827  
FXHW60 PHFO 290202  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
402 PM HST TUE MAR 28 2017  
   
SYNOPSIS
 
 
A SURFACE RIDGE NORTH OF THE STATE WILL MAINTAIN LIGHT TO  
MODERATE EAST TO EAST-SOUTHEAST SURFACE WINDS ACROSS THE ISLANDS  
THROUGH FRIDAY. AN UPPER LEVEL TROUGH, WHICH WILL DIG EAST OF THE  
ISLAND CHAIN LATER TONIGHT AND WEDNESDAY, IS EXPECTED TO BRING AN  
INCREASE IN SHOWERS EARLY WEDNESDAY MORNING THROUGH SATURDAY. A  
SURFACE HIGH BUILDING FAR NORTHEAST OF THE AREA STARTING LATE  
FRIDAY WILL PRODUCE BREEZY TRADES THIS WEEKEND.  
 

 
   
DISCUSSION
 
 
A NEARLY STATIONARY 1031 MB SURFACE HIGH IS CENTERED FAR EAST-  
NORTHEAST OF HAWAII THIS AFTERNOON. A SURFACE RIDGE EXTENDS WEST-  
SOUTHWEST FROM THIS HIGH PASSING ABOUT 300 MILES NORTH OF KAUAI.  
THE PRESSURE GRADIENT SOUTH AND SOUTHWEST OF THESE FEATURES IS  
MAINTAINING LIGHT TO MODERATE EAST TO EAST-SOUTHEAST WIND FLOW  
ACROSS THE STATE. WATER VAPOR IMAGERY SHOWS A SHORTWAVE TROUGH  
NEAR OAHU THIS AFTERNOON DIGGING TOWARDS JUST EAST OF THE STATE.  
AFTERNOON SOUNDINGS AT HILO AND LIHUE SHOW DRY AND STABLE  
MIDLEVELS, STRONG INVERSIONS AND BELOW NORMAL PRECIPITABLE WATER  
VALUES OF 0.8 TO 1 INCH. RADAR SHOWS THAT MUCH OF THE SHOWERS  
FROM THIS MORNING HAVE DISSIPATED THIS AFTERNOON. DUE TO THE LIGHT  
WIND FLOW OVER THE STATE, SEA BREEZES HAVE RESULTED IN CLOUD BUILD  
UPS OVER THE LEEWARD SIDES OF THE ISLANDS THIS AFTERNOON. THESE  
CLOUDS SHOULD GRADUALLY DISSIPATE THIS EVENING.  
 
WATER VAPOR IMAGERY SHOWS A SHORTWAVE TROUGH CURRENTLY PASSING  
NORTH OF OAHU, AND IS EXPECTED TO MERGE INTO A MUCH STRONGER  
TROUGH ALOFT EAST OF THE STATE BY WEDNESDAY MORNING BEFORE SLOWLY  
TRACKING AWAY TO THE EAST THROUGH THIS WEEKEND.  
 
THE SURFACE RIDGE IS EXPECTED TO REMAIN NORTH OF THE ISLANDS THROUGH  
THURSDAY NIGHT, WHICH WILL CONTINUE TO DRIVE THE LIGHT TO MODERATE  
EAST TO EAST-SOUTHEAST FLOW. WITH TRADE WINDS UPSTREAM, MODELS  
SHOW AN INCREASE IN SHOWERS EXPECTED TO PUSH IN FROM THE EAST  
EARLY WEDNESDAY MORNING. THE PRESENCE OF THIS TROUGH ALOFT EAST OF  
THE AREA WILL ENHANCE THE LOW CLOUDS AND PRECIPITATION TRACKING IN  
FROM UPSTREAM THROUGH SATURDAY. THE LIGHT WINDS WILL ALSO CONTINUE  
ON FRIDAY.  
 
