Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 171354
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
354 AM HST WED SEP 17 2014

.SYNOPSIS...
WEAK SURFACE HIGH PRESSURE NORTH OF THE ISLANDS WILL MAINTAIN
GENTLE TRADE WINDS ACROSS THE STATE TODAY...WITH LOCALIZED
AFTERNOON SEA BREEZES DEVELOPING. THE RIDGE WILL BREAK DOWN FROM
THURSDAY INTO THE WEEKEND...ALLOWING FOR WIDESPREAD SEA BREEZES
ALONG WITH AFTERNOON CLOUDS AND SHOWERS. DRY AND STABLE AIR ALOFT
WILL KEEP MOST RAINFALL AMOUNTS LIGHT. A TROPICAL WAVE PASSING
SOUTH OF THE AREA MAY BRING AN INCREASE IN RAINFALL...ESPECIALLY
ACROSS THE BIG ISLAND...WITH GENTLE TRADE WINDS RESUMING ACROSS
THE STATE BY THE LATER HALF OF THE WEEKEND AND INTO EARLY NEXT
WEEK.

&&

.DISCUSSION...
SHOWER ACTIVITY WAS MOST CONCENTRATED ACROSS OAHU DURING THE
NIGHT...WITH MOST WINDWARD AND A NUMBER OF INTERIOR SITES
RECEIVING MEASURABLE RAINFALL. THE OTHER ISLANDS HAVE REMAINED
ALMOST ENTIRELY DRY. CURRENTLY...SHOWERS ARE MOST PREVALENT OVER
THE LEEWARD COASTAL WATERS WELL WEST OF OAHU...AND THE WATERS
SOUTHEAST OF THE BIG ISLAND. SURFACE ANALYSIS DEPICTS LOW PRESSURE
CENTERED FAR NORTHEAST OF THE STATE...WITH A DISSIPATING
FRONT/TROUGH TRAILING SW TO EAST AND SOUTH OF THE BIG ISLAND. A
1022 MB HIGH IS CENTERED ABOUT 1000 MILES NORTHWEST OF KAUAI AND
IS BUILDING SLOWLY EASTWARD...PROVIDING GENTLE TRADE WIND FLOW
ACROSS THE STATE. ALOFT...A WEAK UPPER LEVEL LOW IS CENTERED NEAR
KAUAI...WITH A LARGE MID/UPPER LEVEL RIDGE SPRAWLING NORTHWEST OF
OUR REGION FROM WEST OF THE DATELINE TO NEAR 160W. MIMIC-TPW
SATELLITE IMAGERY SHOWS A STRIPE OF HIGHER MOISTURE ALONG THE
DISSIPATING SURFACE TROUGH EXTENDING ACROSS THE SOUTHERN TIP OF
THE BIG ISLAND AND SOUTH OF THE SMALLER ISLANDS...WITH ANOTHER
AREA OF HIGHER MOISTURE APPROACHING THE STATE FROM THE NORTH. THE
12Z SOUNDINGS DEPICT A RATHER TYPICAL PROFILE...WITH INVERSIONS
BASED AROUND 7000 FEET...AND NEAR NORMAL PWATS BETWEEN 1.3 AND 1.5
INCHES.

SURFACE HIGH PRESSURE WILL BUILD SLOWLY EASTWARD TO THE NORTH OF
THE STATE TODAY AND TONIGHT...MAINTAINING A GENTLE BACKGROUND
TRADE WIND FLOW. TRADES WILL REMAIN LIGHT ENOUGH TO ALLOW
LOCALIZED SEA BREEZES TO DEVELOP ONCE AGAIN THIS AFTERNOON.
EXPECT A HYBRID PATTERN SIMILAR TO THAT OBSERVED ON TUESDAY...
WITH SHOWERS AND CLOUDS FOCUSING ON WINDWARD SLOPES DURING THE
EARLY MORNING AND NIGHTTIME HOURS...AND ACROSS SOME OF THE
LEEWARD AND INTERIOR AREAS DURING THE AFTERNOON. THE AREA OF
INCREASED MOISTURE MOVING SW ACROSS THE WESTERN AND CENTRAL PARTS
OF THE STATE TODAY AND TONIGHT WILL PROBABLY ALLOW FOR A BIT MORE
LEEWARD SHOWER ACTIVITY THIS AFTERNOON...AND WINDWARD SHOWER
ACTIVITY TONIGHT...THAN HAS BEEN OBSERVED DURING THE PAST 24 HOURS.
A DRIER AIRMASS APPEARS LIKELY TO PERSIST ACROSS THE BIG ISLAND...
AND WOULD EXPECT SHOWERS TO BE LESS PREVALENT THERE.

