439 
FXHW60 PHFO 291355
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST SUN SEP 29 2013

.SYNOPSIS...
MODERATE TO LOCALLY STRONG TRADE WINDS WILL CONTINUE TODAY...THEN 
WILL DECREASE LATER TONIGHT AND MONDAY AS A SURFACE HIGH FAR NORTH 
NORTHWEST OF THE ISLANDS WEAKENS. LOW CLOUDS AND SHOWERS CARRIED BY 
THE TRADES WILL FAVOR WINDWARD AND MAUKA AREAS TODAY. AN UPPER LEVEL 
TROUGH APPROACHING THE ISLANDS FROM THE NORTHEAST WILL CAUSE THE 
TRADE SHOWERS TO INCREASE IN COVERAGE AND INTENSITY ACROSS MOST OF 
THE STATE FROM LATE TONIGHT INTO TUESDAY.

&&

.DISCUSSION...
THE TIGHT PRESSURE GRADIENT SOUTH OF A NEARLY STATIONARY 1028 MB 
SURFACE HIGH NEAR 37N 164W...OR ALMOST 1150 MILES NORTH NORTHWEST OF 
HONOLULU...IS MAINTAINING MODERATE TO LOCALLY STRONG TRADE WINDS 
ACROSS THE MAIN HAWAIIAN ISLANDS EARLY THIS MORNING. ALOFT...WATER 
VAPOR IMAGERY INDICATES AN UPPER LEVEL RIDGE IS WEST OF KAUAI NEAR 
168W LONGITUDE...OR MORE THAN 500 MILES WEST OF LIHUE. THIS RIDGE 
ALOFT IS PROPAGATING SLOWLY WESTWARD...SO ITS STABILIZING INFLUENCE 
ON LOCAL ATMOSPHERIC CONDITIONS CONTINUES TO GRADUALLY DIMINISH. 

ELSEWHERE...THE AXIS OF AN UPPER LEVEL TROUGH EXTENDS FROM NEAR 26N 
150W TO 24N 153W TO 24N 157W TO 29N 163W...OR ABOUT 320 MILES NORTH 
NORTHEAST OF HILO. THIS TROUGH ALOFT IS SHIFTING SLOWLY TOWARD THE 
SOUTHWEST. ALTHOUGH THIS UPPER LEVEL FEATURE DOES NOT CURRENTLY 
APPEAR TO BE HAVING ANY DIRECT AFFECT ON WEATHER ACROSS THE STATE... 
IT IS LIKELY ENHANCING THE LOW CLOUDS AND TRADE SHOWERS THAT ARE 
TRAVELING UNDERNEATH IT. THE EARLY MORNING LIHUE AND HILO BALLOON 
SOUNDINGS SHOWED THE AIR MASS ACROSS THE STATE IS BECOMING 
INCREASINGLY MOIST AS THESE CLOUDS AND SHOWERS CONTINUE TO MOVE INTO 
THE AREA. ALSO...THE TRADE WIND INVERSION CONTINUES TO LIFT...AND 
IT IS NOW AROUND 10 THOUSAND FEET IN BOTH ATMOSPHERIC PROFILES.

THE SURFACE HIGH WILL REMAIN IN PLACE FAR NORTH NORTHWEST OF THE 
ALOHA STATE TODAY. AS A RESULT...EXPECT A RATHER TYPICAL TRADE WIND 
WEATHER PATTERN TO PREVAIL OVER THE ISLANDS TODAY...WITH PASSING 
SHOWERS AFFECTING MAINLY WINDWARD AND MAUKA AREAS. THE RELATIVELY 
STRONG TRADES WILL OCCASIONAL CARRY SOME OF THESE SHOWERS OVER TO 
LEEWARD AREAS OF THE SMALLER ISLANDS. THE MOST RECENT SATELLITE 
IMAGERY AND RADAR REFLECTIVITY DATA INDICATE SCATTERED TO BROKEN LOW 
CLOUDS WITH EMBEDDED SCATTERED SHOWERS ARE IN THE VICINITY OF THE 
WINDWARD AND SOUTHEASTERN BIG ISLAND. ELSEWHERE...THERE ARE MAINLY 
SCATTERED TO LOCALLY BROKEN LOW CLOUDS AND ISOLATED SHOWERS OVER THE 
WINDWARD COASTAL WATERS ADJACENT TO MOST OF THE SMALLER ISLANDS. THE 
COVERAGE OF LOW CLOUDS AND SHOWERS INCREASES JUST UPSTREAM FROM THE 
WINDWARD COASTAL WATERS...SO THERE APPEARS TO BE A STEADY SOURCE OF 
ADDITIONAL LOW CLOUDS AND SHOWERS POISED TO MOVE INTO THE ISLANDS BY 
DAYBREAK THIS MORNING. SINCE THE AIR MASS ACROSS THE STATE REMAINS 
RELATIVELY STABLE AND THE TRADE SHOWERS ARE MOVING RAPIDLY... 
RAINFALL TOTALS WILL LIKELY REMAIN RATHER MODEST TODAY. 

