739  
FXHW01 KWNH 231216  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
716 AM EST SAT DEC 23 2017  
 
VALID 00Z SUN DEC 24 2017 - 00Z SUN DEC 31 2017  
 
THIS WEEKEND THE STATE WILL BE WITHIN A BAND OF BELOW  
CLIMATOLOGICAL PRECIPITABLE WATER VALUES, LEADING TO RELATIVELY  
LIGHT RAINFALL WHERE ANY OCCURS. THE MOST FAVORED AREAS FOR SUCH  
ACTIVITY WOULD BE NEAR A DISSIPATING FRONT NEAR THE NORTHWESTERN  
ISLANDS AND LATE WEEKEND OVER THE BIG ISLAND AS MOISTURE DRIFTS IN  
FROM THE EAST. BRISK NORTHEASTERLY WINDS SHOULD WEAKEN A BIT AND  
TURN MORE EASTERLY. TOWARD LATE SUNDAY A FRONT WILL APPROACH FROM  
THE NORTHWEST BUT LIKELY STALL BEFORE REACHING THE NORTHWESTERN  
ISLANDS.  
 
INTO TUESDAY GUIDANCE AGREES FAIRLY WELL THAT AN  
AMPLIFYING/SHARPENING UPPER TROUGH WILL APPROACH THE AREA,  
DEEPENING LOW PRESSURE TO THE NORTHWEST/NORTH AND ENCOURAGING A  
STRENGTHENING FLOW OF MOISTURE FROM THE SOUTH. HOWEVER FROM LATE  
TUESDAY ONWARD MODELS/ENSEMBLES CONTINUE TO SHOW SOME DIVERGENCE  
REGARDING THE DETAILS OF ENERGY WITHIN THE UPPER TROUGH AND TIMING  
OF FRONTAL PASSAGE. AS HAS BEEN THE CASE IN RECENT DAYS THE GFS  
IS ON THE FASTER SIDE OF THE ENVELOPE. THE 00Z GEFS MEAN  
CONTINUES TO ARGUE FOR SLOWER TIMING THAN THE GFS, LEANING CLOSER  
TO THE 00Z ECMWF WITH RESPECT TO ARRIVAL OF LOWER PRECIPITABLE  
WATER VALUES BY THURSDAY-FRIDAY. THE 00Z ECMWF/CMC ENSEMBLE MEANS  
ARE EVEN A LITTLE SLOWER THAN THE ECMWF, SO THE GEFS MEAN/ECMWF  
OFFER THE BEST INTERMEDIATE SOLUTION. THE COMBINATION OF MOISTURE  
AND FAIRLY STRONG DYNAMICS MAY SUPPORT A BAND OF ENHANCED RAINFALL  
WITH THIS MIDWEEK FRONT. IN VARYING WAYS GUIDANCE IS SUGGESTING  
THAT UPPER LEVEL ENERGY MAY CLOSE OFF/LINGER JUST EAST OF THE  
STATE BY NEXT SATURDAY, POTENTIALLY MAINTAINING/BRINGING BACK SOME  
MOISTURE INTO THE BIG ISLAND AND VICINITY. THE NATURE OF  
EVOLUTION AND DISTANT TIME FRAME YIELD LOW CONFIDENCE IN THIS PART  
OF THE FORECAST.  
 
RAUSCH  
 

 
 
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