990  
FXHW60 PHFO 180704  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
904 PM HST WED OCT 17 2018  
   
SYNOPSIS
 
 
LIGHT TO MODERATE TRADES WILL CONTINUE THROUGH THE REMAINDER OF  
THE WORK WEEK. THE TRADES WILL THEN STRENGTHEN TO MODERATE LEVELS  
OVER THE WEEKEND AND POSSIBLY REACH LOCALLY BREEZE LEVELS NEXT  
WEEK AS HIGH PRESSURE BUILDS NORTH OF THE ISLANDS. CLOUDS AND  
SHOWERS ASSOCIATED WITH AND OLD FRONT WILL LINGER NEAR THE BIG  
ISLAND AND MAUI THROUGH THE WEEKEND, KEEPING A SHOWERY TRADE WIND  
PATTERN IN PLACE. MEANWHILE, DRIER CONDITIONS SHOULD PREVAIL  
ACROSS THE WESTERN HALF OF THE STATE WITH MAINLY WINDWARD AND  
MAUKA SHOWERS THROUGH SATURDAY, WITH A MORE SHOWERY TRADE WIND  
PATTERN EXPECTED SUNDAY THROUGH EARLY NEXT WEEK. A DISTURBANCE  
ALOFT WILL BRING UNSTABLE CONDITIONS AND THE POTENTIAL FOR HEAVY  
RAIN AND THUNDERSTORMS TO MAUI AND THE BIG ISLAND FRIDAY NIGHT  
THROUGH SUNDAY AS WELL, WHICH COULD CONTINUE INTO EARLY NEXT WEEK.  
 

 
   
DISCUSSION
 
 
CURRENTLY AT THE SURFACE, A WEAK RIDGE OF HIGH PRESSURE IS  
CENTERED AROUND 475 MILES NORTH OF KAUAI, DRIVING LIGHT TO  
MODERATE TRADE WINDS ACROSS THE ISLAND CHAIN THIS EVENING.  
MEANWHILE, A WEAKENING COLD FRONT IS LOCATED AROUND 550 MILES  
NORTHWEST OF KAUAI. INFRARED SATELLITE IMAGERY SHOWS A BAND OF  
CLOUDS ASSOCIATED WITH AN OLD FRONT OVER AND UPSTREAM OF THE BIG  
ISLAND. MOSTLY CLOUDY SKIES PREVAIL OVER WINDWARD AREAS, WITH  
PARTLY CLOUDY SKIES IN PLACE IN MOST LEEWARD LOCALES. RADAR  
IMAGERY SHOWS ISOLATED TO SCATTERED SHOWERS DRIFTING INTO WINDWARD  
SECTIONS OF THE SMALLER ISLANDS, WITH NUMEROUS SHOWERS AFFECTING  
WINDWARD SECTIONS OF THE BIG ISLAND. MEANWHILE, ASIDE FROM AN  
ISOLATED SHOWER OR TWO OVER LEEWARD AREAS, MOST AREAS ARE RAIN  
FREE.  
 
THE RIDGE OF HIGH PRESSURE NORTH OF THE STATE WILL WEAKEN OVER  
THE NEXT COUPLE DAYS AS A COLD FRONT PASSES BY WELL TO THE NORTH  
OF THE ISLAND CHAIN. LIGHT TO MODERATE TRADES ARE EXPECTED TO  
PREVAIL TONIGHT AND THURSDAY, THEN EASE SLIGHTLY THURSDAY NIGHT  
THROUGH FRIDAY NIGHT WITH LOCAL LAND AND SEA BREEZES POSSIBLE IN  
SOME OF THE MORE SHELTERED AREAS. A RIDGE OF HIGH PRESSURE WILL  
THEN BECOME RE-ESTABLISHED NORTH OF THE STATE OVER THE WEEKEND,  
BRINGING A RETURN OF MODERATE TRADE WINDS, WITH THE TRADES  
STRENGTHENING TO MODERATE AND LOCALLY BREEZY LEVELS DURING THE  
FIRST HALF OF NEXT WEEK.  
 
