ZCZC MIATCDAT3 ALL TTAA00 KNHC DDHHMM TROPICAL STORM ISAAC DISCUSSION NUMBER 3 NATIONAL WEATHER SERVICE MIAMI FL 11 PM EDT THU SEP 21 2000 TROPICAL DEPRESSION THIRTEEN HAS BECOME BETTER ORGANIZED THIS EVENING AND HAS STRENGTHENED INTO TROPICAL STORM ISAAC. BANDING FEATURES HAVE IMPROVED SIGNIFCANTLY DURING THE PAST 6 HOURS...AND THE OUTFLOW PATTERN HAS BECOME BETTER DEVELOPED AND MORE CIRCULAR. A PRONOUNCED POLEWARD OUTFLOW CHANNEL HAS ALSO DEVELOPED. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB SUPPPORT AT LEAST A 35 KT INITIAL INTENSITY. INITIAL MOTION IS 285/13. NOW THAT ISAAC HAS DEVELOPED INTO A DEEPER VERTICAL SYSTEM...IT SHOULD BE STEERED MORE BY THE DEEP LAYER FLOW WHICH TAKES THE SYSTEM OFF TO THE WEST-NORTHWEST THROUGH THE PERIOD. THE UKMET MODEL IS SIMILAR TO THE PREVIOUS FORECAST TRACKS...ONLY SLOWER...AND THE GFDL IS TO THE RIGHT OF THE OFFICIAL TRACK. BY 72 HOURS...THE GLOBAL MODELS DIFFER IN THE POSITION OF THE UPPER-LEVEL LOW CURRENTLY NEAR 55W LONGITUDE...AND ALSO THE DEVELOPMENT OF THE UPPER-LEVEL RIDGE TO THE NORTH OF ISAAC. ALL THREE MODELS AGREE ON MOVING THE UPPER-LEVEL LOW TO THE WEST AND GRADUALLY WEAKENING IT. THE UKMET KEEPS THE UPPER-LEVEL HIGH TO THE NORTHEAST OF ISAAC...WHILE THE AVN MODEL DEVELOPS THE HIGH TO THE NORTHWEST AND BRINGS NORTHWESTLY FLOW ON THE SYSTEM BY 72 HOURS. THE NOGAPS MODEL IS IN BETWEEN THE AVN AND UKMET SOLUTIONS. IF THE AVN MODEL VERIFIES...THEN THE NORTHERLY FLOW WOULD TEND TO TURN ISAAC MORE WESTERLY AFTER 60 TO 72 HOURS. THE OFFICIAL FORECAST IS IN THE SOUTHERN PORTION OF THE FORECAST GUIDANCE ENVELOPE AND ALMOST ON TOP OF THE PREVIOUS FORECAST. ISSAC MAY BE THE ONLY TRUE DEEP-TROPICS TROPICAL CYCLONE SO FAR THIS YEAR WITHOUT ENCOUNTERING ADVERSE SHEAR ACROSS THE MID-ATLANTIC. AS PREVIOUSLY MENTIONED...ALL OF THE GLOBAL MODELS AGREE ON MOVING THE UPPER-LEVEL LOW/TROUGH ALONG 55W WESTWARD AND WEAKENING IT. ALSO...THE DISTURBANCE WEST OF ISSAC CURRENTLY ALONG 40W IS ACTING TO DEFLECT SOME OF THE WESTERLY SHEAR WELL NORTH OF ISAAC...WHICH MAY HAVE ALLOWED THE POLEWARD OUTFLOW CHANNEL TO DEVELOP. THE INTENSITY FORECAST IS HIGHER AND THE INTENSIFICATION RATE FASTER THAN THE PREVIOUS FORECAST...BUT THE OFFICIAL FORECAST IS STILL BELOW THE SHIPS INTENSITY MDOEL. HOWEVER...WITH SUCH A WELL- ORGANIZED CONVECTIVE PATTERN AND POLEWARD OUTFLOW CHANNEL...RAPID INTENSIFICATION AND HIGHER INTENSITIES MAY HAVE TO BE CONSIDERED ON THE NEXT ADVISORY...IF THE DEEP CONVECTION PERSISTS NEAR THE CENTER. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 22/0300Z 12.5N 26.2W 35 KTS 12HR VT 22/1200Z 13.0N 28.2W 45 KTS 24HR VT 23/0000Z 13.6N 30.7W 55 KTS 36HR VT 23/1200Z 14.1N 33.3W 65 KTS 48HR VT 24/0000Z 14.5N 36.0W 70 KTS 72HR VT 25/0000Z 15.5N 41.0W 75 KTS NNNN