274  
FXHW60 PHFO 242013  
AFDHFO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HONOLULU HI  
1013 AM HST TUE APR 24 2018  
   
SYNOPSIS
 
 
STRONG HIGH PRESSURE NORTHEAST OF THE STATE WILL MAINTAIN BREEZY  
TRADES AND A STABLE REGIME OF MAINLY WINDWARD AND MAUKA RAINFALL  
THROUGH TONIGHT. TRADES WILL GRADUALLY WEAKEN ON WEDNESDAY AND  
THURSDAY, THEN SHIFT OUT OF THE SOUTHEAST ON FRIDAY AHEAD OF A  
FRONT. THE FRONT WILL AFFECT THE WESTERN END OF THE STATE ON  
FRIDAY THEN MOVE TO THE EASTERN END OF THE ISLAND CHAIN ON  
SATURDAY, LIKELY BRINGING WET AND UNSETTLED WEATHER TO PORTIONS  
OF THE STATE.  
 

 
   
DISCUSSION
 
 
A SOMEWHAT TYPICAL SPRING TRADE WIND PATTERN IS IN PLACE. A 1034  
MB SURFACE HIGH LOCATED 1,300 MILES NORTH-NORTHEAST OF THE STATE  
IS MAINTAINING A BREEZY AND GUSTY TRADE WIND FLOW. RIDGING ALOFT  
IS PRODUCING STABLE CONDITIONS, AS OVERNIGHT SOUNDINGS AND RECENT  
AIRCRAFT DATA SHOW AN INVERSION BETWEEN 4,500 AND 6,000 FT.  
PRECIPITABLE WATER (PW) RANGES BETWEEN 0.9 TO 1.1 INCHES ON THE  
SOUNDINGS, WHICH IS JUST BELOW APRIL NORMAL, AND RANDOM POCKETS  
OF MOISTURE OBSERVED IN THE TRADE WIND FLOW CONTAIN PW VALUES  
SLIGHTLY ABOVE APRIL NORMAL. ONE SUCH AREA OF MOISTURE IS  
AFFECTING WINDWARD AREAS FROM THE BIG ISLAND TO OAHU THIS MORNING,  
BRINGING UP TO A HALF INCH OF RAINFALL TO THE WETTEST WINDWARD  
SLOPES IN THE LAST THREE HOURS AND LESS THAN ONE TENTH OF AN INCH  
TO MOST WINDWARD AREAS DURING THIS TIME.  
 
ONLY MINOR CHANGES ARE EXPECTED DURING THE NEXT COUPLE OF DAYS.  
THE BREEZY TRADES WILL HOLD THROUGH TONIGHT THEN GRADUALLY DECLINE  
WEDNESDAY AND THURSDAY AS THE SURFACE HIGH TO THE NORTHEAST  
WEAKENS. THE MID LEVEL RIDGE WILL WEAKEN BUT REMAIN STRONG ENOUGH  
TO MAINTAIN SOMEWHAT STABLE CONDITIONS, WITH THE INVERSION SLOWLY  
WEAKENING AND RISING AS AN UPPER LEVEL TROUGH MOVES IN FROM THE  
NORTHWEST. THE CURRENT POCKET OF MOISTURE IN THE TRADE WIND FLOW  
SHOULD CLEAR THE STATE BY EVENING, AND THE GFS AND ECMWF ARE  
SHOWING ANOTHER AREA OF LOW LEVEL MOISTURE MOVING IN ON THURSDAY  
ALONG WITH INCREASING HIGH CLOUDS.  
 
A MARKED SHIFT IN THE WEATHER PATTERN WILL BEGIN THURSDAY NIGHT AS  
A DEEP LOW FORMS ROUGHLY 800 MILES NORTH OF THE STATE AND  
GENERATES A COLD FRONT ABOUT 250 MILES NORTH AND NORTHWEST OF  
KAUAI. TRADES WILL RAPIDLY DECREASE AND SHIFT OUT OF THE SOUTHEAST  
AS THE FRONT DEVELOPS AND ADVANCES TOWARD THE ISLANDS. THE GFS AND  
ECMWF SHOW THE AREA OF LOW LEVEL MOISTURE LINGERING ALONG WINDWARD  
AREAS, AND ALTHOUGH THE ATMOSPHERE DOES NOT APPEAR TO BE  
DESTABILIZING RAPIDLY, WE WILL HAVE TO WATCH OUT FOR THE POTENTIAL  
OF HEAVY SHOWERS, MAINLY OVER THE WESTERN HALF OF THE ISLAND  
CHAIN.  
 
THE FRONT WILL LIKELY AFFECT THE WESTERN HALF OF THE STATE ON  
FRIDAY AND FRIDAY NIGHT. THE GFS AND ECMWF DEPICT THIS FEATURE AS  
SOMEWHAT POORLY DEFINED INITIALLY, THEN MERGING WITH A SURFACE  
TROUGH FORMING IN THE POCKET OF LOW LEVEL MOISTURE POOLED NEAR  
THE ISLANDS. THE SUPPORTING UPPER LEVEL LOW WILL LIKELY BE CLOSEST  
TO THE STATE LATE FRIDAY, AND THE GFS AND ECMWF DEPICT THE  
ISLANDS NEAR THE ENTRANCE REGION OF A FORMING JET STREAM ALOFT.  
THIS POINTS TO THE POTENTIAL FOR LOCALIZED HEAVY RAINFALL ALONG  
THE EASTWARD-MOVING FRONT, AND GIVEN ONGOING MODEL DIFFERENCES,  
THE GREATEST PROBABILITY FOR THIS RAINFALL LOOKS TO BE FROM KAUAI  
TO MOLOKAI DURING THIS TIME.  
 