AS FOR SENSIBLE WEATHER DETAILS, A PLUME OF DEEP MOISTURE  
ASSOCIATED WITH THE REMNANTS OF AN OLD FRONT, WILL LINGER OVER THE  
BIG ISLAND THROUGH THE WEEKEND AND POSSIBLY INTO EARLY NEXT WEEK.  
IN THE NEAR TERM THROUGH FRIDAY, SHOWERY TRADE WIND WEATHER IS  
EXPECTED TO PERSIST OVER THE BIG ISLAND, WITH DRIER TRADE WIND  
WEATHER EXPECTED ACROSS THE SMALLER ISLANDS. CLOUDS AND SHOWERS  
WILL FAVOR WINDWARD AND MAUKA AREAS, BUT WE SHOULD SEE SOME  
INTERIOR AND LEEWARD SHOWER DEVELOPMENT AS A RESULT OF THE  
WEAKENING TRADES ON FRIDAY. ADDITIONALLY, WITH THE AIRMASS  
REMAINING UNSTABLE OVER THE BIG ISLAND THROUGH THE REMAINDER OF  
THE WORK WEEK, WILL KEEP A SLIGHT CHANCE OF THUNDERSTORMS IN THE  
FORECAST EACH AFTERNOON.  
 
THE MODELS ARE ADVERTISING QUITE A DIFFERENT SCENARIO THAN THEY  
WERE 24 HOURS AGO DURING THE WEEKEND AND INTO EARLY NEXT WEEK.  
BOTH THE GFS AND ECMWF NOW SHOW A SHARP UPPER TROUGH DIGGING  
SOUTHWARD OVER THE WESTERN ISLANDS FRIDAY NIGHT, THEN CLOSING OFF  
OVER THE CENTRAL AND EASTERN ISLANDS SATURDAY THROUGH SUNDAY. THE  
MODELS TO DIFFERING DEGREES, THEN SHOW THE WEAKNESS ALOFT  
LINGERING OVER ON IN THE VICINITY OF THE ISLAND CHAIN THROUGH THE  
FIRST HALF OF NEXT WEEK.  
 
DEEP MOISTURE ASSOCIATED WITH THE FRONTAL REMNANTS IS EXPECTED TO  
LINGER OVER THE EASTERN END OF THE STATE THROUGH THE MIDDLE OF  
NEXT WEEK, SO THIS SHOULD KEEP A SHOWERY TRADE WIND PATTERN IN  
PLACE HERE. ACROSS THE WESTERN ISLANDS THE MOISTURE WON'T BE QUITE  
AS DEEP THROUGH SATURDAY NIGHT, SO EXPECT THE SHOWER COVERAGE TO  
NOT BE AS GREAT HERE AT LEAST INITIALLY. THIS WILL LIKELY CHANGE  
ON SUNDAY AS THE DEEPER MOISTURE BEGINS TO LIFT BACK ACROSS  
WESTERN ISLANDS, MAKING FOR A MORE SHOWERY TRADE WIND PATTERN  
WHICH COULD CONTINUE THROUGH THE MIDDLE OF NEXT WEEK.  
 
THE UPPER TROUGH/LOW OVERHEAD COULD ALSO CREATE A FAIRLY UNSTABLE  
AIRMASS FRIDAY NIGHT THROUGH SUNDAY, PARTICULARLY ACROSS MAUI  
COUNTY AND THE BIG ISLAND. THE MAIN HAZARDS WOULD LIKELY BE HEAVY  
RAINFALL AND THUNDERSTORMS. WE'LL NEED TO MONITOR MODEL TRENDS  
CLOSELY OVER THE NEXT COUPLE DAYS HOWEVER, AS THERE IS THE  
POTENTIAL THAT WE COULD SEE SOME SEVERE WEATHER PARTICULARLY  
DURING THE SATURDAY AFTERNOON AND SATURDAY EVENING TIME FRAME AS  
THE COLDEST AIR ALOFT ROTATES OVERHEAD. AT THIS TIME THE MAIN  
SEVERE WEATHER THREATS IF THEY MATERIALIZE, APPEAR TO BE STRONG  
GUSTY WINDS AND HAIL.  
 

 
   
AVIATION
 
 
LIGHT TO OCCASIONALLY MODERATE TRADE WINDS WILL CONTINUE ACROSS  
THE ISLAND CHAIN THROUGH THURSDAY. BRIEF TO TEMPO MVFR CONDITIONS  
WILL MAINLY AFFECT WINDWARD AND MAUKA SECTIONS OVER ALL ISLES.  
THIS WILL ESPECIALLY BE THE CASE OVER THE BIG ISLAND, AND, TO A  
LESSER EXTENT, MAUI, WHERE MORE WIDESPREAD AND PERSISTENT MVFR  
WILL EXIST. LOW LEVEL REMNANT MOISTURE FROM AN OLD FRONT WILL  
AFFECT THOSE ISLES. AN ISOLATED THUNDERSTORM IS STILL A  
POSSIBILITY TOWARD THE EASTERN END OF THE STATE AS WELL. ELSEWHERE  
VFR WILL HOLD SWAY.  
 
