559  
FXHW01 KWNH 221220  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
720 AM EST FRI DEC 22 2017  
 
VALID 00Z SAT DEC 23 2017 - 00Z SAT DEC 30 2017  
 
THERE IS STILL SOME SPREAD AMONG THE GUIDANCE REGARDING A FRONT  
LIKELY TO AFFECT THE STATE AROUND THE MIDDLE OF NEXT WEEK BUT  
DIFFERENCES ARE A LITTLE LESS EXTREME THAN YESTERDAY, WHILE  
PRECEDING EVOLUTION LOOKS MORE AGREEABLE THAN SEEN PREVIOUSLY.  
 
INITIALLY EXPECT A BAND OF DRIER AIR TO DRIFT SOUTH ACROSS THE  
STATE BEHIND A DEPARTING WAVY FRONT AND UPPER SHORTWAVE/COMPACT  
LOW. THEN ON SATURDAY A WEAKENING FRONT WITH ONLY MODEST  
ACCOMPANYING RAINFALL WILL REACH ABOUT AS FAR SOUTH AS THE  
NORTHWESTERN ISLANDS, FOLLOWED IN ANOTHER DAY OR SO BY A WEAK  
FRONT THAT APPROACHES BUT MAY NOT QUITE REACH THE NORTHWESTERN  
ISLANDS. PROGRESSION OF SURFACE HIGHS TO THE NORTH, WITH TRAILING  
ONES WEAKER THAN THE FIRST, WILL LEAD TO BRISK NORTHERLY WINDS  
BECOMING GRADUALLY LESS STRONG AND DIRECTED FROM THE NORTHEAST AND  
EAST-NORTHEAST OVER THE COURSE OF THE WEEKEND. THE TREND TOWARD  
EASTERLY TRADES MAY BRING IN ENOUGH MOISTURE TO SUPPORT A MODEST  
INCREASE IN WINDWARD-FOCUSED SHOWER ACTIVITY SUNDAY ONWARD.  
 
DURING EARLY-MID WEEK MOST GUIDANCE AGREES IN PRINCIPLE THAT A  
SHARPENING UPPER TROUGH SHOULD APPROACH/REACH THE AREA, BUT WITH  
DIFFERENCES IN SHAPE THAT AFFECT THE PROGRESSION OF THE ASSOCIATED  
FRONT. AS NOTED EARLIER THE GUIDANCE SPREAD FOR FRONTAL TIMING  
HAS NARROWED, BY WAY OF THE GFS SLOWING DOWN BY ABOUT A DAY OVER  
THE PAST 24 HOURS AND THE ECMWF SPEEDING UP SOMEWHAT. NOW THE 00Z  
CMC IS ON THE FLAT/FAST SIDE (NOT SUPPORTED BY ITS MEAN) WHILE THE  
00Z ECMWF/ECMWF MEAN ARE SLOWER SIDE BY A SMALL MARGIN. THE 00Z  
GEFS MEAN SHOWS A SLOWER PROGRESSION THAN GFS RUNS AND IN FACT  
TOWARD THE END OF THE PERIOD HOLDS ONTO SOMEWHAT HIGHER  
PRECIPITABLE WATER VALUES OVER THE STATE THAN THE 00Z ECMWF DUE TO  
THE LATTER BEING MORE AMPLIFIED WITH THE SOUTHERN PART OF UPPER  
TROUGH. RECENT TRENDS/CONSENSUS STILL FAVOR PROGRESSION SLOWER  
THAN THE GFS, NEAR AN ECMWF/GEFS MEAN/ECMWF MEAN COMPROMISE. THIS  
FRONT WILL PROVIDE SOME ENHANCEMENT TO RAINFALL BUT WITH SPECIFICS  
DETERMINED BY TIMING THAT STILL HAS A FAIR DEGREE OF UNCERTAINTY.  
 
RAUSCH  
 

 
 
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