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Hawaiian Islands Synoptic Discussion and Guidance

821
FXHW60 PHFO 251947
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST FRI JUL 25 2014

.SYNOPSIS...
TRADE WINDS WILL FOCUS SHOWERS MAINLY ACROSS WINDWARD AND MAUKA
AREAS. AN INCREASE IN SHOWER ACTIVITY IS EXPECTED DURING THE
WEEKEND. TRADES MAY ALSO STRENGTHEN EARLY NEXT WEEK.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS A MID/UPPER LEVEL HIGH OVER/JUST SOUTH OF
THE STATE. AN UPPER LEVEL TROUGH IS LOCATED FAR TO THE NORTHWEST...
WITH A 50 KT WEST-SOUTHWESTERLY JET AHEAD OF IT JUST APPROACHING
KAUAI. AT THE SURFACE...HIGH PRESSURE IS LOCATED FAR NORTH-NORTHEAST
OF THE STATE. 12Z SOUNDINGS FROM HILO AND LIHUE SHOW WELL DEFINED
INVERSIONS NEAR 6KFT. PRECIPITABLE WATERS AROUND 1 1/3 INCHES ARE
NEAR NORMAL FOR THIS TIME OF YEAR. EARLY MORNING MIMIC TOTAL
PRECIPITABLE WATER IMAGERY SHOWS A POCKET OF DRIER AIR ACROSS THE
CENTRAL ISLANDS AND JUST EAST OF THE BIG ISLAND...WITH A LARGER AREA
OF MOISTURE ABOUT 200-300 MILES SOUTHEAST AND EAST OF THE BIG
ISLAND.

SHOWERS HAVE BEEN SOMEWHAT LIMITED OVERNIGHT DUE TO THE DRIER AND
MORE STABLE AIR. KAUAI WAS SLIGHTLY WETTER THAN THE REST OF THE
STATE DUE TO A POCKET OF MOISTURE CURRENT PASSING OVER THEM. A
DRYING TREND WITH IMPROVING CONDITIONS IS EXPECTED AGAIN THIS
AFTERNOON...SIMILAR TO THE TREND WE SAW YESTERDAY. TRADE WINDS WILL
BE LESS BREEZY THAN THE PAST COUPLE DAYS. THE SURFACE HIGH FAR TO
THE NORTHEAST HAS WEAKENED...AND SURFACE PRESSURES ACROSS THE
ISLANDS HAVE DROPPED ABOUT 1-2MB OVER THE PAST 24 HOURS. TRADES
SHOULD REMAIN IN THE GENERAL MODERATE CATEGORY THROUGH THE WEEKEND.

THE AREA OF MOISTURE SOUTHEAST AND EAST OF THE BIG ISLAND WILL MOVE
STEADILY WESTWARD. GFS 850-700MB RELATIVE HUMIDITY DOES A REASONABLE
JOB DEPICTING THIS AREA...AND BRINGS IT TO THE BIG ISLAND EARLY
SATURDAY. BOTH GFS AND ECMWF ARE RELATIVELY SIMILAR WITH THIS
TIMING. AT THE SAME TIME...THE UPPER TROUGH NORTHWEST OF THE STATE
WILL MOVE EASTWARD NORTH OF THE AREA. 12Z MODELS SHOW THE TROUGH
PASSING JUST NORTH OF KAUAI ON SUNDAY BEFORE SLOWING AND WEAKENING.
THE CURRENT FORECAST HAS AN INCREASE IN RAINFALL CHANCES THIS
WEEKEND FROM EAST TO WEST BEGINNING NEAR THE BIG ISLAND LATE
TONIGHT. MOST SHOWERS WILL BE FOCUSED ACROSS WINDWARD AND MAUKA
AREAS...BUT SOME WILL SPREAD LEEWARD AS WELL. THE INCREASE IN
MOISTURE AND RISE IN INVERSION HEIGHT COULD LEAD TO BRIEF HEAVIER
SHOWERS...BUT WITH PERSISTENT MODERATE TRADES WE DO NOT EXPECT ANY
HEAVY SHOWERS TO BECOME ANCHORED.

DURING THE FIRST HALF OF NEXT WEEK...THE HIGH NORTHEAST OF THE STATE
WEAKENS...BUT A POTENTIAL SYSTEM PASSING FAR SOUTH OF THE STATE
COULD ENHANCE THE GRADIENT ACROSS THE ISLANDS. THIS MAY LEAD TO AN
INCREASE IN WINDS STARTING AS EARLY AS SUNDAY NIGHT. 12Z MODELS HAVE
BACKED OFF ON A SURGE OF TROPICAL MOISTURE THAT HAD BEEN ADVERTISED
TO SPREAD ACROSS THE STATE DURING THE MID-WEEK TIME FRAME. THE 12Z
GFS INSTEAD BRINGS SOME MOISTURE JUST AS FAR NORTH AS THE BIG ISLAND
WEDNESDAY/THURSDAY. SPECIFICS ABOUT ANY INCREASE IN SHOWERS DURING
MID TO LATE WEEK REMAIN UNCERTAIN.

&&

.AVIATION...
VFR CONDITIONS WILL PREDOMINATE ALL AREAS TODAY. ISOL MVFR CONDS ARE
POSSIBLE IN PASSING TRADE WIND SHOWERS BUT SHOULD REMAIN LIMITED
TO WINDWARD SLOPES AND NOT AFFECT TERMINAL LOCATIONS. GUSTY TRADE
WINDS ALONG EXPOSED AND THROUGH CHANNELED AREAS ARE EXPECTED ONCE
AGAIN THIS AFTERNOON BUT THE PRESSURE GRADIENT ACROSS THE ISLANDS
IS SLACKENING SO INTENSITIES SHOULD BE NOTABLY LOWER THAN THE LAST
COUPLE DAYS.

&&

.MARINE...
WINDS MAY INCREASE TO SMALL CRAFT ADVISORY LEVELS AS EARLY AS SUNDAY
NIGHT FOR THE TYPICALLY WINDIER AREAS NEAR MAUI AND THE BIG ISLAND.
EVEN AS THE SURFACE HIGH FAR NORTHEAST OF THE STATE WEAKENS
SLIGHTLY...A POTENTIAL SYSTEM PASSING FAR SOUTH OF THE AREA COULD
ENHANCE THE GRADIENT ACROSS THE ISLANDS. SMALL CRAFT ADVISORY WINDS
ARE THEN POSSIBLE FOR THOSE AREAS THROUGH MUCH OF THE NEW WEEK.

NO SIGNIFICANT SWELLS ARE EXPECTED THROUGH THE NEXT SEVERAL DAYS.
SMALL LONG-PERIOD BACKGROUND SWELL FROM THE SSW AND SHORT-PERIOD
SWELL FROM THE SE REPRESENT THE SWELL SOURCES FROM THE SOUTHERN
HEMISPHERE...WHILE SHORT PERIOD WIND WAVES FROM THE EAST REPRESENT
THE ONLY SOURCE OF SWELL FROM THE NORTHERN HEMISPHERE.

&&

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

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...BEDAL



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