982  
FXHW60 PHFO 231954  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
954 AM HST SAT FEB 23 2019  
   
SYNOPSIS
 
 
A SURFACE TROUGH WILL LINGER IN THE VICINITY OF THE BIG ISLAND  
THROUGH THE WEEKEND. COUPLED WITH IMPULSES PASSING HIGH OVERHEAD,  
LOCALLY HEAVY SHOWERS AND ISOLATED THUNDERSTORMS ARE LIKELY FOR  
THE BIG ISLAND AND SCATTERED SHOWERS FOR MAUI OVER THE REST OF THE  
WEEKEND. HIGH PRESSURE NORTHWEST OF THE ISLANDS WILL MAINTAIN A  
MOSTLY DRY WEATHER PATTERN FOR OAHU AND KAUAI THROUGH TODAY. AN  
UPTICK IN WINDWARD SHOWERS IS POSSIBLE TONIGHT THROUGH MONDAY AS  
SOME OF THE LOW LEVEL MOISTURE SPREADS WESTWARD FROM MAUI AND THE  
BIG ISLAND. THE HIGH PRESSURE WILL STRENGTHEN EARLY IN THE NEW  
WORK WEEK, WITH STRENGTHENING NORTH TO NORTHEAST WINDS, ALONG WITH  
COOLER TEMPERATURES.  
 

 
   
DISCUSSION
 
 
IT IS ALREADY AN ACTIVE MORNING WITH LOCALLY HEAVY SHOWERS  
OCCURRING ALONG THE HAMAKUA COAST OF THE BIG ISLAND. THEREFORE,  
THE FORECAST WAS UPDATED TO INCLUDE LOCALLY HEAVY SHOWERS FOR THE  
LOWER ELEVATION OF THE BIG ISLAND FOR THE REST OF TODAY. ALSO,  
CONDITIONS ARE RIPE FOR SCATTERED EVENING SHOWERS, LOCALLY HEAVY  
AT TIMES, FOR THE LEE SIDE OF THE BIG ISLAND THROUGH MIDNIGHT  
TONIGHT. WE ARE WATCHING OUT FOR MAUI, AND MOLOKAI, WHERE SHOWERS  
ARE ALREADY PRESENT. WE WON'T RULE OUT ANOTHER FORECAST UPDATE TO  
INCLUDE LOCALLY HEAVY SHOWERS FOR THIS AFTERNOON, SHOULD IT TREND  
TOWARD THAT DIRECTION IN THE COMING HOURS.  
 
THE 2 AM HST HILO SOUNDING SHOW A MOIST AND SOMEWHAT UNSTABLE AIR  
MASS. THERE IS AN INVERSION AT 25K FEET, THAT WILL DETER  
THUNDERSTORMS, BUT AN APPROACHING SHORT WAVE HIGH ALOFT, WILL  
LIKELY DESTROY THIS HIGH INVERSION, LEADING TO A MORE FAVORABLE  
AIR MASS FOR THUNDERSTORMS THIS AFTERNOON AND EVENING.  
 
THE SHORT WAVE TROUGH HIGH ABOVE, OR UPPER LEVEL DISTURBANCE, IS  
A SERIES OF TWO OVER THE WEEKEND. THE ONE APPROACHING THE ISLANDS  
NOW IS FORECAST TO DAMPEN OUT OVER THE BIG ISLAND THIS EVENING. A  
STRONGER ONE IS FORECAST TO DROP IN EARLY SUNDAY MORNING, THEN  
GRADUALLY MOVE EAST, EXITING THE BIG ISLAND EARLY SUNDAY EVENING.  
THIS SECOND SHORT WAVE WILL BE STRONG ENOUGH TO PUSH THE SURFACE  
AS PER BOTH THE ECMWF AND GFS SOLUTIONS. TIMING IS CRITICAL, AND  
WHAT THE GFS IS SAYING, IS THAT THIS SECOND SHORTWAVE WILL CLEAR  
THINGS OUT BEFORE THE MAXIMUM DAYTIME HEATING SUNDAY AFTERNOON,  
LEADING TO A NICER DAY (SUNDAY) THAN TODAY FOR MAUI AND THE BIG  
ISLAND, IN PARTICULAR. THE SURFACE TROUGH ALSO GETS NUDGED EAST  
OF THE BIG ISLAND BY SUNDAY EVENING. THE TROUGH FINALLY STALLS  
SOME 100 MILES OFF THE BIG ISLAND COAST ON MONDAY, AND STAYS THERE  
THROUGH TUESDAY.  
 
