High Surf Advisory 33, NWS Honolulu, HI

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
535 AM HST MON AUG 29 2011

.UPDATE...
ISSUED A HIGH SURF ADVISORY FOR SOUTH FACING SHORES OF ALL HAWAIIAN
ISLANDS STARTING AT NOON HST FOR RISING SURF. UPDATED MARINE SECTION
BELOW.

&&

.SYNOPSIS...
SURFACE HIGH PRESSURE FAR NORTH OF THE ISLANDS WILL MAINTAIN
MODERATE TRADE WIND SPEEDS THROUGH FRIDAY...WITH LIGHTER WINDS
POSSIBLE NEXT WEEKEND. TRADE SHOWERS WILL FAVOR WINDWARD AND MAUKA
AREAS. AN UPPER LEVEL LOW PASSING JUST NORTH OF THE ALOHA STATE MAY
ENHANCE SHOWERS...WITH LOCALLY HEAVY RAINFALL POSSIBLE FROM TONIGHT
THROUGH TUESDAY NIGHT. EXPECT SLIGHTLY DRIER TRADE WIND WEATHER
DURING THE SECOND HALF OF THE WEEK.

&&

.DISCUSSION...
THE PRESSURE GRADIENT SOUTH OF A NEARLY EAST TO WEST ORIENTED
SUBTROPICAL RIDGE LOCATED ABOUT 850 MILES NORTH OF HONOLULU IS
MAINTAINING MODERATE TRADE WIND SPEEDS ACROSS THE STATE EARLY THIS
MORNING. PRESSURE TENDENCIES OVER THE PAST 24 HOURS AT OBSERVING
STATIONS ACROSS THE ALOHA STATE ARE DOWN ABOUT ONE-HALF MILLIBAR.
HOWEVER...WE EXPECT LITTLE CHANGE IN WIND SPEEDS THIS WEEK...WITH
MODERATE TRADE WINDS TO PREVAIL...AT LEAST THROUGH FRIDAY.

A TYPICAL TRADE WIND WEATHER PATTERN OF WINDWARD AND MAUKA SHOWERS
CONTINUES. MID-LEVEL RIDGING IS MAINTAINING A SUBSIDENCE INVERSION
NEAR 7 TO 8 THOUSAND FEET ACCORDING TO THE EARLY MORNING BALLOON
SOUNDINGS FROM HILO AND LIHUE. PRECIPITABLE WATER VALUES IN THESE
SOUNDINGS RANGED FROM 1.3 TO 1.4 INCHES. SINCE THERE IS CURRENTLY A
LACK OF ORGANIZED FEATURES IN THE CLOUDS UPSTREAM OF THE ISLANDS...
WE EXPECT THE AREAL COVERAGE OF LOW CLOUDS AND SHOWERS TO REMAIN
SIMILAR TO THE DISTRIBUTIONS WE HAVE BEEN SEEING RECENTLY. SOME
LEEWARD AREAS OF THE SMALLER ISLANDS MAY EXPERIENCE BRIEF PASSING
SHOWERS EARLY THIS MORNING...WITH MOST LINGERING LEEWARD SHOWERS
LIKELY TAPERING OFF A FEW HOURS AFTER SUNRISE.

ALOFT...AN UPPER TROPOSPHERIC LOW IS CENTERED NEAR 26.5N 154W...OR
ABOUT 480 MILES NORTH NORTHEAST OF HILO. THIS FEATURE...WHICH IS
MOVING SOUTH SOUTHWESTWARD AT NEARLY 20 MPH...WILL PASS JUST NORTH
OF THE ISLANDS TONIGHT AND TUESDAY. ONCE THIS FEATURE IS CLOSE
ENOUGH...THERE IS A POSSIBILITY OF ENHANCED TRADE SHOWER COVERAGE
AND INTENSITY STARTING TONIGHT AND CONTINUING INTO TUESDAY NIGHT.
THE GREATEST INSTABILITY AND COLDEST TEMPERATURES ALOFT ASSOCIATED
WITH THE UPPER LEVEL LOW ARE EXPECTED TO REMAIN JUST NORTH OF THE
ISLANDS. HOWEVER...LOWER TROPOSPHERIC RIDGING WILL PERSIST...WHILE
THE MODELS DO NOT APPEAR TO BRING IN ANY SIGNIFICANT MOISTURE
SOURCES. BASED ON ALL OF THIS EVIDENCE...IT IS ASSUMED THERE WILL BE
A WEAKENED SUBSIDENCE INVERSION OVER THE STATE WITH A MODEST
INCREASE IN SHOWER COVERAGE AS SOME SHOWERS MAY BE CARRIED OVER TO
LEEWARD SECTIONS OF THE SMALLER ISLANDS. THERE IS A POSSIBILITY
SHOWERS COULD BE BRIEFLY HEAVY...SO THE MENTION OF LOCALLY HEAVY
SHOWERS REMAINS IN THE LATEST FORECAST. A SLIGHT CHANCE OF
THUNDERSTORMS REMAINS IN THE FORECAST FOR PORTIONS OF THE COASTAL
AND OFFSHORE WATERS IMMEDIATELY NORTH OF KAUAI AND OAHU.

