000 WTPZ42 KNHC 191450 TCDEP2 TROPICAL STORM IVO DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP122007 800 AM PDT WED SEP 19 2007 SATELLITE IMAGERY THIS MORNING SHOWS THAT IVO HAS A SOMEWHAT RAGGED CENTRAL DENSE OVERCAST WITH CLOUD TOPS OF -70C TO -80C. SATELLITE INTENSITY ESTIMATES ARE 55 KT FROM TAFB AND SAB...SO THE INITIAL INTENSITY IS INCREASED TO 55 KT. THE CIRRUS OUTFLOW REMAINS GOOD IN THE SOUTHEASTERN SEMICIRCLE...BUT HAS DECREASED A LITTLE OVER THE NORTHWESTERN SEMICIRCLE. THE INITIAL MOTION IS 295/10. IVO REMAINS SOUTH OF A MID-LEVEL RIDGE EXTENDING WESTWARD FROM NORTHERN MEXICO. THE LARGE-SCALE MODELS ARE IN GOOD AGREEMENT THAT A DEEP-LAYER LOW CURRENTLY MOVING SOUTHWARD INTO CALIFORNIA WILL DROP FAR ENOUGH SOUTH TO BREAK THE RIDGE NORTH OF IVO...ALLOWING THE STORM TO RECURVE TO THE NORTHEAST DURING THE NEXT 72-96 HR. HOWEVER...IT APPEARS THE LOW WILL NOT COME FAR ENOUGH SOUTH TO ACCELERATE THE STORM NORTHEASTWARD...SO THE MOTION AFTER RECURVATURE IS LIKELY TO BE SLOW. THE NEW TRACK FORECAST IS AN UPDATE OF THE PREVIOUS TRACK FOR THE FIRST 72 HR... THEN IS SHIFTED TO THE NORTHEAST AT 96 AND 120 HR IN A COMPROMISE BETWEEN THE GFDL...HWRF...UKMET...AND FSU SUPERENSEMBLE MODELS. IT SHOULD BE NOTED THAT WHILE THE LARGE-SCALE MODELS AGREE ON THE EVOLUTION OF THE STEERING PATTERNS...THEY DO NOT AGREE ON THE MOTION. THE GFS AND ECMWF BOTH SHOW IVO INTERACTING WITH ANOTHER DISTURBANCE TO THE WEST WHICH PREVENTS THE STORM FROM RECURVING IN THOSE MODELS. IVO IS FORECAST TO REMAIN IN A LIGHT VERTICAL WIND SHEAR ENVIRONMENT FOR THE NEXT 36 HR OR SO...SO CONTINUED STRENGTHENING IS FORECAST IN AGREEMENT WITH THE SHIPS MODEL. BEYOND THAT TIME...THE LARGE-SCALE MODELS DISAGREE ON HOW MUCH WIND SHEAR IVO WILL ENCOUNTER. THE SHIPS AND GFDL MODELS FORECAST 20-25 KT OF NORTHWESTERLY SHEAR...WHILE THE OTHER GLOBAL MODELS SHOW LESS SHEAR. FOR NOW...THE INTENSITY FORECAST WILL FOLLOW SHIPS AND CALL FOR GRADUAL WEAKENING AFTER 48 HR. HOWEVER...THERE ARE TWO ALTERNATIVES. FIRST...THE NEW FORECAST TRACK BRINGS IVO OVER WARMER WATER BY 120 HR...AND IF THE GFS IS WRONG ABOUT THE AMOUNT OF SHEAR IVO COULD BE STRONGER THAN FORECAST AT THAT TIME. SECOND...IF IVO TRACKS NORTH OF THE FORECAST TRACK...IT WOULD LIKELY ENCOUNTER BOTH STRONGER SHEAR AND COLDER WATER...WHICH SHOULD CAUSE A FASTER WEAKENING. FORECAST POSITIONS AND MAX WINDS INITIAL 19/1500Z 15.3N 111.1W 55 KT 12HR VT 20/0000Z 15.9N 112.1W 65 KT 24HR VT 20/1200Z 16.8N 113.2W 70 KT 36HR VT 21/0000Z 17.7N 113.9W 75 KT 48HR VT 21/1200Z 18.6N 114.0W 75 KT 72HR VT 22/1200Z 20.5N 113.0W 70 KT 96HR VT 23/1200Z 21.5N 111.5W 60 KT 120HR VT 24/1200Z 22.0N 110.0W 55 KT $$ FORECASTER BEVEN