FROM THURSDAY INTO THE WEEKEND...ANOTHER COLD FRONT IS FORECAST
TO DROP SOUTHWARD ACROSS THE NORTH CENTRAL PACIFIC. THIS WILL
PUSH THE SUBTROPICAL RIDGE SOUTH AND EAST...DISRUPTING THE
PRESSURE GRADIENT ACROSS THE STATE. LOCAL TRADE WIND FLOW SHOULD
WEAKEN FURTHER OR DISAPPEAR ENTIRELY DURING THIS TIME...ALLOWING
FOR A PERIOD OF MORE WIDESPREAD DAYTIME SEA BREEZES AND NIGHTTIME
LAND BREEZES. THIS PATTERN WILL BRING MOSTLY CLEAR NIGHTS AND
MORNINGS WITH SPOTTY SHOWERS OVER INTERIOR AREAS DURING THE
AFTERNOONS AND EVENINGS. THE LIGHT WINDS WILL ALLOW VOLCANIC HAZE
TO BUILD UP AROUND THE BIG ISLAND...BUT IT DOES NOT APPEAR AS IF
THERE WILL BE ENOUGH SOUTHERLY COMPONENT IN THE FLOW TO CARRY THE
HAZE OVER THE SMALLER ISLANDS.

FORECAST UNCERTAINTY INCREASES DURING THE WEEKEND AND INTO NEXT
WEEK...WHEN THE MODELS BRING A TROPICAL WAVE AND ASSOCIATED SURGE
OF DEEP MOISTURE NORTHWESTWARD TOWARD THE STATE. THE 00Z GUIDANCE
HAS GENERALLY TRENDED A BIT SOUTH WITH THE WAVE...WITH THE DEEPER
MOISTURE AFFECTING MAINLY JUST THE BIG ISLAND OR POSSIBLY
REMAINING SOUTH OF THE STATE ALTOGETHER SUNDAY INTO MONDAY. IT
ALSO APPEARS THAT THE INCREASED PRESSURE GRADIENT BETWEEN LOWER
PRESSURE TO THE SOUTH...AND A NARROW HIGH PRESSURE RIDGE BETWEEN
THE ISLANDS AND A STALLED FRONTAL BOUNDARY TO OUR NORTH SHOULD
INDUCE SOME INCREASE IN EASTERLY FLOW OVER THE STATE DURING THIS
TIME. HAVE NUDGED POPS DOWN A BIT FOR THE SMALLER ISLANDS...
KEEPING THE HIGHEST POPS OVER THE BIG ISLAND.

UNCERTAINTY INCREASES FURTHER FOR NEXT TUESDAY AND BEYOND...WHEN
THE GFS DROPS ANOTHER COLD FRONT SOUTHWARD INTO THE CENTRAL
PACIFIC...WEAKENING AND VEERING OUR LOCAL WIND FLOW TOWARD THE
SOUTHEAST. THE ECMWF HAS A COMPLETELY DIFFERENT SOLUTION FOR THIS
TIME PERIOD WITH A BUILDING HIGH PRESSURE RIDGE IN THE CENTRAL
PACIFIC...CAUSING INCREASING TRADES OVER THE STATE. WILL HAVE TO
AWAIT FUTURE MODEL RUNS WHICH WILL HOPEFULLY RESOLVE THESE
DIFFERENCES.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL FOR THE NEXT COUPLE OF DAYS WITH ONLY
LOCALIZED MVFR CEILINGS OVER THE SLOPES. CLOUDS AND SHOWERS WILL BE
MOST PREVALENT OVER WINDWARD SLOPES DURING THE NIGHTS AND MORNINGS
AND OVER THE LEEWARD SLOPES DURING THE AFTERNOONS AND EVENINGS.

&&

.MARINE...
OVERNIGHT OBSERVATIONS FROM THE BARBERS POINT BUOY SHOW THE SOUTH
SWELL CONTINUING TO SLOWLY DECLINE...WITH PEAK ENERGY VALUES
GRADUALLY SHIFTING INTO THE SHORTER WAVE PERIOD BANDS. THIS SWELL
WILL SLOWLY FADE AWAY THROUGH FRIDAY. SURF ALONG SOUTH FACING
SHORES WILL REMAIN SOMEWHAT ELEVATED THROUGH TODAY AS ANOTHER
SMALL SOUTH SWELL ARRIVES...BUT IS EXPECTED TO REMAIN BELOW
ADVISORY LEVELS. OTHERWISE...A SMALL NORTHWEST SWELL ARRIVED LATE
YESTERDAY...AS INDICATED BY WAIMEA BAY BUOY DATA. ANOTHER SMALL
NORTHWEST SWELL WILL ARRIVE LATER TODAY...BUT SURF ALONG NORTH
FACING SHORES WILL REMAIN WELL BELOW ADVISORY LEVELS.

AN 0806Z ASCAT PASS FOUND GENTLE TO LOCALLY MODERATE TRADE WINDS
ACROSS THE HAWAIIAN COASTAL WATERS...WITH A SMALL AREA OF WINDS
GREATER THAN 15 KNOTS IN THE ALENUIHAHA CHANNEL. THESE WILL
PROBABLY BE THE STRONGEST WINDS SEEN IN OUR WATERS THROUGH THE
NEXT SEVERAL DAYS...WITH WIND SPEEDS EXPECTED TO BEGIN DECREASING
LATER TODAY. WINDS AND SEAS ARE EXPECTED TO REMAIN BELOW SMALL
CRAFT ADVISORY LEVELS THROUGH AT LEAST EARLY NEXT WEEK.

&&

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

&&

$$

JACOBSON






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