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Hawaiian Islands Synoptic Discussion and Guidance

899
FXHW60 PHFO 230135
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST MON SEP 22 2014

.SYNOPSIS...
MODERATE TRADE WINDS WILL FOCUS CLOUDS AND MODEST SHOWERS OVER
WINDWARD AREAS DURING THE NEXT COUPLE OF DAYS. ASIDE FROM THE KONA
SLOPES...LEEWARD AREAS WILL BE RATHER DRY AND HOT...THOUGH HUMIDITY
WILL BE DOWN COMPARED TO THE PAST SEVERAL DAYS. A WET TRADE WIND
PATTERN WILL DEVELOP LATE WEDNESDAY AND THURSDAY AND WILL PERSIST
THROUGH THE WEEKEND.

&&

.DISCUSSION...
MODERATE TRADE WINDS ARE BACK AND WILL PERSIST INTO THE WEEKEND.
ISLAND SURFACE PRESSURE READINGS ARE UP AN AVERAGE OF AROUND TWO
MILLIBARS FROM THIS TIME YESTERDAY AS THE PRESSURE GRADIENT TIGHTENS
ACROSS THE REGION. THE RESURGENT TRADES ARE BEING DRIVEN BY SURFACE
HIGH PRESSURE SITTING ABOUT 1000 MILES TO THE NORTHWEST OF KAUAI.
THE HIGH WILL REMAIN IN PLACE FOR THE NEXT DAY THEN MOVE SLOWLY
EASTWARD THROUGH THE REST OF THE WEEK. AS A RESULT...TRADES MAY
INCREASE SLIGHTLY ON TUESDAY AND AGAIN OVER THE NORTHERN ISLANDS ON
THURSDAY...FOLLOWED BY A DECREASE HEADING INTO THE WEEKEND.

THE RETURNING TRADE WINDS HAVE BROUGHT SOME RELIEF FROM THE RECENT
HUMID CONDITIONS. A DRIER AIR MASS USHERED IN ON THE TRADES CONTAINS
DEW POINTS A FEW DEGREES DOWN FROM WHAT WE EXPERIENCED RECENTLY...
LEADING TO A SLIGHT DECREASE AFTERNOON RELATIVE HUMIDITY COMPARED TO
YESTERDAY. EVEN THOUGH LEEWARD AREAS WILL REMAIN SEASONABLY HOT...
AROUND 90...DURING THE NEXT COUPLE OF DAYS...THE SLIGHT DROP IN
HUMIDITY ALONG WITH THE MODERATE TRADES WILL CONTINUE TO PROVIDE
SOME RELIEF.

THROUGH WEDNESDAY...MODEST SHOWER ACTIVITY WILL BE FOCUSED OVER
WINDWARD AREAS...AS WELL AS THE KONA SLOPES EACH AFTERNOON. ALTHOUGH
A WELL-DEFINED INVERSION IS ABSENT IN THE AFTERNOON SOUNDINGS...MID
LEVEL RIDGING IS PRODUCING STABLE CONDITIONS...AND WITH NO
SIGNIFICANT FEATURES OBSERVED IN THE TRADE WIND FLOW IMMEDIATELY
UPWIND OF THE ISLANDS...WINDWARD RAINFALL WILL BE MODEST WHILE
LEEWARD PORTIONS OF THE SMALLER ISLANDS REMAIN DRY.

AN INCREASE IN SHOWER ACTIVITY IN THE TRADE WIND FLOW IS DUE DURING
THE SECOND HALF OF THE WEEK. A BAND OF MOISTURE ASSOCIATED WITH
REMNANTS OF A FRONT WILL DRIFT SOUTHWARD AND STALL OVER THE ISLANDS
WEDNESDAY NIGHT AND THURSDAY AS THE SURFACE HIGH PASSES BY TO THE
NORTH. MOISTURE FROM THE BAND WILL LINGER OVER PORTIONS OF THE STATE
THROUGH SUNDAY...PROVIDING ENHANCED WINDWARD RAINFALL AND LIKELY
SPREADING SOME SHOWERS TO LEEWARD AREAS. A MID TO UPPER LEVEL TROUGH
IS EXPECTED TO MOVE NEAR THE STATE FROM SOUTHEAST FRIDAY AND
SATURDAY...AND THIS COULD TRIGGER SOME HEAVY SHOWERS ON THE BAND.
FOR NOW...THE FORECAST REMAINS BROAD BRUSHED WITH HIGHER POPS AND NO
MENTION OF HEAVY RAINFALL. THE DETAILS WILL BECOME CLEARER OVER THE
NEXT COUPLE OF DAYS.

&&

.AVIATION...
TRADE WINDS WILL CONTINUE TO STRENGTHEN THROUGH TUESDAY. CLOUDS AND
SHOWERS WILL BE FOCUSED PRIMARILY ACROSS WINDWARD AND MAUKA
AREAS...WITH BRIEF MVFR CONDITIONS OCCURRING IN SHOWERS. GFS
FORECAST SOUNDINGS SHOW LOW-LEVEL WINDS APPROACHING 25 KNOTS BRIEFLY
OVERNIGHT. THE INVERSION IS EXPECTED TO STRENGTHEN FURTHER
TONIGHT...AND CONDITIONS WILL BE INCREASINGLY CONDUCIVE FOR
LOW-LEVEL TURBULENCE IN THE LEE OF THE ISLANDS. IF THESE CONDITIONS
DEVELOP...WE MAY NEED TO ISSUE AN AIRMET FOR MODERATE LOW-LEVEL
TURBULENCE FOR LATE TONIGHT/TUESDAY.

&&

.MARINE...
TRADES WILL BE AT BORDER LINE SMALL CRAFT ADVISORY LEVELS ACROSS
TYPICALLY WINDY WATERS AROUND THE BIG ISLAND AND MAUI THROUGH AT
LEAST FRIDAY. A RECENT ASCAT PASS CONFIRMED THESE CONDITIONS...AND
LITTLE CHANGE IS EXPECTED IN THE SHORT TERM. WINDS MAY BRIEFLY
INCREASE A NOTCH OVER THE NORTHERN HALF OF THE STATE AS A BAND OF
MOISTURE STALLS OVER THE CENTRAL PORTION OF THE ISLAND CHAIN ON
THURSDAY.

SURF WILL REMAIN AROUND BACKGROUND LEVELS THROUGH MUCH OF THE WEEK.
A SLIGHTLY HIGHER SWELL...POSSIBLY UP TO 4 FEET...IS EXPECTED FROM
THE NNE THURSDAY THROUGH SATURDAY. A MODERATE SSW IS DUE THIS
WEEKEND.

&&

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

&&

$$

PUBLIC AND MARINE...WROE
AVIATION...BRAVENDER



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