Weather Service Report


655 
FXHW60 PHFO 280130
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
330 PM HST FRI MAR 27 2015

.SYNOPSIS...
MODERATE TO LOCALLY FRESH TRADE WINDS WILL CONTINUE OVER THE WEEKEND 
AND EARLY NEXT WEEK...BEFORE DIMINISHING TUESDAY. MOSTLY DRY WEATHER 
WILL CONTINUE FOR MOST OF THE WEEKEND...WITH A SLIGHT INCREASE IN 
WINDWARD SHOWERS EXPECTED LATE SUNDAY AND MONDAY...ESPECIALLY ON 
KAUAI AND OAHU. A DISTURBANCE PASSING NORTH OF THE ISLANDS ON 
TUESDAY WILL WEAKEN THE TRADE WINDS...AND MAY BRIEFLY LEAD TO 
INCREASED WINDWARD SHOWERS. THE DISTURBANCE WILL MOVE EAST OF THE 
AREA ON WEDNESDAY AND THURSDAY...WITH TYPICAL TRADE WIND WEATHER 
EXPECTED THROUGH THE END OF NEXT WEEK. 

&&

.DISCUSSION...
MODERATE TO LOCALLY FRESH TRADE WINDS ARE BEING SUPPLIED BY A 1030 
MB SURFACE HIGH FAR NE OF THE ISLANDS...AND AN ASSOCIATED RIDGE 
ABOUT 500 MILES N OF MAUI. A RIDGE ALOFT IS SUPPORTING A STABLE 
ATMOSPHERE...AND A MOSTLY DRY TRADE WIND WEATHER PATTERN IS IN PLACE 
OVER THE ISLANDS. AFTERNOON SOUNDINGS SHOW A STRONG SUBSIDENCE 
INVERSION BASED NEAR 6 KFT...AND PWAT VALUES NEAR 0.8 INCHES...WHICH 
IS ABOUT ONE STANDARD DEVIATION BELOW NORMAL FOR MARCH/APRIL. WHILE 
STABLE LOW CLOUDS HAVE BEEN BANKED UP ALONG WINDWARD SLOPES THROUGH 
THE DAY...AND ALONG LEEWARD SLOPES OF THE BIG ISLAND THIS 
AFTERNOON...VERY LITTLE RAINFALL HAS BEEN OBSERVED. EXPECT A SLIGHT 
MOISTENING OF THE TRADE FLOW OVERNIGHT TO ALLOW FOR A FEW WINDWARD 
SHOWERS...BUT THEY ARE EXPECTED TO BE BRIEF AND LIGHT.

A MOSTLY DRY TRADE WIND WEATHER PATTERN IS EXPECTED THROUGH MOST OF 
THE WEEKEND...AS A MID LEVEL RIDGE REMAINS OVERHEAD...AND TRADE 
WINDS GRADUALLY DIMINISH AS THE SURFACE RIDGE N OF THE ISLANDS 
RETREATS EASTWARD. A NEW HIGH BUILDING NW OF THE AREA ON SUNDAY WILL 
PUSH REMNANT FRONTAL MOISTURE TOWARD THE ISLANDS FROM THE N ON 
SUNDAY AND MONDAY. ALTHOUGH THIS MOISTURE IS NOT EXPECTED TO MAKE IT 
AS FAR S AS THE ISLANDS...WEAKLY CONVERGENT LOW LEVEL FLOW WILL LEAD 
TO INCREASED TRADE WIND SHOWERS...MAINLY OVER KAUAI AND OAHU...FROM  
LATE SUNDAY INTO MONDAY. THE ATMOSPHERE WILL CONTINUE TO BE 
RELATIVELY STABLE SO THESE SHOWERS ARE NOT EXPECTED TO BE HEAVY. 

