931  
FXHW01 KWNH 261215  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
714 AM EST FRI JAN 26 2018  
 
VALID 00Z SAT JAN 27 2018 - 00Z SAT FEB 03 2018  
 
INTO THE WEEKEND THE MODELS HAVE DEVELOPED SOME DISAGREEMENT OVER  
THE WESTWARD EXTENT OF ENHANCED MOISTURE FORECAST TO BE MOST  
CONCENTRATED EAST OF THE STATE, INFLUENCING THE DURATION OF  
RAINFALL OVER/NEAR THE BIG ISLAND. THE ECMWF WITH HIGHER  
PRECIPITABLE WATER VALUES AND RAINFALL DID TREND SOMEWHAT LOWER  
VERSUS ITS PRIOR RUN WHILE THE 00Z GEFS MEAN IS SIMILAR TO GFS  
RUNS SO A COMPROMISE SEEMS APPROPRIATE AT THIS TIME. REGARDLESS  
OF MOISTURE LEVELS, SOUTHEASTERLY FLOW AHEAD OF A FRONT  
APPROACHING/CROSSING THE STATE SHOULD PROVIDE SOME ENHANCEMENT TO  
BIG ISLAND RAINFALL.  
 
BEHIND THE WEEKEND FRONT EXPECT WINDS TO TURN  
NORTHEASTERLY/EASTERLY WHILE A POCKET OF DRIER AIR WILL LIKELY  
KEEP ANY SCATTERED SHOWERS ON THE LIGHT SIDE INTO TUESDAY. ALBEIT  
WITH SOME UNCERTAIN DETAILS, GUIDANCE IS CONSISTENT AT LEAST WITH  
THE IDEA OF THE PATTERN BECOMING MORE ACTIVE BY MIDWEEK AS FAIRLY  
DEEP MEAN TROUGHING ALOFT BECOMES ESTABLISHED TO THE NORTHWEST OF  
THE ISLANDS. INTO WEDNESDAY THERE IS REASONABLE CONSENSUS THAT A  
FRONT APPROACHING THE NORTHWESTERN ISLANDS WILL BE ACCOMPANIED BY  
A BAND OF RAINFALL THAT COULD BE LOCALLY MODERATE/HEAVY FOR A TIME  
BEFORE WEAKENING AS BEST DYNAMICS ALOFT LIFT NORTHEASTWARD.  
GUIDANCE IS STILL SUGGESTING THAT THIS FRONT MAY STALL OVER THE  
ISLANDS AS AN UPSTREAM FRONT APPROACHES, ALLOWING FOR A CONTINUED  
SOUTHWEST TO NORTHEAST FLOW OF MOISTURE ALOFT.  
 
GUIDANCE DIFFERENCES BECOME MORE PRONOUNCED FOR THE UPSTREAM FRONT  
BY THURSDAY-FRIDAY, WITH GFS RUNS BECOMING QUITE PROGRESSIVE  
RELATIVE TO MOST OTHER GUIDANCE DUE TO HAVING GREATER SOUTHEASTERN  
AMPLITUDE FOR THE UPPER TROUGH. THE 00Z GEFS MEAN HEDGES A BIT  
FASTER THAN THE ECMWF/CMC AND THEIR MEANS BUT IN PRINCIPLE IS MUCH  
CLOSER TO THOSE SOLUTIONS THAN THE GFS. THEREFORE WOULD RECOMMEND  
A COMPROMISE AMONG THE GEFS MEAN AND ECMWF/ECMWF MEAN AS THE BEST  
STARTING POINT AT THIS TIME. THIS SCENARIO WOULD KEEP MORE  
MOISTURE OVER THE STATE THAN GFS RUNS BY THE END OF THE PERIOD  
NEXT FRIDAY.  
 
RAUSCH  
 

 
 
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