AN AIRMET FOR MOUNTAIN OBSCURATION REMAINS IN EFFECT FOR THE  
WINDWARD AREAS OF THE BIG ISLAND.  
 

 
   
MARINE
 
 
TRADE WINDS ARE EXPECTED TO CONTINUE WEAKENING THROUGH FRIDAY.  
THE TRADE WINDS ARE EXPECTED TO STRENGTHEN OVER THE WEEKEND INTO  
EARLY NEXT WEEK, BUT THE CURRENT FORECAST DOES NOT HAVE SMALL  
CRAFT ADVISORY (SCA) CONDITIONS OVER ANY OF THE COASTAL WATERS OR  
CHANNELS THROUGH MONDAY.  
 
UNSTABLE ATMOSPHERIC CONDITIONS MAY DEVELOP OVER THE EASTERN  
ISLANDS TOWARD THE END OF THE WORK WEEK. THIS MAY CAUSE LOCALLY  
HEAVY DOWNPOURS, AND ALLOW THUNDERSTORMS TO DEVELOP ACROSS THE  
EASTERN PART OF THE STATE FROM LATE FRIDAY INTO THIS WEEKEND.  
MARINERS SHOULD MONITOR FUTURE WEATHER FORECASTS AS THIS  
SITUATION EVOLVES, PARTICULARLY NEAR MAUI AND THE BIG ISLAND.  
 
THE CURRENT NORTH-NORTHWEST SWELL IS GRADUALLY DECLINING, BUT  
SURF REMAINS ELEVATED THIS EVENING. THEREFORE, A HIGH SURF  
ADVISORY (HSA) REMAINS IN EFFECT ALONG MOST NORTH AND WEST FACING  
SHORES THROUGH EARLY THURSDAY MORNING. THE NORTH-NORTHWEST SWELL  
WILL CONTINUE TO SLOWLY SUBSIDE FROM THURSDAY THROUGH THIS  
WEEKEND. AS THE TRADE WINDS CONTINUE TO WEAKEN, EXPECT SURF TO  
SLOWLY DECREASE ALONG MOST EAST FACING SHORES. HOWEVER, SOME EAST  
FACING SHORES WITH A NORTHERLY EXPOSURE MAY CONTINUE TO SEE SMALL  
BREAKERS FROM THE NORTH-NORTHWEST SWELL ENERGY DURING THE NEXT  
COUPLE OF DAYS.  
 
A NEW LONG-PERIOD SOUTH SWELL ARRIVING FRIDAY, IS EXPECTED TO  
PEAK THIS WEEKEND. BASED ON THE LATEST GUIDANCE, SURF WILL LIKELY  
REACH THE HSA CRITERIA ALONG SOUTH FACING SHORES STARTING SATURDAY.  
THERE IS ALSO A POSSIBILITY SURF MAY APPROACH THE HIGH SURF  
WARNING THRESHOLD ALONG SOUTH FACING SHORES THIS WEEKEND, BEFORE  
THE SWELL BEGINS TO GRADUALLY DECLINE EARLY NEXT WEEK.  
 
SEE THE LATEST COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST FOR  
OAHU (SRDHFO), WHICH WAS UPDATED EARLIER THIS AFTERNOON, FOR MORE  
DETAILS ON SWELLS AND SURF.  
 

 
   
HFO WATCHES/WARNINGS/ADVISORIES
 
 
HIGH SURF ADVISORY UNTIL 6 AM HST THURSDAY FOR NIIHAU-KAUAI  
WINDWARD-KAUAI LEEWARD-WAIANAE COAST-OAHU NORTH SHORE-OAHU  
KOOLAU-OLOMANA-MOLOKAI-MAUI WINDWARD WEST-MAUI CENTRAL VALLEY-  
WINDWARD HALEAKALA-BIG ISLAND NORTH AND EAST.  
 

 
 

 
 
DISCUSSION...JELSEMA  
AVIATION...KINEL  
MARINE...HOUSTON  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HI Page Main Text Page