200  
FXHW01 KWNH 211221  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
720 AM EST THU DEC 21 2017  
 
VALID 00Z FRI DEC 22 2017 - 00Z FRI DEC 29 2017  
 
THE THREAT FOR LOCALLY HEAVY RAINFALL WILL QUICKLY DIMINISH AFTER  
THURSDAY AS AN UPPER TROUGH, LIKELY FORMING A CLOSED LOW BY  
TONIGHT, PROGRESSES EAST OF THE ISLANDS FRIDAY ONWARD. THE  
GRADIENT BETWEEN LEADING COLD FRONT AND TRAILING HIGH PRESSURE  
WILL SUPPORT A PERIOD OF NORTHEASTERLY WINDS OVER THE STATE WITH  
TERRAIN-FOCUSED SHOWER ACTIVITY, LIKELY EXTENDING THROUGH THE  
WEEKEND. LOCATIONS OVER/NEAR THE BIG ISLAND MAY SEE SOMEWHAT  
HIGHER RAINFALL TOTALS RELATIVE TO OTHER AREAS CORRESPONDING TO  
RELATIVELY HIGHER PRECIPITABLE WATER VALUES. MOISTURE SHOULD NOT  
BE ABUNDANT IN ABSOLUTE TERMS THOUGH.  
 
FROM ABOUT SUNDAY ONWARD GUIDANCE DIVERGES WITH RESPECT TO  
MID-LATITUDE PACIFIC SYSTEMS AND INFLUENCE OF TRAILING FRONTS ON  
THE STATE. FOR LOW PRESSURE TRACKING WELL NORTH AS OF SUNDAY,  
GFS/GEFS GUIDANCE IS SOUTHWARD AND WEAKER THAN MOST OTHER  
GUIDANCE. THEN THE GFS IS MUCH STRONGER/NORTHWARD THAN OTHER  
SOLUTIONS WITH THE NEXT DEVELOPING SYSTEM. THE TRAILING TROUGH IN  
THE GFS AND TO SOME EXTENT GEFS PASSES THROUGH THE ISLANDS ON THE  
FASTER SIDE OF THE GUIDANCE AROUND TUESDAY. ON THE OTHER HAND 00Z  
ECMWF SHORTWAVE DETAILS LEAD TO A SLOWER EVOLUTION THAT BRINGS UP  
MORE MOISTURE FROM THE SOUTH WEDNESDAY-THURSDAY VERSUS THE CURRENT  
MAJORITY OR THE PREVIOUS ECMWF RUN. WITH MINIMAL CONFIDENCE, FOR  
THE PORTION OF THE FORECAST FROM SUNDAY ONWARD WILL RECOMMEND A  
COMPROMISE BETWEEN THE 00Z GEFS AND ECMWF MEANS WITH A SLIGHT TILT  
TOWARD THE ECMWF MEAN. BY NEXT THURSDAY THIS WOULD LEAD TO A  
PATTERN BETWEEN THE EXTREMES, THE GFS THAT BUILDS DECENT HIGH  
PRESSURE NORTH OF THE ISLANDS AND THE 00Z ECMWF THAT WOULD HAVE  
MOIST FLOW FROM THE SOUTH/SOUTHEAST.  
 
RAUSCH  
 

 
 
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