THE REMNANTS OR TAIL END OF A FRONT IS FORECAST TO REACH KAUAI  
MONDAY MORNING, THEN STALL OVER THE CENTRAL ISLANDS MONDAY EVENING.  
IT IS A SHALLOW FRONT WITH CLOUD TOPS NO HIGHER THAN 10K FEET.  
THIS MEANS THE WINDWARD AND MOUNTAIN AREAS WILL BE SOCKED IN,  
WHILE MUCH CLEARER CONDITIONS PREVAIL ON THE LEE SIDE. THINGS GETS  
MESSY THEREAFTER, TUESDAY THROUGH WEDNESDAY AS ANOTHER SURFACE  
BOUNDARY DROPS IN WITH AIDED BY A VIGOROUS LOOKING UPPER LEVEL  
TROUGH, LEADING TO A SLIGHT CHANCE OF THUNDERSTORMS. ALSO, THIS  
UPPER LEVEL IMPULSE WILL CARRY A POCKET OF VERY COLD TEMPS OF  
MINUS 19 DEGREE C AT 500 MB OVER THE ISLANDS. SO, WE COULD HAVE  
ANOTHER ROUND OF WINTER WEATHER FOR THE SUMMITS AND UPPER SLOPES  
OF THE BIG ISLAND, AND POTENTIALLY MAUI.  
 
AS FOR THE SURFACE WINDS, THE PREVAILING WIND FLOW WILL BE A  
LIGHT NORTHEAST BREEZE ACROSS THE SMALLER ISLANDS AND LIGHT AND  
VARIABLE OVER AND IN IMMEDIATE VICINITY OF THE BIG ISLAND TODAY  
THROUGH TONIGHT. THE NORTHEAST WINDS STRENGTHEN SLIGHTLY AS WELL  
OVERNIGHT TO MODERATE SPEED. AS THE SURFACE TROUGH OVER THE BIG  
ISLAND GETS PUSHED EAST OF THE ISLAND SUNDAY EVENING, A LIGHT TO  
MODERATE NORTHEAST FLOW FILLS AND LINGERS INTO NEXT WEEK,  
INCLUDING THROUGH THE NEXT ROUND OF POTENTIALLY UNSETTLED WEATHER.  
 
AFTER THIS SHORTWAVE PASSES, MODELS SUGGEST THAT THE LONGWAVE  
PATTERN NEAR AND NORTH OF THE ISLANDS WILL ONCE AGAIN TAKE THE  
FORM IT HAS FOR MOST OF FEBRUARY, WITH HIGH PRESSURE NORTHWEST  
OF THE ISLANDS AND LOW PRESSURE TO THE NORTHEAST. THIS SETUP  
FAVORS A COOL AND BREEZY NORTHWEST TO NORTH WIND FLOW. SHALLOW  
BANDS OF CLOUDS AND SHOWERS WOULD LIKELY PERIODICALLY MOVE OVER  
THE ISLANDS FROM THE NORTH IF THIS LARGE SCALE PATTERN WERE TO  
HOLD  
 

 
   
AVIATION
 
 
A SURFACE TROUGH NEAR THE BIG ISLAND WILL KEEP WINDS LIGHT  
ACROSS THE REGION TODAY WITH LAND BREEZES DEVELOPING IN A WEAK  
NORTHERLY BACKGROUND FLOW. EXPECT SHRA WITH OCCASIONAL +SHRA,  
ASSOCIATED WITH THE FEATURE TO AFFECT MAUI AND THE BIG ISLAND  
THROUGH THE REMAINDER OF THIS AFTERNOON. LOWERED VIS AND MOUNTAIN  
OBSCURATION WILL CAUSE MVFR CONDITIONS AT TIMES. THE REST OF THE  
ISLANDS MAY HAVE ISOL LOW CLOUDS, ESPECIALLY INLAND AND OVER THE  
MOUNTAIN RIDGES AND SLOPES, BUT GENERALLY VFR CONDITIONS.  
 
