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Hawaiian Islands Synoptic Discussion and Guidance

569
FXHW60 PHFO 191359
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST SUN APR 19 2015

.SYNOPSIS...
HIGH PRESSURE NORTH OF THE STATE WILL BRING BREEZY TRADE WINDS
THROUGH THE FIRST HALF OF THE WEEK. THE TRADE WINDS WILL FOCUS
CLOUDS AND SHOWERS MAINLY OVER WINDWARD AREAS. A DISTURBANCE
DEVELOPING FAR WEST OF THE ISLANDS MAY RESULT IN SLIGHTLY WEAKER
WINDS BY THE SECOND HALF OF THE WEEK.

&&

.DISCUSSION...
TRADE WIND WEATHER CONTINUES OVER THE ISLANDS EARLY THIS MORNING AS
A SURFACE HIGH PRESSURE RIDGE STAYS NORTH OF THE STATE. LOW CLOUDS
AND SHOWERS DID INCREASE A TAD OVERNIGHT AS EXPECTED...AND AFFECT
MAINLY WINDWARD AREAS. LATEST SATELLITE IMAGERY INDICATED THERE IS A
LINE OF LOW CLOUDS REACHING THE WINDWARD BIG ISLAND EARLY THIS
MORNING AND WILL SPREAD TO MAUI COUNTY AND OAHU IN THE NEXT FEW
HOURS. EARLY MORNING LIHUE AND HILO SOUNDINGS CONTINUE TO REVEAL A
STABLE AIR MASS IN THE ISLAND VICINITY...WITH LOW LEVEL MOISTURE
EXTENDING TO SLIGHTLY ABOVE 8K FEET ON THE HILO SOUNDING.
THEREFORE...EXPECT MORE PASSING SHOWERS TO AFFECT MAINLY WINDWARD
AREAS THROUGH THE REST OF THE MORNING...WITH SOME SHOWERS REACHING
THE LEE AREAS OF THE SMALLER ISLANDS AT TIMES UNDER THE BREEZY TRADE
WINDS. RAINFALL AMOUNTS WILL BE SOMEWHAT LIMITED UNDER THE
UNFAVORABLE ATMOSPHERIC CONDITIONS AS WELL AS THE BRISK TRADE WINDS.

A STRENGTHENING SURFACE HIGH PRESSURE FAR NORTHWEST OF THE STATE
WILL PASS NORTH OF THE STATE IN THE NEXT COUPLE OF DAYS...BRINGING A
TIGHTENING OF LOW LEVEL PRESSURE GRADIENT ACROSS THE AREA. IN
TURN...A FURTHER SLIGHT INCREASE OF TRADE WINDS IS EXPECTED TODAY
THROUGH MONDAY. A MID LEVEL HIGH WILL PERSIST IN THE ISLAND
VICINITY...KEEPING AIR MASS STABLE OVER THE AREA. BREEZY TRADE WIND
WEATHER WILL PREVAIL IN THE NEXT COUPLE OF DAYS...WITH PASSING LOW
CLOUDS AND SHOWERS AFFECTING MAINLY WINDWARD AREAS. LEE AREAS OF THE
SMALLER ISLANDS MAY SEE SOME SHOWERS AT TIMES...DUE TO THE STRONG
WINDS. FURTHERMORE...THE WINDS MAY REACH ADVISORY LEVEL FOR SOME
WIND-PRONE AREAS ACROSS THE ISLANDS UNDER THE STABLE AIR MASS.

THE AFOREMENTIONED SURFACE HIGH WILL MOVE TO FAR NORTHEAST OF THE
STATE BY TUESDAY...THEN WEAKEN SOMEWHAT BUT SAG SOUTH CLOSER TO THE
ISLANDS. LATEST MODEL SOLUTIONS CONTINUE TO ALIGN CLOSER TO EACH
OTHER...WITH A SURFACE LOW PRESSURE SYSTEM EXPECTING TO DEVELOP FAR
WEST OF THE STATE BY THE LATER HALF OF THE WEEK. THIS SYSTEM MAY
STILL BE CLOSE ENOUGH TO THE STATE SUCH THAT WINDS MAY TURN A LITTLE
MORE SOUTHEASTERLY ACROSS THE AREA. MEANWHILE...A SURFACE HIGH
PRESSURE IS EXPECTED TO PERSIST NORTHEAST OF THE STATE...KEEPING
RATHER ROBUST WINDS OVER THE ISLANDS. MODEL SOLUTIONS STILL SHOW A
SLIGHT INCREASE IN MOISTURE ACROSS THE AREA...WITH GFS CONTINUES TO
HAVE A WETTER FORECAST. PREVIOUS FORECAST PACKAGE WAS ON
TRACK...HENCE HAVE ONLY DONE SOME MINOR TWEAKING TO THE WINDS AS
WELL AS INTRODUCING MORE BREAKS IN THE CHANCE OF PRECIPITATION
COVERAGE FOR THIS ONE. TRADE WIND WEATHER WILL PROBABLY CONTINUE FOR
THE LATER HALF OF THE WEEK...THROUGH WITH WINDS TURNING A LITTLE
MORE SOUTHEAST AND SLIGHTLY WEAKER WHILE THE ISLANDS MAY SEE A
SLIGHT INCREASE IN CLOUDS AND SHOWERS AS WELL.

