061  
FXHW01 KWNH 281218  
PMDHI  
 
HAWAII EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
717 AM EST SUN JAN 28 2018  
 
VALID 00Z MON JAN 29 2018 - 00Z MON FEB 05 2018  
 
GRADUAL PROGRESSION OF THE UPPER SHORTWAVE CURRENTLY CROSSING THE  
STATE WILL ALLOW FOR A BRIEF PERIOD OF DRIER CONDITIONS OVER MOST  
LOCATIONS FOR MONDAY INTO TUESDAY, WHILE LOW LEVEL WINDS TURN FROM  
NORTHEASTERLY/EASTERLY TO SOUTHERLY AS A FRONT APPROACHES FROM THE  
NORTHWEST. NOTE THAT EVEN SO EARLY IN THE FORECAST, THERE ARE  
SOME TIMING DIFFERENCES FOR THE SHORTWAVE. THE 00Z UKMET/CMC  
RECOMMEND A COMPROMISE BETWEEN THE FAST 00Z-06Z GFS/00Z GEFS AND  
SLOW 00Z ECMWF/ECMWF ENSEMBLES.  
 
THE FRONT APPROACHING ON TUESDAY WILL HERALD THE START OF A  
GENERALLY WETTER PATTERN AS UPPER TROUGHING BECOMES ESTABLISHED TO  
THE NORTHWEST OF THE STATE. HOWEVER GUIDANCE THUS FAR HAS VARIED  
WITHIN RUNS AND FROM DAY TO DAY WITH RESPECT TO DETAILS OF FRONTAL  
TIMING/POSITION AT ANY PARTICULAR TIME AND THE AMOUNT OF MOISTURE.  
THUS CONFIDENCE HAS BEEN LOWER THAN DESIRED IN PINPOINTING  
PARTICULAR TIME FRAMES AND EXTENT OF HEAVIER RAINFALL EPISODES.  
 
INTO WEDNESDAY EXPECT THE AFOREMENTIONED FRONT AND ACCOMPANYING  
RAINFALL, WITH POTENTIAL FOR SOME LOCALLY MODERATE-HEAVY ACTIVITY,  
TO PROGRESS INTO THE CENTRAL ISLANDS. THIS FRONT SHOULD  
STALL/WEAKEN AS HEIGHTS ALOFT STABILIZE OR SLOWLY RISE WITH THE  
APPROACH OF UPSTREAM ENERGY ALOFT AND ANOTHER FRONT. THE TRAILING  
FRONT HAS SHOWN GREATER TIMING DIFFERENCES OVER RECENT DAYS. GFS  
AND TO SOME DEGREE GEFS MEANS HAVE BEEN ON THE FAST SIDE OF THE  
SPREAD ONLY TO TREND SLOWER TOWARD OTHER SOLUTIONS THE NEXT DAY.  
SLOWER GFS TRENDS INTO TODAY HAVE LED TO BETTER AGREEMENT THAT A  
SOUTHWEST-NORTHEAST BAND OF ENHANCED MOISTURE SHOULD PERSIST OVER  
THE AREA LATE WEEK INTO THE WEEKEND. HOWEVER IT REMAINS TO BE  
SEEN IF PRECIPITABLE WATER VALUES REACH LEVELS SEEN IN THE GFS, AS  
OTHER GUIDANCE SHOWS SOMEWHAT HIGHER HEIGHTS ALOFT AND THUS LESS  
CONCENTRATION OF DEEP MOISTURE/RAINFALL. ON THE OTHER HAND THE  
GENERAL PATTERN MAY SUPPORT GREATER MOISTURE/RAINFALL THAN  
INDICATED BY THE 00Z ECMWF. RECENT LATE-PERIOD GFS BIASES MAY  
COME INTO PLAY AGAIN BY NEXT SUNDAY AS IT BRINGS MUCH LOWER  
HEIGHTS ALOFT OVER/NORTH OF THE STATE VERSUS OTHER SOLUTIONS,  
PUSHING YET ANOTHER FRONT FARTHER EASTWARD THAN MOST SOLUTIONS--  
ASIDE FROM THE 00Z CMC WHICH INTERESTINGLY IS SLOWER THAN THE GFS  
WITH THE UPPER TROUGH.  
 
RAUSCH  
 

 
 
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