THE SURFACE HIGH FAR NORTH NORTHWEST OF THE ALOHA STATE WILL WEAKEN 
FROM TONIGHT THROUGH MONDAY. ALSO...AS THE UPPER LEVEL TROUGH 
CONTINUES TO PROPAGATE TOWARD THE ISLANDS...IT WILL CAUSE THE 
PRESSURE GRADIENT TO BE DISRUPTED. THIS WILL RESULT IN A NOTICEABLE 
DECREASE IN TRADE WIND SPEEDS ACROSS THE ISLANDS DURING THE START OF 
THE NEW WORK WEEK. ONCE THE UPPER LEVEL TROUGH SETTLES INTO A 
POSITION JUST NORTH AND WEST OF THE ISLANDS THIS WEEK...THE CLOSE 
PROXIMITY OF THE COOL AIR ALOFT AND DYNAMIC LIFT ASSOCIATED WITH 
THIS SYSTEM ARE EXPECTED TO DESTABILIZE THE AIR MASS ACROSS MOST OF 
THE STATE...PARTICULARLY OVER THE SMALLER HAWAIIAN ISLANDS. AN 
INCREASE IN LOW-LEVEL MOISTURE IS ALSO EXPECTED AT THE SAME TIME. 
THESE FACTORS WILL LIKELY PRODUCE ENHANCED TRADE WIND SHOWERS ACROSS 
PARTS OF THE STATE STARTING LATE TONIGHT AND EARLY MONDAY MORNING. 
IN FACT...THERE MAY BE ENOUGH INSTABILITY TO PRODUCE A SLIGHT CHANCE 
OF THUNDERSTORMS AND LOCALLY HEAVY RAINFALL...ESPECIALLY IN THE 
VICINITY OF KAUAI AND JUST NORTH OF OAHU FROM MONDAY INTO TUESDAY.  

RELATIVELY WEAK SURFACE HIGH PRESSURE WILL PERSIST FAR NORTH OF THE 
ISLANDS NEXT WEEK...MAINTAINING LIGHT TRADE WINDS ACROSS THE AREA. 
ALTHOUGH THE UPPER LEVEL TROUGH IS EXPECTED TO REMAIN JUST WEST OF 
THE STATE THROUGH MOST OF THE NEW WEEK...THE FORECAST GUIDANCE 
SUGGESTS A DRIER AIR MASS MAY RETURN TO THE ISLANDS BY THE SECOND 
HALF OF THE WEEK. THIS DECREASING MOISTURE SHOULD BE A LIMITING 
FACTOR FOR RAINFALL OVER THE STATE BY MID-WEEK. 

&&

.AVIATION...
WITH PASSING SHOWERS...TEMPO MVFR CONDITIONS WILL OCCUR...ESPECIALLY 
IN WINDWARD AND MAUKA SECTIONS. VFR WILL PREVAIL IN LEEWARD AREAS.
 
TRADE WINDS REMAIN STRONG ENOUGH TO CONTINUE AIRMET TANGO FOR 
MODERATE LOW LEVEL TURBULENCE DOWNWIND FROM THE MOUNTAINS.

&&

.MARINE...
TRADE WIND SPEEDS ARE FORECAST TO REMAIN NEAR THE SMALL CRAFT 
ADVISORY /SCA/ CRITERIA ACROSS THE WINDIER WATERS ADJACENT TO THE 
ISLANDS OF MAUI AND HAWAII COUNTIES THROUGH TONIGHT. THEREFORE...A 
SCA REMAINS IN EFFECT FOR THESE MARINE ZONES UNTIL EARLY MONDAY 
MORNING. NO OTHER SCA CONDITIONS ARE IN THE FORECAST DURING THE NEW 
WORK WEEK.

THE CURRENT NORTHWEST SWELL WILL CONTINUE TO PRODUCE SMALL SURF 
ALONG MOST NORTH AND WEST FACING SHORES OF THE SMALLER ISLANDS 
TODAY. THIS SWELL WILL SLOWLY SUBSIDE THROUGH TONIGHT. A NEW MODEST 
NORTH SWELL IS EXPECTED TO AFFECT MOST NORTH FACING SHORES OF THE 
ISLANDS ON WEDNESDAY...FOLLOWED BY A MODEST NORTH NORTHWEST SWELL 
DUE INTO THE ISLANDS EARLY NEXT WEEKEND.

THE TRADE WIND SWELL WILL CONTINUE TO PRODUCE CHOPPY SURF ALONG EAST 
FACING SHORES THROUGH TONIGHT...BUT THIS SWELL WILL DIMINISH 
SLIGHTLY DURING THE BEGINNING OF THE NEW WORK WEEK AS THE TRADE 
WINDS WEAKEN. ELSEWHERE...SMALL SOUTHEAST SWELLS WILL MAINTAIN 
MODEST SURF ALONG EXPOSED SOUTH FACING SHORES AT LEAST THROUGH 
MID-WEEK. 

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST MONDAY FOR MAALAEA BAY-PAILOLO 
CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG ISLAND 
SOUTHEAST WATERS.

&&

$$

DISCUSSION/MARINE...HOUSTON
AVIATION...KINEL