AIRMET SIERRA REMAINS IN EFFECT FOR NORTH TO SOUTHEAST SECTIONS  
OF MAUI AND THE BIG ISLAND DUE TO MOUNTAIN OBSCURATION FROM LOW  
CLOUDS AND SHRA ABOVE 2500 FT.  
 

 
   
MARINE
 
 
GENTLE NORTHEAST WINDS WILL PREVAIL OVER MOST OF THE ISLAND CHAIN  
INTO MONDAY. A WEAK SURFACE HIGH PASSING NORTH OF THE STATE IS  
DRIVING THE WINDS, WHILE A SURFACE TROUGH STRETCHING ACROSS THE  
BIG ISLAND IS LEADING TO VARIABLE WINDS TO THE SOUTH OF THE BIG  
ISLAND. THE TROUGH WILL DRIFT EAST SLOWLY, BUT OTHERWISE, LITTLE  
CHANGE IS EXPECTED THROUGH THE WEEKEND, WITH THUNDERSTORMS  
POSSIBLE AROUND THE BIG ISLAND INTO SUNDAY MORNING.  
 
WINDS WILL INCREASE THROUGH THE FIRST HALF OF THE WORK WEEK.  
WINDS WILL BEGIN TO STRENGTHEN LATE MONDAY OR MONDAY NIGHT AS A  
DISSIPATING FRONT SAGS SOUTHWARD OVER THE ISLANDS AND ANOTHER  
SURFACE HIGH PASSES TO THE NORTH. WINDS WILL SHIFT OUT OF THE  
NORTH AND BUILD TO FRESH TO POSSIBLY STRONG LEVELS LATE TUESDAY  
AND WEDNESDAY AS A DIFFUSE FRONT MOVES DOWN THE ISLAND CHAIN.  
SMALL CRAFT ADVISORY LEVEL WINDS WILL BE POSSIBLE DURING THIS  
TIME, FOLLOWED BY A DECREASE IN THE NORTH WINDS ON THURSDAY.  
 
SMALL SURF IS EXPECTED ON ALL SHORES THIS WEEKEND AS AN EAST  
SWELL FADES AND TINY NORTHWEST SWELL HOLDS WELL BELOW WINTER TIME  
AVERAGE. A PULSE OF MODERATE NORTHWEST SWELL IS EXPECTED ON  
MONDAY, FOLLOWED BY A LARGE, LONG PERIOD NORTHWEST SWELL THAT WILL  
BUILD ON TUESDAY AND PEAK ON WEDNESDAY AND EARLY THURSDAY. THIS  
LARGE SWELL WILL PRODUCE SURF AROUND HIGH SURF WARNING LEVELS FOR  
NORTH AND WEST FACING SHORES. IN ADDITION, THE COMBINATION OF  
FRESH TO POTENTIALLY STRONG NORTH WINDS AND SEAS FROM THE WINDS,  
THE NORTHWEST SWELL, AND A SHORT PERIOD NORTH SWELL WILL LEAD TO  
ROUGH CONDITIONS THAT WILL LIKELY REQUIRE A SMALL CRAFT ADVISORY  
FOR MOST WATERS. THOUGH WINDS WILL DECLINE ON THURSDAY, SEAS WILL  
REMAIN ELEVATED.  
 

 
   
HFO WATCHES/WARNINGS/ADVISORIES
 
 
NONE.  
 

 
 

 
 
H LAU/THOMAS/WROE  
 
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