917  
FXHW60 PHFO 201355  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
400 AM HST MON AUG 20 2018  
   
SYNOPSIS
 
 
A DRIER AIRMASS WILL PERSIST ACROSS THE STATE THROUGH TUESDAY, WITH  
TRADE WINDS STRENGTHENING. FOR THE REMAINDER OF THE WEEK, THE  
FORECAST WILL BE HIGHLY DEPENDENT ON THE TRACK AND INTENSITY OF  
HURRICANE LANE. AN INCREASE IN WINDS AND SHOWERS WILL BE THE LIKELY  
RESULT BASED ON THE LATEST FORECAST FOR LANE.  
 

 
   
DISCUSSION
 
 
PLEASE NOTE THAT TROPICAL CYCLONE OPERATIONS ARE IN EFFECT. SO AS  
TO INCORPORATE THE LATEST FORECAST INFORMATION FROM THE CENTRAL  
PACIFIC HURRICANE CENTER (CPHC), PUBLIC FORECAST UPDATES FROM THE  
HONOLULU FORECAST OFFICE WILL COME SHORTLY AFTER FORECAST UPDATES  
FROM CPHC, NAMELY AFTER 5 AND 11 AM/PM, INSTEAD OF THE NORMAL 4  
AM/PM.  
 
DRIER AIRMASS HAS SPREAD ACROSS THE STATE SINCE LAST NIGHT, THOUGH  
EARLY MORNING LIHUE SOUNDING ONLY INDICATED A SLIGHT DRYING, WHILE  
HILO SOUNDING HAS A MORE TYPICAL TRADE WIND WEATHER TYPE PROFILE.  
LATEST RADAR DATA REVEALED NOT MUCH SHOWERS, THOUGH SATELLITE  
IMAGERY, EVEN WITH SOME HIGH CLOUDS OBSCURING, INDICATED THERE ARE  
EMBEDDED LOW CLOUDS JUST UPSTREAM EAST OF THE ISLANDS. A SURFACE  
HIGH PRESSURE FAR NORTHEAST OF THE STATE PROVIDING THE TRADE WINDS  
CONTINUES TO DROP SOUTH-SOUTHWESTWARD. MEANWHILE ANOTHER HIGH  
PRESSURE FAR NORTHWEST OF THE STATE IS ADVANCING EAST AS WELL. THIS  
WILL ALLOW A TIGHTENING OF PRESSURE GRADIENT ACROSS THE AREA,  
RESULTING IN AN INCREASE IN TRADE WINDS FOR THE STATE. REGARDLESS,  
THE MORE TYPICAL TRADE WIND WEATHER IS BACK ACROSS AREA, WITH  
PASSING LOW CLOUDS AND SHOWERS AFFECTING MAINLY WINDWARD AND  
MOUNTAIN AREAS. HIGH CLOUDS WILL STREAM OVER THE AREA AT TIMES TODAY  
AND WILL KEEP SKIES A LITTLE MORE OPAQUE AT TIMES.  
 
THE AFOREMENTIONED TWO HIGHS WILL GRADUALLY MERGE AND TAKE UP A  
POSITION FAR TO OUR NORTH BY TUESDAY. THIS WILL LEAD TO A FURTHER  
STRENGTHENING IN TRADE WIND SPEEDS ACROSS AREA. THE SOMEWHAT DRIER  
AND MORE STABLE AIRMASS WILL PERSIST IN THE AS WELL. THEREFORE,  
EXPECT BREEZY TRADE WIND WEATHER FOR THE ISLANDS TUESDAY, WITH  
PASSING LOW CLOUDS AND SHOWERS AFFECTING MAINLY WINDWARD AND  
MOUNTAIN AREAS. LEE AREAS OF THE SMALLER ISLANDS MAY ALSO SEE SOME  
CLOUDS AND SHOWERS AS WINDS MAY BE STRONG ENOUGH TO CARRY THEM OVER.  
 
AS FOR WEDNESDAY THROUGH THE WEEKEND TIME FRAME, OUR WEATHER WILL BE  
HIGHLY DEPENDENT ON THE TRACK AND INTENSITY OF HURRICANE LANE. IT IS  
STILL TOO EARLY TO DETERMINE THE WEATHER IMPACTS FROM LANE WILL HAVE  
ON THE STATE. THE LAST FORECAST TRACK FROM THE CENTRAL PACIFIC  
HURRICANE CENTER STILL HAS LANE PASSING SOUTH OF THE MAIN HAWAIIAN  
ISLANDS, BUT HAS IT TRACKED CLOSER TO THE ISLAND CHAIN. IF THIS  
TRACK HOLDS, THE POTENTIAL IMPACTS ON THE ISLANDS WEATHER WOULD BE  
AN INCREASE IN WIND SPEEDS DUE TO A TIGHTENED PRESSURE GRADIENT  
BETWEEN HIGH PRESSURE NORTH OF THE STATE AND LANE TRAVERSING TO OUR  
SOUTH. ALSO MOISTURE ASSOCIATED WITH LANE WOULD LIKELY BRING AN  
INCREASE IN SHOWER ACTIVITY OVER THE BIG ISLAND WEDNESDAY, MAINLY  
EAST AND SOUTHEAST AND THEN EVENTUALLY SPREADING UP THE ISLAND CHAIN  
THROUGH THE REMAINDER OF THE WEEK.  
 
