BEGINNING OF JUNE...AS THE DEVELOPING EL NINO FAVORS LOW PRESSURE
DEVELOPMENT NEAR AND EAST OF NEW ZEALAND. A PEAK IN SOUTH SHORE SURF
NEAR ADVISORY LEVELS IS EXPECTED THROUGH TUESDAY. LATEST BUOY
OBSERVATIONS SHOW INCONSISTENT SPIKES IN ENERGY IN THE 20-22
SECOND BAND AS WELL AS THE 14 SECOND BAND. CONFIDENCE IS NOT HIGH
ENOUGH TO ISSUE AN ADVISORY AT THIS TIME...BUT THE FACT THAT TWO
SEPARATE SWELLS WITH DIFFERENT PEAK PERIODS WILL BE ARRIVING MEANS
THAT SURF WILL BE MORE CONSISTENT THAN USUAL ALONG S FACING SHORES.
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 PRODUCE SURF NEAR ADVISORY LEVELS.

&&

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

&&

$$

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



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