970  
FXHW60 PHFO 190628  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
828 PM HST WED SEP 18 2019  
   
SYNOPSIS
 
 
TRADE WINDS WILL CONTINUE THROUGH FRIDAY WITH A HIGH PRESSURE  
RIDGE NORTH OF THE REGION. CLOUDS AND PASSING SHOWERS WILL FAVOR  
WINDWARD AND MAUKA SLOPES OF EACH ISLAND. WET AND HUMID WEATHER  
REMAINS IN THE FORECAST FROM FRIDAY THROUGH MONDAY AS TROPICAL  
MOISTURE MOVES OVER THE ISLANDS AS AN UPPER LEVEL LOW DRIFTS FROM  
EAST TO WEST ACROSS THE STATE. A MORE STABLE TRADE WIND WEATHER  
PATTERN DEVELOPS NEXT WEEK TUESDAY AND WEDNESDAY AS HIGH PRESSURE  
BUILDS BACK IN OVER THE ISLAND CHAIN.  
 

 
   
DISCUSSION
 
 
THE SATELLITE IMAGERY THIS EVENING SHOWS A LOW PRESSURE AREA  
ROUGHLY 300 TO 500 MILES NORTHWEST OF KAUAI MOVING NORTHWARD AWAY  
FROM THE HAWAIIAN ISLANDS. ICY CIRRUS CLOUDS FROM THIS LOW  
PRESSURE AREA ARE DRIFTING OVER THE WESTERN HALF OF THE STATE. ONE  
THOUSAND MILES TO THE NORTHEAST AN UPPER LEVEL LOW IS DEEPENING  
AND DRIFTING SLOWLY SOUTHWESTWARD. BANDS OF LOW CLOUDS ARE PILING  
UP ALONG EAST FACING SLOPES OF ALL ISLANDS THIS EVENING. LOCAL  
RADAR IMAGERY SHOWS SCATTERED SHOWERS PRODUCED BY CONVERGENT  
CLOUD BANDS DOWNSTREAM OF EACH ISLAND. EAST TO SOUTHEASTERLY WIND  
FLOW OVER THE WESTERN ISLANDS IS CAUSING THESE SHOWERS TO  
OCCASIONALLY DRIFT INTO SOUTHERN SECTIONS OF OAHU AND KAUAI THIS  
EVENING.  
 
A FAIRLY DRY TRADE WIND PATTERN WILL CONTINUE INTO FRIDAY ACROSS  
THE REGION. CLOUDS AND SHOWERS WILL FAVOR WINDWARD AND MAUKA  
AREAS TRENDING INTO THE SCATTERED RANGE IN THE OVERNIGHT AND EARLY  
MORNING HOURS. THE LARGE SCALE PATTERN BEGINS TO CHANGE ON FRIDAY  
AS AN UPPER LEVEL LOW APPROACHES HAWAII FROM THE NORTHEAST  
DIRECTION. WIND SPEEDS WILL ALSO DECREASE FROM FRIDAY INTO THE  
WEEKEND AS THE LIFT ASSOCIATED WITH THIS UPPER LOW WEAKENS THE  
SURFACE PRESSURE GRADIENTS AROUND THE ISLANDS.  
 
FRIDAY THROUGH MONDAY...EXPECT A TURN TOWARDS CLOUDY WET WEATHER  
CONDITIONS AS A COLD CORE UPPER LEVEL LOW TRACKS FROM EAST TO WEST  
ACROSS THE ISLANDS OVER A PLUME OF WARM MOISTURE MOVING IN FROM  
THE DEEP TROPICS. WINDS WILL BECOME LIGHT AND VARIABLE FROM  
SATURDAY TO SUNDAY AS THIS UPPER LOW WEAKENS THE SURFACE PRESSURE  
GRADIENTS. PRECIPITABLE WATER (PW) LEVELS WITHIN THIS TROPICAL  
MOISTURE PLUME REMAIN IN THE ABUNDANT 2.0 TO 2.5 INCH RANGE. PW IS  
A MEASURE OF THE TOTAL WATER CONTENT OF THE ATMOSPHERE IF ALL THE  
WATER IN THE COLUMN WERE TO INSTANTLY FALL AS RAIN. THE  
COMBINATION OF THESE COLD AND WARM SYSTEMS WILL RESULT IN A VERY  
UNSTABLE ATMOSPHERE WITH CLOUDS AND NUMEROUS SHOWERS SPREADING  
ACROSS THE STATE. RAINFALL WITH THIS EVENT WILL SPREAD EQUALLY  
INTO BOTH WINDWARD AND DRIER LEEWARD AREAS. CONVECTIVE CLOUDS WILL  
FORM OVER ISLAND INTERIORS IN THE AFTERNOON TO EARLY EVENING DUE  
TO ISLAND HEATING EFFECTS. LOCALLY HEAVY RAIN IS LIKELY WITH THIS  
SETUP ALONG WITH A FEW THUNDERSTORMS. STILL TOO EARLY TO ADD  
THUNDERSTORMS OR HEAVY SHOWERS INTO THE FORECAST GRIDS, AS THE  
TIMING AND LOCATION IMPACTS WILL LIKELY EVOLVE AS FORECAST TIME  
PERIOD GROWS SHORTER.  
 
