467  
FXHW01 KWNH 191230  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
730 AM EST TUE DEC 19 2017  
 
VALID 00Z WED DEC 20 2017 - 00Z WED DEC 27 2017  
   
..HEAVY RAIN THREAT WEDNESDAY INTO THURSDAY
 
 
THE SYNOPTIC-SCALE PATTERN WILL FEATURE A BROAD UPPER LOW  
POSITIONED TO THE NORTH OF THE HAWAI`IAN ISLAND CHAIN. THIS WILL  
ALLOW SLOW MOVING HEIGHT FALLS TO TRAVERSE THROUGH THE REGION WITH  
THE PARENT SURFACE FRONT BRIEFLY STALLING IN THE VICINITY OF THE  
BIG ISLAND ON WEDNESDAY. A GRADUAL EASTWARD JOG OF THIS UPPER  
TROUGH SHOULD EVENTUALLY ALLOW THE COLD FRONT TO SWEEP SOUTH OF  
HAWAI`I BY LATE WEDNESDAY NIGHT. BEFORE THIS OCCURS, A  
PARTICULARLY WET PERIOD IS IN STORE FOR THE REGION AS A 1.75 TO 2  
INCH PRECIPITABLE WATER AIR MASS STREAMS THROUGH. BASED ON THE 00Z  
ECMWF, SOME 6-HOUR RAINFALL TOTALS COULD REACH THE 1 TO 2 INCH  
RANGE. THIS WOULD PARTICULARLY BE THE CASE OVER THE HIGHER TERRAIN  
WHERE LOCAL OROGRAPHIC FORCING WOULD COME INTO PLAY.  
 
BY THURSDAY/FRIDAY, IN THE WAKE OF THE LONGWAVE TROUGH PASSAGE, A  
RELAXING OF THE FLOW IS LIKELY AS MID-LEVEL WINDS SHIFT TO A MORE  
WEST-NORTHWESTERLY DIRECTION. THIS WILL FOCUS A PERIOD OF COLD  
ADVECTION ACROSS THE ISLANDS WHICH FAVORS A MORE NORTHERLY  
COMPONENT IN THE SURFACE WIND FIELDS. THERE MAY BE AN  
INTENSIFICATION OF THESE WINDS INTO THE EARLY PART OF THE WEEKEND  
AS A SURFACE RIDGE BUILDS TO THE NORTH. ADDITIONALLY, A GENERAL  
SHIFT TO A MORE NORTHEASTERLY DIRECTION IS EXPECTED ALTHOUGH THE  
00Z CMC SHOWS WARM ADVECTION RATHER ALTHOUGH ITS SOLUTION APPEARS  
TO BE AN OUTLIER. LOOKING TOWARD NEXT WEEK, A RETURN TO AMPLIFIED  
FLOW IS LIKELY ALTHOUGH DETAILS REMAIN UNCERTAIN WITH THE  
EVOLUTION. THERE IS PLENTY OF SUPPORT VIA ENSEMBLE SPAGHETTI PLOTS  
SHOWING A LONGWAVE TROUGH POSITION JUST EAST OF 170W BY NEXT  
TUESDAY MORNING. THIS SHOULD ULTIMATELY LEAD TO AN INTENSE PERIOD  
OF WARM ADVECTION ALTHOUGH MODELS VARY WITH THE UPSTREAM FRONTAL  
POSITION. EVENTUALLY, ANOTHER HEAVY RAINFALL THREAT WOULD BE  
EXPECTED BY NEXT TUESDAY ACROSS KAUAI BASED ON THE 06Z GFS/00Z  
ECMWF/00Z CMC SOLUTIONS.  
 
RUBIN-OSTER  
 

 
 
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