601  
FXHW60 PHFO 201939  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
939 AM HST THU JUL 20 2017  
   
SYNOPSIS
 
 
A PERSISTENT RIDGE TO OUR NORTH WILL KEEP BREEZY TRADES BLOWING  
THROUGH FRIDAY. THE TRADES WILL EASE THIS WEEKEND AS THE RIDGE  
WEAKENS. TROPICAL MOISTURE WILL OVERSPREAD THE ISLANDS THIS  
WEEKEND, LEADING TO HIGHER HUMIDITY AND AN INCREASE IN SHOWERS.  
THE REMNANTS OF FERNANDA ARE EXPECTED TO PASS JUST NORTH OF THE  
ISLANDS ON MONDAY, LIKELY WEAKENING OUR WINDS, AND INTRODUCING  
MUGGY CONDITIONS.  
 

 
   
DISCUSSION
 
 
A RIDGE OF HIGH PRESSURE EXTENDS FROM A 1031 MB HIGH NEAR 40N180W  
THROUGH 35N155W AND 33N140W, FAR NORTH OF THE MAIN HAWAIIAN  
ISLANDS. THIS RIDGE HAS BEEN A FIXTURE WITH OUR WEATHER FOR THE  
PAST FEW DAYS, KEEPING TRADES ON THE BREEZY SIDE AND PUSHING  
CLOUDS AND SHOWERS ACROSS WINDWARD AND MAUKA AREAS. WE EXPECT THIS  
PATTERN TO PERSIST THROUGH FRIDAY. TROUGHING NORTH OF THE RIDGE  
WILL DIG SOUTHWARD BY THIS WEEKEND, PUTTING A DENT IN THE RIDGE  
BETWEEN 160W AND 180W, WHICH WILL RESULT IN WEAKENED TRADES FOR  
THE ISLANDS.  
 
TROPICAL STORM FERNANDA IS ABOUT 1000 MILES EAST OF HILO AS OF  
THIS WRITING, WEAKENING AS IT IS POISED TO CROSS 140W. SATELLITE  
LOOP SHOWS SKIES ACROSS THE MAIN HAWAIIAN ISLANDS ARE CLEARING  
SOMEWHAT THIS MORNING, BUT INCREASED TROPICAL MOISTURE AHEAD OF  
FERNANDA IS NOTED EAST OF ABOUT 152W. FERNANDA IS FORECAST TO PASS  
NORTH OF THE ISLANDS ON MONDAY, BUT THE EASTERN EDGE OF THE LOW  
CLOUD FIELD WHICH MERGES WITH THAT OF FERNANDA'S WILL REACH THE  
EASTERN ISLANDS TONIGHT. ALSO, MODELS SHOW AN INCREASE IN DEEP  
MOISTURE FOR THE EASTERN END OF THE STATE ON FRIDAY WHICH WILL  
BRING AN INCREASE IN TRADE WIND SHOWERS THERE, WITH DRY TRADE  
WIND WEATHER EXPECTED TO CONTINUE ACROSS THE CENTRAL AND WESTERN  
ISLANDS.  
 
WHILE TRADE WINDS ARE FORECAST TO DECREASE FRIDAY NIGHT THROUGH  
THE WEEKEND, WE STILL EXPECT INCREASED CLOUDS AND SHOWERS  
ASSOCIATED WITH FERNANDA TO FAVOR WINDWARD AND MAUKA AREAS.  
HOWEVER, LEEWARD SHOWERS MAY DEVELOP AT TIMES DURING THE  
AFTERNOONS IF LOCAL SEA BREEZES FORM.  
 
AS THE REMNANTS OF FERNANDA PASS BY TO THE NORTH OF THE ISLANDS  
ON MONDAY, TRADE WINDS WILL WEAKEN FURTHER AND SEA BREEZE  
DEVELOPMENT IS EXPECTED ACROSS MOST AREAS. THIS LIGHT WIND REGIME,  
WITH OVERNIGHT LAND BREEZES AND DAYTIME SEA BREEZES, MAY CONTINUE  
INTO TUESDAY BEFORE THE REMNANTS OF FERNANDA PASS BY AND HIGH  
PRESSURE STRENGTHENS NORTH OF THE STATE. BREEZY TRADE WINDS WILL  
THEN RETURN BY THE MIDDLE OF NEXT WEEK. MODELS SHOW THE MAIN  
AREA OF MOISTURE ASSOCIATED WITH FERNANDA WILL PASS NORTH OF THE  
ISLANDS ON MONDAY BUT, WITH THE HUMID CONDITIONS AND LIGHT WINDS  
IN PLACE, AFTERNOON AND EVENING SHOWERS ARE EXPECTED PARTICULARLY  
OVER THE INTERIOR OF THE ISLANDS DURING THE FIRST HALF OF NEXT  
WEEK. A DRIER, LESS HUMID, AND MORE STABLE AIRMASS IS THEN  
EXPECTED TO RETURN BY THE MIDDLE OF NEXT WEEK.  
 

