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Hawaiian Islands Synoptic Discussion and Guidance

965
FXHW60 PHFO 232005
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1005 AM HST WED JUL 23 2014

.SYNOPSIS...
DRIER AND LOCALLY BREEZY TRADE WIND WEATHER WILL PREVAIL ACROSS THE
STATE THROUGH THURSDAY...WITH TRADE WINDS DIMINISHING SLIGHTLY AS WE
HEAD INTO THE WEEKEND. ONLY BRIEF WINDWARD SHOWERS ARE EXPECTED THE
NEXT COUPLE OF DAYS...WITH A CHANCE THAT WINDWARD SHOWERS WILL
INCREASE TOWARDS THE LATER PART OF THE WEEKEND AND INTO EARLY NEXT
WEEK.

&&

.DISCUSSION...
CURRENTLY...MODERATE TO LOCALLY STRONG TRADE WINDS OVER THE ISLANDS
ARE BEING SUPPORTED BY A 1032 MB HIGH CENTERED ABOUT 1500 MILES TO
THE N. THE ISLAND AIR MASS IS IN THE PROCESS OF DRYING OUT...WITH
SURFACE DEW POINTS DOWN ABOUT 7-8 DEGREES F OVER THE PAST 24
HOURS...AND DRIER AIR IMMEDIATELY UPSTREAM. WHILE EARLY MORNING
SOUNDINGS SHOW A SUBSIDENCE INVERSION IN PLACE BETWEEN 7 AND 8
KFT...THEY ALSO INDICATE A FAIR AMOUNT OF INSTABILITY/CAPE ABOVE
ABOUT 12 KFT. LATEST VISIBLE IMAGERY SHOWS A FAIRLY TYPICAL
DISTRIBUTION OF LOW CLOUDS OVER THE ISLANDS AND NEAR SHORE WATERS
THIS MORNING...EXCEPT THE LOWER WINDWARD AND LEEWARD SLOPES OF THE
BIG ISLAND ARE A LITTLE CLOUDIER THAN NORMAL.

THE FORECAST CALLS FOR MID AND UPPER LEVEL RIDGING TO BECOME BETTER
DEVELOPED OVER THE ISLANDS IN THE SHORT TERM...AS DRIER AIR MOVES IN
FROM THE E. THIS WILL LEAD TO A FAIRLY TYPICAL AND BREEZY TRADE WIND
PATTERN INTO FRIDAY...WITH BRIEF WINDWARD SHOWERS FAVORING NIGHTS
AND MORNINGS...AND TYPICAL AFTERNOON CLOUDS AND SHOWERS OVER THE BIG
ISLAND LEEWARD SLOPES. WITH THE INSTABILITY ALOFT NOTED IN OVERNIGHT
SOUNDINGS...THERE IS A CHANCE TODAY THAT WE SEE A REPEAT OF
YESTERDAY AFTERNOON...WHEN THUNDERSTORMS AND HEAVY SHOWERS DEVELOPED
OVER INTERIOR AND LEEWARD SLOPES OF THE BIG ISLAND. WILL BE UPDATING
THE FORECAST TO INCREASE AFTERNOON POPS AND SKY COVER IN THAT AREA...
BUT WILL BE OMITTING THE THUNDERSTORM THREAT.

THE TRADE WIND-SUPPLYING HIGH TO THE N IS FORECAST TO WEAKEN AND
SLIDE E LATE THURSDAY INTO FRIDAY...WITH TRADE WINDS GRADUALLY
DIMINISHING IN RESPONSE. ALTHOUGH A WEAKNESS IN THE RIDGE PREVAILS N
OF THE ISLANDS INTO NEXT WEEK...GLOBAL FORECAST MODELS ARE
ANTICIPATING THAT A WEAK TROPICAL SYSTEM WILL PASS WELL S OF THE
ISLANDS THIS WEEKEND AND EARLY NEXT WEEK. WHILE ANY INCIPIENT SYSTEM
IS NOT EXPECTED TO THREATEN THE ISLANDS...IT/S PRESENCE COULD
TIGHTEN THE LOCAL PRESSURE GRADIENT AND PRODUCE STRONGER TRADE WINDS
THAN CURRENTLY FORECAST.

A SLIGHT INCREASE IN TRADE SHOWERS IS POSSIBLE OVER THE WEEKEND AND
EARLY NEXT WEEK AS RIDGING ALOFT GIVES WAY TO WEAK TROUGHING...AND
LOW LEVEL MOISTURE INCREASES FROM THE ESE SUNDAY AND MONDAY. TYPICAL
TRADE WIND WEATHER ON TUESDAY MAY GIVE WAY TO WETTER CONDITIONS
WEDNESDAY AND THURSDAY AS MORE TROPICAL MOISTURE POTENTIALLY ADVECTS
IN FROM THE ESE.

&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ REMAINS IN EFFECT FOR THE TYPICALLY
WINDY WATERS AROUND THE BIG ISLAND AND MAUI DUE TO FRESH TO LOCALLY
STRONG TRADE WINDS. WINDS ARE EXPECTED TO DIMINISH BELOW SCA SPEEDS
FROM FRIDAY INTO THE WEEKEND...ALTHOUGH THERE IS THE OUTSIDE CHANCE
A TROPICAL SYSTEM PASSING WELL S OF THE ISLANDS KEEPS THE LOCAL
GRADIENT TIGHTER THAN CURRENTLY EXPECTED...WHICH WOULD LEAD TO
STRONGER TRADE WINDS.

NO SIGNIFICANT SWELLS ARE EXPECTED THROUGH THE UPCOMING WEEK. SMALL
LONG-PERIOD BACKGROUND SWELL FROM THE SSW AND SHORT-PERIOD SWELL
FROM THE SE REPRESENT THE SWELL SOURCES FROM THE S HEMISPHERE...
WHILE SHORT PERIOD WIND WAVES FROM THE E REPRESENT THE ONLY SOURCE
OF SWELL FROM THE N HEMISPHERE.

&&

.AVIATION...
MODERATE AND GUSTY TRADE WINDS WILL CONTINUE THROUGH THE PERIOD.
AIRMET TANGO IS POSTED FOR LOW LEVEL TURB IN THE LEE OF THE TERRAIN
FOR ALL ISLANDS BELOW 8 KFT.

OVERALL...ISLAND TERMINALS WILL REMAIN PREDOMINATELY VFR. TRADES
WILL CARRY PASSING SHOWERS...MAINLY AFFECTING WINDWARD AND MAUKA
AREAS. SOME SHOWERS MAY PRODUCE BRIEF MVFR CONDITIONS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY THROUGH 6 PM HST THURSDAY FOR THE PAILOLO
AND ALENUIHAHA CHANNELS...MAALAEA BAY AND WATERS SOUTH OF THE BIG
ISLAND.

&&

$$

DISCUSSION/MARINE...BIRCHARD
AVIATION...EATON



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