EVERYONE IS ENCOURAGED TO MONITOR THE LATEST ADVISORIES ON HURRICANE  
LANE FROM THE CENTRAL PACIFIC HURRICANE CENTER.  
 

 
   
AVIATION
 
 
A HIGH PRESSURE RIDGE NORTH OF THE ISLANDS WILL PRODUCE STRONGER  
TRADE WINDS TODAY WITH WITH IMPROVING WEATHER TRENDS AS DRIER MORE  
STABLE AIR MOVES IN FROM THE EAST.  
 
NO AIRMETS IN EFFECT. AS WINDS STRENGTHEN AND THE ATMOSPHERE  
BECOMES MORE STABLE THIS AFTERNOON, TURBULENCE IN THE LEE OF THE  
MOUNTAINS MAY BECOME MODERATE AND AIRMET TANGO MAY BE NEEDED.  
 

 
   
MARINE
 
 
PLEASE NOTE THAT TROPICAL CYCLONE OPERATIONS ARE IN EFFECT. SO AS TO  
INCORPORATE THE LATEST FORECAST INFORMATION FROM THE CENTRAL PACIFIC  
HURRICANE CENTER (CPHC), MARINE FORECAST UPDATES FROM THE HONOLULU  
FORECAST OFFICE WILL COME SHORTLY AFTER FORECAST UPDATES FROM CPHC,  
NAMELY SHORTLY AFTER 5 AND 11 AM/PM, INSTEAD OF THE NORMAL 4 AND 10  
AM/PM.  
 
A WEAK SURFACE TROUGH W OF KAUAI WILL MOVE W AWAY FROM THE AREA  
TODAY AND TONIGHT AS HIGH PRESSURE BUILDS TO THE N. THIS WILL RESULT  
IN A GRADUALLY INCREASING TRADE WIND FLOW THROUGH TUESDAY. A SMALL  
CRAFT ADVISORY (SCA) HAS BEEN POSTED FOR THE WINDIER AREAS AROUND  
MAUI AND THE BIG ISLAND IN ANTICIPATION OF THE INCREASING TRADE WIND  
FLOW. AS HURRICANE LANE MOVES INTO AN AREA S OF THE ISLANDS BY  
MID-WEEK, THE PRESSURE GRADIENT WILL LIKELY INCREASE, LIKELY LEADING  
TO INCREASING TRADE WINDS, AND POTENTIALLY AN EXPANSION OF THE SCA.  
MORE SIGNIFICANTLY, TROPICAL STORM AND/OR HURRICANE WATCHES MAY BE  
REQUIRED FOR PORTIONS OF THE SOUTHERN COASTAL WATERS LATER TODAY.  
THERE IS SIGNIFICANT UNCERTAINTY REGARDING THE POSITION AND  
INTENSITY OF LANE LATER IN THE WEEK, SO IT IS TOO EARLY TO DETERMINE  
WHAT IMPACTS LANE MAY EVENTUALLY BRING, AND MARINERS NEED TO  
CONTINUE TO CLOSELY MONITOR FORECASTS.  
 
LONG PERIOD ESE SWELLS FROM HURRICANE LANE ARE BUILDING AT BUOY  
51004 TO THE SE OF THE BIG ISLAND THIS MORNING, WHILE THE PACIOOS  
BUOY IN HILO IS JUST BEGINNING TO DETECT LONG-PERIOD ENERGY FROM THE  
E. A HIGH SURF ADVISORY FOR E FACING SHORES OF MAUI AND THE BIG  
ISLAND IS NOW IN EFFECT. AS HURRICANE LANE MOVES INTO AN AREA S OF  
THE ISLANDS LATER THIS WEEK, S SWELL WILL LIKELY INCREASE, WITH A  
WIDE VARIETY OF WAVE PERIODS LEADING TO ROUGH AND POTENTIALLY  
DAMAGING SURF. STILL SIGNIFICANT UNCERTAINTY AND DIFFICULT TO KNOW  
THE LEVEL OF IMPACTS, BUT THERE IS POTENTIAL FOR LARGE SEAS/SWELLS  
FROM LANE TO LEAD TO WARNING-LEVEL SURF ALONG E THROUGH S FACING  
SHORES OF THE ISLANDS. A SMALL NNW SWELL IS EXPECTED TO ARRIVE  
MONDAY AND PEAK ON TUESDAY BEFORE GRADUALLY DIMINISHING WEDNESDAY  
AND THURSDAY.  
 
SEE THE LATEST OAHU SURF DISCUSSION (SRDHFO) FOR MORE DETAILS ON  
SURF AND SWELL.  
 

 
   
HFO WATCHES/WARNINGS/ADVISORIES
 
 
HIGH SURF ADVISORY UNTIL 6 PM HST TUESDAY FOR WINDWARD HALEAKALA-  
SOUTH BIG ISLAND-BIG ISLAND NORTH AND EAST.  
 
SMALL CRAFT ADVISORY UNTIL 6 PM HST TUESDAY FOR MAALAEA BAY-  
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG  
ISLAND SOUTHEAST WATERS.  
 

 
 

 
 
SYNOPSIS/DISCUSSION...HUI  
AVIATION...BOHLIN  
MARINE...BIRCHARD  
 
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