THEREFORE, EXPECT WETTER TRADE WIND TYPE WEATHER AFTER TONIGHT  
WITH THE RELATIVELY WEAK LOW LEVEL FLOW FOCUSING MOST OF THESE LOW  
CLOUDS AND SHOWERS OVER WINDWARD AND SOUTHEAST FACING SLOPES.  
WHILE LEEWARD AREAS SHOULD STAY RELATIVELY DRY AT NIGHT, SEA BREEZES  
WILL CONTINUE TO DOMINATE OVER THE NEXT SEVERAL DAYS AND PRODUCE  
CLOUD BUILD UPS AND SHOWERS. GIVEN THE RELATIVE ATMOSPHERIC STABILITY  
OVER HAWAII, SHOWERS ENHANCED BY THE UPSTREAM TROUGH WILL LIKELY  
BE LOOSING THEIR INTENSITY AS THEY REACH THE ISLANDS. THEREFORE,  
WHILE IT WILL LIKELY BE WETTER THAN IT HAS BEEN, WE DO NOT  
ANTICIPATE A FLOODING-TYPE SITUATION.  
 
THE SURFACE RIDGE TO OUR NORTH IS EXPECTED TO ERODE SOME TIME ON  
THURSDAY, AS A COLD FRONT APPROACHES FROM THE NORTHWEST. THIS  
FRONT WILL LIKELY STALL AND WEAKEN TWO TO THREE HUNDRED MILES  
NORTH-NORTHWEST OF KAUAI BY LATE FRIDAY, SO IT IS NOT EXPECTED TO  
EVER REACH THE STATE. HOWEVER, THE FORECAST MODELS CONTINUE TO  
SUGGEST A BAND OF CLOUDS AND SHOWERS WILL FORM NEAR THE WESTERN  
END OF THE STATE TO THE SOUTHEAST OF THE FRONT. THESE LOW CLOUDS  
AND SHOWERS MIGHT PRODUCE MODERATE RAINS OVER PARTS OF THE STATE,  
PARTICULARLY THE WESTERN ISLANDS, LATE THURSDAY AND FRIDAY.  
 
SURFACE HIGH PRESSURE IS EXPECTED TO BUILD FAR NORTHEAST OF THE REGION  
THIS WEEKEND, WHICH WILL LIKELY PRODUCE BREEZY TRADE WINDS ACROSS  
THE HAWAIIAN ISLANDS BEGINNING FRIDAY NIGHT THROUGH EARLY NEXT  
WEEK. CLOUDS AND SHOWERS WILL CONTINUE ON SATURDAY, BUT A SLIGHTLY  
DRIER AIR MASS IS EXPECTED OVER MOST OF THE STATE BEGINNING ON  
SUNDAY.  
 

 
   
AVIATION
 
 
AIRMET SIERRA FOR MOUNTAIN OBSCURATION WAS RECENTLY CANCELLED AS  
LINGERING CLOUDS AND SHOWERS HAVE CONTINUED TO THIN AFTER A BRIEF  
UPTICK IN COVERAGE. THE BIG ISLAND HAS THE MOST CLOUDS OVERHEAD  
THIS AFTERNOON, BUT EXPECT CLOUDS TO THIN OVERNIGHT. THE CLOUDS  
ARE TOPPING OUT AROUND 9000 FT. THE NORTH TO SOUTH ORIENTED JET  
FORECAST TO BE OVER THE ISLANDS THIS AFTERNOON HAS NOT BEEN AS  
STRONG AS THE MODELS INDICATED, SO HAVE NOT ISSUED A SIGMET. WILL  
CONTINUE TO MONITOR THE SITUATION.  
 