THE TRADE WIND FLOW COULD BE DISRUPTED AROUND TUESDAY AS A 
PROGRESSIVE MID-LEVEL TROUGH PASSES N OF THE ISLANDS...AND INDUCES 
THE FORMATION OF A SURFACE TROUGH NEAR THE ISLANDS. THIS IS SEEN IN 
DETERMINISTIC GFS GUIDANCE...BUT MOST GFS ENSEMBLE MEMBERS ARE 
WEAKER THAN THE DETERMINISTIC RUN WITH THE DEPTH OF THE MID-LEVEL 
TROUGH...AND THE STRENGTH OF THE SURFACE TROUGH. LATEST ECMWF 
GUIDANCE SHOWS THE TROUGH ALOFT PASSING N OF THE ISLANDS...WITH 
LITTLE IN THE WAY OF A SURFACE REFLECTION. GIVEN THESE SOLUTIONS... 
THE FORECAST LEANS AWAY FROM THE LIGHTER WINDS DEPICTED BY THE 
DETERMINISTIC GFS AND LEANS TOWARD THE GEFS/ECMWF...WHICH KEEPS 
LIGHT TO MODERATE TRADE WINDS FOR MOST OF NEXT WEEK. THE PASSING 
DISTURBANCE MAY BRIEFLY DESTABILIZE THE ATMOSPHERE ON TUESDAY AND 
WEDNESDAY...LEADING TO INCREASED WINDWARD SHOWERS. TRADES COULD BE 
LIGHT ENOUGH ON TUESDAY FOR SEA BREEZES TO DRIVE AFTERNOON LEEWARD 
CLOUD FORMATION.   
 
&&

.AVIATION...
PREDOMINANTLY VFR CONDITIONS ARE EXPECTED AT ALL TERMINALS THROUGH 
29/00Z. NO AIRMETS ARE IN EFFECT AND NONE ARE ANTICIPATED TONIGHT. 

&&

.MARINE...
A LATE MORNING SCATTEROMETER PASS THAT SAMPLED WATERS E OF OAHU 
SHOWED WINDS AT OR ABOVE SMALL CRAFT ADVISORY CRITERIA IN WATERS N 
AND S OF THE BIG ISLAND...BUT BELOW CRITERIA IN OTHER MARINE ZONES. 
LATEST GUIDANCE FROM GLOBAL AND MESOSCALE MODELS INDICATE THAT WINDS 
NEAR 25 KT WILL CONTINUE OVER WATERS N AND S OF THE BIG ISLAND 
OVERNIGHT...WHERE A SCA IS IN EFFECT THROUGH 6 AM SATURDAY. TRADE 
WINDS ARE STILL EXPECTED TO GRADUALLY EASE THROUGH SATURDAY...AND 
WHILE A SLIGHT INCREASE IN WIND SPEEDS IS EXPECTED SUNDAY/ 
MONDAY...THE EXPECTATION IS THAT WINDS WILL REMAIN BELOW SCA SPEEDS 
BEFORE WINDS DIMINISH AGAIN SOMEWHAT TUESDAY. 

NOT MUCH SURF TO SPEAK OF CURRENTLY...AS LINGERING SSW AND NNW 
SWELLS GRADUALLY FADE AWAY THROUGH SATURDAY...AND A SMALL 
LONG-PERIOD SSE SWELL BUILDS. A LONG-PERIOD NW SWELL WILL BUILD AND 
PEAK ON SUNDAY...WITH PEAK SURF HEIGHTS NEARING ADVISORY LEVELS 
ALONG EXPOSED N AND W SHORES. THIS SWELL WILL GRADUALLY DIMINISH IN 
HEIGHT AND PERIOD THROUGH TUESDAY. AT ITS PEAK...THIS SWELL WILL 
BRING COMBINED SEAS CLOSE TO THE 10 FT THRESHOLD FOR SCA ISSUANCE. 
SWELLS ARRIVING FROM THE S WILL BE RELATIVELY LOW IN MAGNITUDE...BUT 
SUFFICIENTLY LARGE TO KEEP SOME SURF ALONG S FACING SHORES THROUGH 
THE PERIOD...WITH THE STRONGEST SOURCE BEING A SSE SWELL THAT IS 
CURRENTLY ON THE RISE. THE RECENTLY UPDATED OAHU SURF DISCUSSION 
/SRDHFO/ CONTAINS DETAILS ON THE SOURCES OF THE EXPECTED SWELLS. 

&&

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

&&

$$

DISCUSSION/MARINE...BIRCHARD
AVIATION...JELSEMA



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