257  
WTPZ44 KNHC 051453  
TCDEP4  
 
HURRICANE ISELLE DISCUSSION NUMBER 21  
NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014  
800 AM PDT TUE AUG 05 2014  
 
ISELLE HAS CHANGED LITTLE IN ORGANIZATION DURING THE PAST SIX  
HOURS. THE EYE REMAINS WELL DEFINED, WITH CLOUD TOPS TO -70C  
OCCURRING IN THE EYEWALL. ANALYSES FROM THE SHIPS MODEL AND FROM  
CIMSS AT THE UNIVERSITY OF WISCONSIN INDICATE 15-20 KT OF  
SOUTHWESTERLY VERTICAL WIND SHEAR OVER THE SYSTEM. HOWEVER, THIS  
HAS NOT YET DISRUPTED THE INNER CORE. THE INITIAL INTENSITY REMAINS  
110 KT BASED ON A COMBINATION OF SUBJECTIVE AND OBJECTIVE SATELLITE  
INTENSITY ESTIMATES. AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT  
IS SCHEDULED TO INVESTIGATE ISELLE NEAR 06/0600Z, WHICH WILL PROVIDE  
GROUND TRUTH FOR THE INTENSITY.  
 
THE INITIAL MOTION ESTIMATE IS 270/8 KT. THE SUBTROPICAL RIDGE TO  
THE NORTH OF ISELLE SHOULD STRENGTHEN DURING THE NEXT SEVERAL DAYS,  
WHICH SHOULD STEER THE CYCLONE GENERALLY WEST-NORTHWESTWARD WITH AN  
INCREASE IN FORWARD SPEED. THE TRACK MODEL GUIDANCE REMAINS  
TIGHTLY CLUSTERED AND FORECASTS ISELLE TO PASS NEAR OR OVER THE  
HAWAIIAN ISLANDS IN 72-84 HOURS. BASED ON THIS, THE NEW FORECAST  
TRACK IS AGAIN JUST AN UPDATE OF THE PREVIOUS TRACK, AND IT LIES IN  
THE CENTER OF THE TRACK GUIDANCE ENVELOPE.  
 
THE ANALYZED AND 24-HOUR FORECAST SHEAR HAVE BOTH INCREASED SINCE  
THE PREVIOUS ADVISORY, AND IT IS LIKELY THAT ISELLE WILL LOSE ITS  
ANNULAR STRUCTURE LATER TODAY OR TONIGHT. THIS SHOULD RESULT IN  
WEAKENING ALONG THE LINES OF THE PREVIOUS FORECAST SINCE A  
SIGNIFICANT PART OF THE CIRCULATION IS OVER SEA SURFACE TEMPERATURES  
OF LESS THAN 26C. THE INTENSITY FORECAST BECOMES MORE UNCERTAIN  
AFTER 36 HOURS. THE CYCLONE WILL BE MOVING OVER INCREASING SEA  
SURFACE TEMPERATURES WEST OF 152W. THE DYNAMICAL MODELS ARE IN POOR  
AGREEMENT ON HOW MUCH SHEAR THE ISELLE WILL EXPERIENCE, WITH THE  
UKMET FORECASTING STRONG SHEAR WHILE THE GFS/ECMWF FORECAST LESS  
SHEAR. ONE LAST FACTOR IS THAT THE CYCLONE IS LIKELY TO ENCOUNTER A  
VERY DRY AIR MASS AS IT APPROACHES HAWAII. THE INTENSITY GUIDANCE  
RESPONDS TO THESE FACTORS BY DIVERGING. THE SHIPS AND FLORIDA STATE  
SUPERENSEMBLE FORECAST AN INTENSITY NEAR 40 KT WHEN ISELLE GETS NEAR  
HAWAII, WHILE THE GFDL AND HWRF FORECAST IT TO BE A HURRICANE. THE  
INTENSITY FORECAST FROM 48-120 HOURS HAS BEEN NUDGED UPWARD IN  
AGREEMENT WITH THE LGEM MODEL AND THE INTENSITY CONSENSUS, AND IT  
CALLS FOR ISELLE TO BE JUST BELOW HURRICANE STRENGTH NEAR HAWAII.  
 
INTERESTS IN THE HAWAIIAN ISLANDS SHOULD CLOSELY MONITOR THE  
PROGRESS OF ISELLE. HOWEVER, IT IS IMPORTANT NOT TO FOCUS TOO  
CLOSELY ON THE EXACT TRACK AND INTENSITY FORECASTS BECAUSE THE  
AVERAGE TRACK ERROR 72 HOURS OUT IS ABOUT 100 MILES, THE AVERAGE  
INTENSITY ERROR IS ABOUT 15 KT. IN ADDITION, THE HAZARDS OF A  
TROPICAL CYCLONE CAN EXTEND OVER A BROAD AREA WELL AWAY FROM THE  
CENTER.  
 
THIS IS THE LAST ADVISORY ISSUED BY THE NATIONAL HURRICANE CENTER ON  
ISELLE. FUTURE PUBLIC ADVISORIES WILL BE ISSUED BY THE CENTRAL  
PACIFIC HURRICANE CENTER UNDER AWIPS HEADER HFOTCPCP3 AND WMO HEADER  
WTPA33 PHFO, WHILE FORECAST/ADVISORIES WILL BE ISSUED UNDER  
AWIPS HEADER HFOTCMCP3 AND WMO HEADER WTPA23 PHFO.  
 
FORECAST POSITIONS AND MAX WINDS  
 
INIT 05/1500Z 16.0N 139.5W 110 KT 125 MPH  
12H 06/0000Z 16.3N 141.1W 100 KT 115 MPH  
24H 06/1200Z 16.9N 143.9W 85 KT 100 MPH  
36H 07/0000Z 17.6N 147.2W 70 KT 80 MPH  
48H 07/1200Z 18.3N 150.4W 65 KT 75 MPH  
72H 08/1200Z 20.0N 156.5W 55 KT 65 MPH...NEAR HAWAII  
96H 09/1200Z 21.0N 162.0W 45 KT 50 MPH  
120H 10/1200Z 21.5N 166.5W 35 KT 40 MPH  
 

 
FORECASTER BEVEN  
 
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