 
   
MARINE
 
 
THE WEST-NORTHWEST (290-310 DEG) SWELL THAT PEAKED SUNDAY INTO  
MONDAY WILL HOLD THROUGH MID WEEK DUE TO A SLIGHT REINFORCEMENT  
TONIGHT INTO WEDNESDAY. A COMBINATION OF THIS SWELL AND MODERATE  
TO FRESH BREEZES OVER THE CHANNELS AND WATERS AROUND THE BIG  
ISLAND WILL CONTINUE TO GENERATE ROUGH BOATING CONDITIONS.  
ALTHOUGH THE WINDS HAVE DROPPED SLIGHTLY BELOW SMALL CRAFT  
ADVISORY LEVELS (25 KT), SEAS WILL HOLD AROUND 10 FT THROUGH THE  
DAY TODAY ACROSS WINDWARD WATERS BETWEEN THE BIG ISLAND AND OAHU.  
AS A RESULT, THE SMALL CRAFT ADVISORY WILL REMAIN UP ACROSS THESE  
AREAS FOR SEAS TODAY. SURF ASSOCIATED WITH THE WEST-NORTHWEST  
SWELL SHOULD REMAIN JUST BELOW ADVISORY LEVELS ALONG EXPOSED  
NORTH AND WEST FACING SHORES THROUGH MID WEEK.  
 
WINDS SHOULD REMAIN BELOW ADVISORY LEVELS THROUGH FRIDAY AS THE  
RIDGE OF HIGH PRESSURE NORTH OF THE STATE SLIGHTLY WEAKENS IN  
RESPONSE TO A TWO WEAK COLD FRONTS PASSING NORTH OF THE AREA.  
ADVISORY- LEVEL WINDS WILL LIKELY RETURN ACROSS PORTIONS OF THE  
WATERS OVER THE WEEKEND AS HIGH PRESSURE BUILDS NORTH OF THE STATE  
BEHIND THE SECOND FRONT.  
 
SURF ALONG EAST FACING SHORES WILL REMAIN SMALL AND CHOPPY THROUGH  
THE WEEK DUE TO PERSISTENT MODERATE TO FRESH ONSHORE WINDS, BUT  
INCREASE OVER THE WEEKEND AS WINDS STRENGTHEN AGAIN LOCALLY. A  
SLIGHT INCREASE IN SURF ALONG SOUTH FACING SHORES WILL BE POSSIBLE  
THURSDAY THROUGH SATURDAY DUE TO A LONG-PERIOD SWELL (190 DEG)  
ASSOCIATED WITH RECENT ACTIVITY ACROSS THE SOUTHERN PACIFIC.  
 
THE MAIN FORECAST CHALLENGE WILL BE CENTERED ON ANOTHER, SLIGHTLY  
LARGER, WEST-NORTHWEST (290-310 DEG) SWELL THAT WILL IMPACT THE  
ISLANDS THURSDAY NIGHT THROUGH THE WEEKEND. THIS IS DUE TO A POWERFUL  
STORM-FORCE LOW CURRENTLY TRACKING EAST-NORTHEASTWARD ACROSS THE  
NORTHWEST PACIFIC. RECENT ASCAT AND ALTIMETER OBS SUPPORT LATEST MODEL  
MODEL RUNS WITH REGARD TO WIND SPEEDS AND RESULTING SEA HEIGHTS.  
THE LARGE WEST-NORTHWEST SWELL ASSOCIATED WITH THIS SYSTEM COULD  
REACH THE ISLANDS AS EARLY AS THURSDAY NIGHT, PEAK THROUGH THE DAY  
FRIDAY AND FRIDAY NIGHT, THEN SLOWLY EASE OVER THE UPCOMING  
WEEKEND. WARNING-LEVEL SURF AND ADVISORY-LEVEL SEAS ARE EXPECTED  
FRIDAY INTO THE WEEKEND BEFORE TRENDING DOWN THROUGH THE BEGINNING  
OF NEXT WEEK. THE SOUTHERN SHORES COULD SEE IMPACTS FROM THIS  
SWELL AGAIN AT SELECT SPOTS DUE TO WRAPPING AROUND THE ISLANDS.  
 

 
   
HFO WATCHES/WARNINGS/ADVISORIES
 
 
NONE.  
 

 
 

 
 
DISCUSSION...FOSTER  
AVIATION...M BALLARD  
MARINE...TS  
 
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