&&

.AVIATION...
THE SOUTHERN FRINGE TO A PATCH OF LOW CLOUDS WITH EMBEDDED SHOWERS
INCREASED AND MOVED ASHORE ON THE BIG ISLAND PROMPTING AN AIRMET
SIERRA FOR MTN OBSCURATION ACROSS THE ISLAND/S NE FACING SLOPES
ABOVE 25 HUNDRED FEET. THIS PATCH IS JUST PART OF A LONG EXTENSION
OF AREAS OF SHOWER BEARING CLOUDS AIMED AT THE ISLANDS...PRIMARILY
THE BIG ISLAND AND MAUI. THIS EXTENSION STRETCHED UPWIND OF THE
ISLANDS FOR AT LEAST 400 MILES. UNLIKE THE BIG ISLAND...POCKETS OF
LOW CLOUDS MOSTLY DISSIPATED UPON REACHING THE REMAINING ISLANDS
WITH BRIEF MVFR CIGS AND VIS NOTED. THUS...NO AIRMET SIERRA
ISSUED...BUT WE WILL BE MONITORING CLOSELY FOR THOSE CLOUDS/SHOWERS
UPWIND OF THE ISLANDS. LEE SECTIONS OF THE SMALLER ISLANDS WILL
CONTINUE DRY ALTHO A STRAY SHOWER OR TWO MAY ENTER THE LEEWARD ZONE.
LEEWARD BIG ISLAND WILL HAVE SOME MINOR AFTERNOON CLOUDS AND
ISOLATED SHOWER AT BEST.

AIRMET TANGO FOR LO LVL TURB BLO 080 WILL CONT THRU AT LEAST 22Z FOR
ALL ISLANDS. THIS IS DUE TO A MOD TO STRONG TRADE WIND FLOW WITH A
MODERATELY STRONG INVERSION BETWEEN 6 AND 7K FEET.

&&

.MARINE...
SMALL CRAFT WIND CONDITIONS WILL PREVAIL FOR ALL COASTAL WATERS AS
THE WINDS ARE EXPECTED TO INCREASE A NOTCH TODAY...WITH NEAR GALE
FORCE WINDS IN CHANNELS AROUND MAUI AND THE BIG ISLAND.
MOREOVER...THE RATHER STRONG WINDS WILL CONTINUE THROUGH THE EARLY
PART OF THIS WEEK. THUS A SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR
ALL COASTAL WATERS THROUGH EARLY TUESDAY.

THE STRONG TRADE WINDS WILL ALSO BRING NEAR ADVISORY LEVEL SURF
ALONG EAST FACING SHORES IN THE NEXT FEW DAYS...WITH RATHER ROUGH
AND CHOPPY SURF UNDER THE WINDS. A NEW SMALL NORTHWEST SWELLS ARE
EXPECTED ON TODAY AND TUESDAY...THOUGH SURF HEIGHTS FROM THE SWELLS
WILL REMAIN BELOW ADVISORY LEVELS. A LONG-PERIOD SOUTH-SOUTHWEST
SWELL IS EXPECTED TO ARRIVE TONIGHT...PEAKING LATE MONDAY INTO
TUESDAY...THEN SLOWLY DIMINISH THROUGH THE REST OF THE WEEK...WITH
NEAR ADVISORY LEVEL SURF ALONG SOUTH FACING SHORES DURING THE PEAK
OF THE SWELL.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR ALL HAWAIIAN WATERS.

&&

$$

AVIATION...LAU
REST...HUI



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