TUESDAY THROUGH WEDNESDAY...THE UPPER LOW WILL DRIFT NORTHWARD  
AWAY FROM THE ISLANDS ALLOWING THE HIGH PRESSURE RIDGE TO BUILD  
BACK IN OVER THE STATE. MODERATE TRADE WINDS AND DRIER TRENDS WILL  
RETURN AS HIGHER PRESSURE BUILDS INTO THE REGION WITH INCREASING  
STABILITY SPREADING FROM EAST TO WEST ACROSS THE HAWAIIAN ISLANDS.  
PW LEVELS DROP BACK DOWN INTO THE 1.0 TO 1.5 INCH RANGE. ISOLATED  
TO SCATTERED SHOWER ACTIVITY WILL FAVOR THE TYPICAL WINDWARD AND  
MAUKA AREAS OF EACH ISLAND.  
 

 
   
AVIATION
 
 
A PREDOMINATE VFR SITUATION IS EXPECTED THROUGH THURSDAY MORNING,  
WITH THE WINDS BECOMING MORE TRADES ON THURSDAY WHILE HOLDING AT  
MODERATE TO LOCALLY STRONG.  
 
CURRENT LOW LEVEL WIND FLOW IS SOUTHEAST OVER ON KAUAI, EAST OVER  
THE CENTRAL ISLANDS, THEN EAST- NORTHEAST OVER ON THE BIG ISLAND.  
SPEEDS ARE LIGHT TO MODERATE, EXCEPT LOCALLY STRONG OVER THE NEAR-  
SHORE WATERS OF MAUI COUNTY AND THE BIG ISLAND. WINDS ARE EXPECTED  
TO BECOME TRUE TRADES, OUT OF THE EAST ON THURSDAY, AT MODERATE TO  
LOCALLY STRONG.  
 
CLOUD DISTRIBUTION IS BASICALLY SCATTERED TO BROKEN AREA-WIDE,  
INCLUDING THE SURROUND WATERS. THERE IS OVERCAST COVERAGE, MAINLY  
OVER THE WINDWARD AND LEEWARD WATERS AND COAST OF THE BIG ISLAND,  
BETWEEN WAIPIO VALLEY AND THE PUNA DISTRICT, AND FROM KAWAIHAE TO  
SOUTH CAPE. SCATTERED SHOWERS ARE PRESENT WITH THESE CLOUDS,  
MAINLY OVER THE WATERS BUT ALSO OVER LAND INCLUDING THE KAILUA-  
KONA AREA OF THE BIG ISLAND AND WINDWARD KAUAI. WE EXPECT LITTLE  
CHANGE OVERNIGHT WITH THESE CLOUDS AND SHOWERS FAVORING THE  
WINDWARD AND MOUNTAIN AREAS OF ALL OF THE LARGER ISLANDS. THE  
SHOWERS ON THE KONA COAST ARE EXPECTED TO LINGER TO SHORTLY AFTER  
MIDNIGHT BEFORE DISSIPATING. TOPS OF THESE SHOWERS ARE AROUND 9K  
FEET, AND WILL CAUSE BRIEF MVFR CEILINGS AND VIS.  
 
NO AIRMETS IN EFFECT AND NONE IS EXPECTED THROUGH THURSDAY  
MORNING.  
 

 
   
MARINE
 
 
FRESH TO LOCALLY STRONG, EAST TO EAST-SOUTHEAST WINDS WILL CONTINUE  
DUE TO HIGH PRESSURE TO THE NORTHEAST AND A TROPICAL DISTURBANCE  
MOVING NORTHWARD TO THE WEST OF THE STATE, BUT SHIFT BACK TO A  
MORE TYPICAL EAST TO NORTHEAST DIRECTION THURSDAY THROUGH FRIDAY.  
A SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR THE TYPICAL WINDY  
AREAS AROUND MAUI AND THE BIG ISLAND THROUGH THURSDAY NIGHT.  
GUIDANCE SHOWS A RETURN OF LIGHT TO MODERATE SOUTHEAST FLOW LATE  
SATURDAY THROUGH SUNDAY DUE TO AN APPROACHING TROUGH FROM THE  
EAST-SOUTHEAST, WHICH WOULD SUPPORT LOCALIZED LAND AND SEA BREEZE  
CONDITIONS NEAR THE COASTS.  
 
A SERIES OF SMALL SWELLS FROM THE SOUTHWEST AND SOUTH ARE EXPECTED  
THROUGH THE REST OF THIS WEEK. THE LARGEST OF THESE LOOKS TO BE  
TOWARDS THE END OF THE WEEK AS A LONG-PERIOD SOUTH-SOUTHWEST SWELL  
AND SHORT-PERIOD WEST-SOUTHWEST SWELL COMBINE ACROSS THE AREA. THIS  
WILL LIKELY BOOST SURF ALONG SOUTH AND WEST FACING SHORES LATE  
THURSDAY THROUGH FRIDAY. A SMALL NORTHWEST SWELL IS POSSIBLE LATE  
IN THE WEEKEND INTO EARLY NEXT WEEK. A SMALL SWELL FROM TROPICAL  
CYCLONE KIKO SHOULD FILL IN THURSDAY ALONG EXPOSED EAST FACING  
SHORES BEFORE DROPPING OVER THE WEEKEND FOR ONLY A SLIGHT BOOST IN  
SURF HEIGHTS. SURF HEIGHTS ARE EXPECTED TO BE BELOW ADVISORY  
LEVELS WITH ALL OF THESE SWELLS.  
 

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

 
 

 
 
DISCUSSION...BOHLIN  
AVIATION...LAU  
MARINE...FOSTER  
 
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