UNCERTAINTY INCREASES HEADING INTO THE WEEKEND, THOUGH THERE IS  
HIGH POTENTIAL FOR WET AND UNSETTLED WEATHER OVER PORTIONS OF THE  
STATE. THE GFS AND ECMWF HAVE YET TO SHOW A CONSISTENT DEPICTION  
OF THE FRONT DURING THIS TIME, THOUGH THE LATEST RUNS SUGGEST THAT  
THE FEATURE WILL STALL NEAR THE BIG ISLAND LATE SATURDAY. PW  
VALUES IN EXCESS OF 1.75 INCHES ARE EXPECTED ALONG THE STALLED  
FRONT AS DEEP SOUTHEASTERLY FLOW DRAWS UP DEEP MOISTURE FROM THE  
TROPICS, AND FLOODING RAINFALL IS POSSIBLE FOR MOST LIKELY THE  
EASTERN HALF OF THE STATE. TO THE WEST OF THE FRONT, A DRIER AND  
MORE STABLE NORTHERLY FLOW WILL FILL IN. AND WHILE UNCERTAINTY IN  
THE FRONTAL POSITION REMAINS, KAUAI AND OAHU HAVE THE GREATEST  
CHANCE OF EXPERIENCING DRIER CONDITIONS AS EARLY AS SATURDAY.  
 

 
   
AVIATION
 
 
FRESH TO STRONG TRADE WINDS WILL CARRY IN AN AREA OF LOW CLOUDS  
AND SHOWERS TO THE ISLANDS TODAY. THE CLOUDS AND SHOWERS WILL  
FOCUS ALONG WINDWARD AND MOUNTAIN AREAS FROM THE BIG ISLAND TO  
OAHU, WITH SOME SHOWERS TRACKING TO LEEWARD SIDES OF THE SMALLER  
ISLANDS AT TIMES. BRIEF MOUNTAIN OBSCURATIONS WILL BE LIKELY  
ACROSS WINDWARD AREAS TODAY, WITH AIRMET SIERRA FOR TEMPO MOUNTAIN  
OBSCURATIONS LIKELY.  
 
AIRMET TANGO IS IN EFFECT THROUGH TODAY OVER AND WEST OF THE  
MOUNTAINS ON ALL ISLANDS DUE TO THE STRONG TRADE WINDS.  
 

 
   
MARINE
 
 
A SMALL CRAFT ADVISORY IS CURRENTLY POSTED THROUGH TONIGHT FOR  
ALL HAWAIIAN COASTAL WATERS DUE TO STRONG HIGH PRESSURE FAR NORTH  
OF THE AREA. WINDS ARE EXPECTED TO DECREASE A BIT WEDNESDAY AND  
THURSDAY AS THE HIGH WEAKENS.  
 
THE CURRENT SMALL NORTHWEST SWELL WILL CONTINUE TO LOWER TODAY  
THROUGH WEDNESDAY. THE FIRST IN A SERIES OF NORTH NORTHWEST SWELLS  
IS EXPECTED TO ARRIVE ON THURSDAY, PEAK ON FRIDAY, THEN LOWER  
GRADUALLY ON SATURDAY. A RATHER STRONG LOW PRESSURE SYSTEM IS  
FORECAST TO DEVELOP NORTH OF THE AREA THIS WEEKEND. SURF HEIGHTS  
LATER THIS WEEKEND AND ON INTO EARLY NEXT WEEK WILL BE HIGHLY  
DEPENDENT ON THE LOCATION OF THE LOW CENTER AND THE FETCH  
ORIENTATION AIMED AT THE ISLANDS. CURRENT GUIDANCE SUGGESTS  
ADVISORY LEVEL SURF FROM THIS LOW BUT CONFIDENCE IN SURF HEIGHTS  
REMAINS RATHER LOW SINCE MUCH WILL DEPEND ON THE STRENGTH OF THE  
LOW AND THE FETCH ORIENTATION.  
 
EASTERLY TRADE WINDS WILL CONTINUE TO PRODUCE SHORT PERIOD CHOPPY  
SURF ALONG EAST FACING SHORES THROUGH THURSDAY WITH A DOWNWARD  
TREND EXPECTED LATER IN THE WEEK AND ON THROUGH THE WEEKEND.  
SMALL, MAINLY BACKGROUND SOUTH SWELLS ARE EXPECTED THROUGH  
SATURDAY WITH A SLIGHTLY LARGER SOUTH SWELL EXPECTED SUNDAY AND  
MONDAY.  
 

 
   
HFO WATCHES/WARNINGS/ADVISORIES
 
 
SMALL CRAFT ADVISORY UNTIL 6 AM HST WEDNESDAY FOR ALL HAWAIIAN  
WATERS.  
 

 
 

 
 
DISCUSSION...WROE  
AVIATION...WROE  
MARINE...BURKE  
 
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