 
   
AVIATION
 
 
MODERATE TO BREEZY TRADE WINDS WILL PERSIST AS A HIGH PRESSURE  
RIDGE REMAINS FAR TO THE NORTH OF THE ISLANDS. BRIEF MVFR  
CONDITIONS, LOWER CEILINGS AND PASSING SHOWERS, WILL MAINLY  
AFFECT WINDWARD AREAS.  
 
WITH THE OCCASIONALLY STRONG TRADES, AN AIRMET IS CURRENTLY POSTED  
FOR MODERATE LOW LEVEL TURBULENCE OVER AND LEE OF ALL MOUNTAINS.  
 

 
   
MARINE
 
 
SMALL CRAFT ADVISORY (SCA) CONDITIONS CAN BE EXPECTED FOR WATERS  
SOUTH OF THE BIG ISLAND, ALL WATERS AROUND MAUI COUNTY, AND WATERS  
SOUTH OF OAHU THROUGH EARLY FRIDAY MORNING, AND THE TYPICALLY  
WINDIER WATERS AROUND MAUI COUNTY AND THE BIG ISLAND THROUGH EARLY  
SATURDAY MORNING.  
 
A RAPIDLY WEAKENING TROPICAL STORM FERNANDA IS FORECAST TO PASS  
JUST NORTH OF THE ISLANDS FROM SUNDAY THROUGH LATE MONDAY. THE  
WIND FORECAST AT THIS TIME CONTINUES TO CARRY LIGHT TO MODERATE  
TRADE WINDS.  
 
A HIGH SURF ADVISORY IS IN PLACE FOR THE EAST FACING SHORES OF  
THE BIG ISLAND AND MAUI THROUGH THIS AFTERNOON DUE TO A MID- TO  
LONG- PERIOD EAST SWELL ASSOCIATED WITH A CAPTURED FETCH FROM WHEN  
FERNANDA WAS A MAJOR HURRICANE. GUIDANCE SHOWS THIS SWELL  
SUBSIDING LATER TODAY, BUT CONTINUES TO INDICATE REINFORCING EAST  
SWELLS FROM FERNANDA ARRIVING ALONG THE EAST FACING SHORES FROM  
AS EARLY AS FRIDAY. HIGH SURF ADVISORY CONDITIONS FOR THE EAST  
FACING SHORES WILL BECOME LIKELY AGAIN FOR THE BIG ISLAND FRIDAY  
OR SATURDAY AND SPREAD WEST TO THE SMALLER ISLANDS. THE CURRENT  
HIGH SURF ADVISORY WILL NEED TO BE REVISITED LATER TODAY.  
 
THE LARGEST HIGH TIDES OF THE MONTH (KING TIDES) WILL OCCUR EACH  
AFTERNOON DURING THE NEXT SEVERAL DAYS. NOTE THAT THESE ABNORMALLY  
HIGH TIDES COMBINED WITH WAVE RUN UP DUE TO THE EAST SWELL  
PRODUCED BY FERNANDA MAY RESULT IN COASTAL FLOODING IN SOME AREAS,  
ESPECIALLY FROM FRIDAY INTO MONDAY.  
 
ELSEWHERE, NO SIGNIFICANT SWELLS ARE EXPECTED. SMALL SOUTH SWELLS  
WILL CONTINUE TO PRODUCE SMALL BACKGROUND SURF ALONG SOUTH FACING  
SHORES INTO NEXT WEEK.  
 

 
   
HFO WATCHES/WARNINGS/ADVISORIES
 
 
HIGH SURF ADVISORY UNTIL 6 PM HST THIS EVENING FOR WINDWARD  
HALEAKALA-SOUTH BIG ISLAND-BIG ISLAND NORTH AND EAST.  
 
SMALL CRAFT ADVISORY UNTIL 6 AM HST SATURDAY FOR MAALAEA BAY-  
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG  
ISLAND SOUTHEAST WATERS.  
 
SMALL CRAFT ADVISORY UNTIL 6 AM HST FRIDAY FOR OAHU LEEWARD  
WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-MAUI COUNTY  
LEEWARD WATERS.  
 

 
 

 
 
DISCUSSION...POWELL  
AVIATION...KINEL  
MARINE...EATON  
 
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