ONCE THE UPPER LEVEL LOW MOVES TO THE NORTHWEST AWAY FROM THE MAIN
HAWAIIAN ISLANDS...RIDGING ALOFT WILL GRADUALLY REBUILD FROM EAST TO
WEST OVER THE ISLANDS BY MID-WEEK. THEREFORE...EXPECT STABLE
ATMOSPHERIC CONDITIONS TO RETURN STARTING WEDNESDAY. FORECAST MODELS
ALSO SUGGEST SLIGHTLY DRIER CONDITIONS WITH BELOW NORMAL
PRECIPITATION EXPECTED.

BASED ON THE LATEST FORECAST MODEL OUTPUT...HIGH PRESSURE CENTERED
FAR NORTHEAST OF THE STATE WILL BE SLOWLY DRIFTING SOUTHEASTWARD AND
WEAKENING BY THIS WEEKEND. AS A RESULT...TRADE WIND SPEEDS MAY
BECOME LIGHTER HEADING INTO THE WEEKEND.

&&

.AVIATION...
PERIODS OF MVFR CONDITIONS DUE TO LOW CEILINGS AND REDUCED
VISIBILITIES IN SHOWERS ARE EXPECTED OVER WINDWARD AREAS...
INCLUDING THE PHTO VICINITY...THIS MORNING. VFR CONDITIONS WILL
CONTINUE TO PREVAIL OVER NEARLY ALL LEEWARD AREAS OF THE STATE.

&&

.MARINE...
NO SMALL CRAFT ADVISORIES...SCA...ARE POSTED OVER ANY HAWAIIAN
WATERS. HOWEVER...TRADE WIND SPEEDS ACROSS THE TYPICALLY WINDY
WATERS AND CHANNELS ADJACENT TO THE ISLANDS OF MAUI COUNTY AND THE
BIG ISLAND WILL LIKELY REMAIN JUST BELOW THE SCA CRITERIA THROUGH
TONIGHT. A SLIGHT DECREASE IN TRADE WIND SPEEDS IS POSSIBLE TUESDAY
AND WEDNESDAY DUE TO THE PRESENCE OF THE UPPER LEVEL LOW TO THE
NORTH OF THE ISLANDS. OTHERWISE...NO SIGNIFICANT CHANGES ARE
ANTICIPATED THROUGH THE REMAINDER OF THE WORK WEEK.

SOUTHERN HEMISPHERE SWELL SOURCES WILL PRODUCE A NOTICEABLE BOOST IN
SURF HEIGHTS ALONG SOUTH FACING SHORES OF THE STATE THIS WEEK. SURF
FROM A BUILDING SOUTH SWELL WILL REACH THE HIGH SURF ADVISORY
THRESHOLD OF 8 FEET AFTER NOON HST TODAY. A LARGER...LONG-PERIOD
REINFORCING SOUTH SWELL IS EXPECTED TO INCREASE SURF HEIGHTS ALONG
SOUTH FACING SHORES OF THE STATE WELL ABOVE THE HIGH SURF ADVISORY
CRITERIA FROM LATER TONIGHT THROUGH WEDNESDAY. THIS SOUTH SWELL WILL
GRADUALLY SUBSIDE THURSDAY AND FRIDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY STARTING AT NOON HST FOR RISING SURF ALONG SOUTH
FACING SHORES OF ALL HAWAIIAN ISLANDS.

&&

HOUSTON