Tropical Storm IVO

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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
 
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