734  
FXHW01 KWNH 241213  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
713 AM EST SUN DEC 24 2017  
 
VALID 00Z MON DEC 25 2017 - 00Z MON JAN 01 2018  
 
EXPECT MODERATE NORTHEASTERLY TRADES FROM NOW INTO CHRISTMAS DAY,  
WITH ONLY LIGHT/SCATTERED WINDWARD-FOCUSED SHOWERS GIVEN  
RELATIVELY LOW PRECIPITABLE WATER VALUES. THERE MAY ALSO BE A  
BAND OF LIGHT RAINFALL IN THE VICINITY OF A WEAK FRONT THAT  
BRIEFLY STALLS NEAR KAUAI. GUIDANCE IS COMING INTO BETTER  
AGREEMENT REGARDING THE FRONT PASSING THROUGH THE AREA AROUND  
MIDWEEK, IN ASSOCIATION WITH AN UPPER TROUGH THAT  
AMPLIFIES/SHARPENS JUST WEST OF THE STATE INTO TUESDAY AND CROSSES  
THE REGION WEDNESDAY-THURSDAY. NOW THE GUIDANCE SPREAD FOR  
FRONTAL TIMING IS GENERALLY DETERMINED BY SMALL SCALE SHORTWAVE  
DIFFERENCES THAT ARE TYPICALLY NOT WELL RESOLVED MORE THAN A  
COUPLE DAYS OUT IN TIME, FAVORING A COMPROMISE APPROACH. THE  
VIGOROUS DYNAMICS ALOFT AND SOUTHERLY FLOW OF MOISTURE AHEAD OF  
THE FRONT MAY PRODUCE AREAS OF HEAVY/CONVECTIVE RAINFALL. SHOWER  
ACTIVITY WILL TREND LIGHTER LATE WEEK INTO THE WEEKEND. THE  
PRIMARY FORECAST UNCERTAINTY NOW INVOLVES THE POTENTIAL FOR SOME  
UPPER LEVEL ENERGY TO CLOSE OFF A LOW OVER OR EAST OF THE ISLANDS  
FRIDAY ONWARD. BASED ON THE FULL ARRAY OF MODEL/ENSEMBLE  
GUIDANCE, AN AVERAGE OF THE PAST COUPLE GFS/ECMWF RUNS PROVIDES A  
GOOD STARTING POINT WITH A PATH TO THE EAST OF THE ISLANDS BUT THE  
GFS RUNS MAY BE TOO DEEP WITH THE UPPER LOW. THE 00Z CMC IS MORE  
OPEN/EASTWARD WHILE SOME 00Z GEFS MEMBERS AND ONE OR TWO CMC/ECMWF  
ENSEMBLE MEMBERS SHOW AN UPPER LOW TRACK TO THE WEST/SOUTHWEST OF  
THE GFS/ECMWF. ASSUMING AN UPPER LOW SOMEWHAT WEAKER THAN  
DEPICTED IN THE GFS, BY NEXT WEEKEND ANTICIPATE LESS SURFACE  
TROUGHING JUST EAST OF THE STATE AND MORE TYPICAL TRADE FLOW SUCH  
AS FORECAST BY THE ECMWF AND GEFS/ECMWF MEANS.  
 
RAUSCH  
 

 
 
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