790  
FXHW01 KWNH 241221  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
820 AM EDT TUE APR 24 2018  
 
VALID 00Z WED APR 25 2018 - 00Z WED MAY 02 2018  
 
GUIDANCE AGREES IN PRINCIPLE REGARDING THE PATTERN EVOLUTION THAT  
WILL PROMOTE INCREASED POTENTIAL FOR SIGNIFICANT RAINFALL OVER THE  
CENTRAL/EASTERN ISLANDS BY THE WEEKEND AND EARLY NEXT WEEK, BUT  
DIFFER IN SOME OF THE SPECIFIC DETAILS.  
 
THROUGH WEDNESDAY AND TO SOME EXTENT INTO THURSDAY EXPECT HIGH  
PRESSURE NORTHEAST OF THE STATE TO SUPPORT BRISK TRADES WITH  
SHOWER ACTIVITY TENDING TO FAVOR WINDWARD AREAS. THEN A RAPIDLY  
AMPLIFYING CENTRAL PACIFIC TROUGH ALOFT WILL LIKELY CLOSE OFF AN  
EMBEDDED LOW TO THE NORTH OF THE AREA, AROUND 30-35N LATITUDE,  
WHILE DEVELOPING SURFACE LOW PRESSURE ANCHORS A FRONT THAT MOVES  
INTO THE ISLANDS. AMONG CURRENT GUIDANCE THE 00Z/06Z GFS RUNS  
SHOW A FARTHER SOUTH/SOUTHWEST UPPER LOW POSITION THAN MOST OTHER  
SOLUTIONS IN THE FRIDAY-SUNDAY TIME FRAME. A MORE INTERMEDIATE  
POSITION IN THE 00Z GEFS MEAN SUGGESTS HEDGING AWAY FROM THE GFS  
SCENARIO. MEANWHILE THERE ARE DIFFERENCES IN SOUTHEASTWARD EXTENT  
OF THE FRONT THAT WILL SERVE AS THE FOCUS FOR POTENTIALLY ENHANCED  
RAINFALL IN ASSOCIATION WITH MOISTURE BEING PULLED UP FROM THE  
SOUTH (PRECIPITABLE WATER VALUES OF AT LEAST 1.75-2.00 INCHES).  
PERHAPS DUE IN PART TO DIFFERENCES ALOFT, THE GFS GENERALLY HOLDS  
THE FRONT A LITTLE FARTHER NORTHWEST THAN OTHER GUIDANCE,  
GENERALLY OVER THE EASTERN HALF OF THE STATE. ON THE OTHER HAND  
BY EARLY NEXT WEEK THE 00Z GEFS MEAN TRACKS THE SURFACE LOW AND  
TRAILING FRONT/MOISTURE AXIS A LITTLE SOUTHEAST OF CONSENSUS.  
OVERALL A SOLUTION CLOSE TO OR JUST A FRACTION SLOWER THAN THE 00Z  
ECMWF SEEMS TO PROVIDE THE BEST INTERMEDIATE FORECAST AT THE  
SURFACE AND ALOFT. EVEN WITH THE DIFFERENCES IN FRONTAL POSITION  
AND CORRESPONDING RAINFALL DURATION/INTENSITY OVER EASTERN  
LOCATIONS, THERE IS DECENT CONFIDENCE IN A DRIER PATTERN WITH  
NORTH/NORTHEASTERLY WINDS OVER THE WESTERN ISLANDS BY THE WEEKEND  
AND EARLY NEXT WEEK.  
 
RAUSCH  
 

 
 
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