000 WTPZ42 KNHC 190835 TCDEP2 TROPICAL STORM IVO DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP122007 200 AM PDT WED SEP 19 2007 IVO CONTINUES TO STRENGTHEN AS IT EXHIBITS A FAIRLY SYMMETRIC CDO AND WELL-DEFINED UPPER-LEVEL OUTFLOW PATTERN. THE CURRENT INTENSITY IS SET AT 50 KT...WHICH IS THE AVERAGE OF DVORAK ESTIMATES FROM TAFB AND SAB. MICROWAVE IMAGERY SHOWS A DEVELOPING INNER CORE BUT NO EYE...YET. IT IS EXPECTED THAT THE TROPICAL CYCLONE WILL REMAIN AN ENVIRONMENT OF LOW VERTICAL WIND SHEAR AND HIGH OCEANIC HEAT CONTENT FOR THE NEXT COUPLE OF DAYS. THUS THE OFFICIAL INTENSITY FORECAST SHOWS STRENGTHENING TO A HURRICANE IN A DAY OR TWO. THIS IS IN REASONABLE AGREEMENT WITH THE LATEST GFDL MODEL PREDICTION. THE SHIPS GUIDANCE IS A LITTLE HIGHER THAN THE OFFICIAL FORECAST THROUGH 72 HOURS. BY DAYS 4 AND 5...AN INCREASE IN WESTERLY SHEAR IS LIKELY TO CAUSE SOME WEAKENING AND THE OFFICIAL FORECAST IS QUITE SIMILAR TO THE SHIPS OUTPUT IN THE LATTER PART OF THE PERIOD. IF IVO MOVES FARTHER NORTH THAN EXPECTED IT WOULD ENCOUNTER STRONGER UPPER-LEVEL WESTERLIES...AND WOULD MOST LIKELY WEAKEN FASTER THAN INDICATED HERE. THERE HAS BEEN A SLIGHT SLOWING IN THE ESTIMATED MOTION...WHICH IS NOW ABOUT 300/9. DURING THE NEXT 48-72 HOURS...IVO IS FORECAST TO MOVE AROUND THE WESTERN PERIPHERY OF A MID-LEVEL SUBTROPICAL ANTICYCLONE. BEYOND THAT TIME FRAME...GLOBAL MODELS DEPICT A STRONG MID- TO UPPER-TROPOSPHERIC LOW DROPPING SOUTHWARD AND SOUTHEASTWARD INTO SOUTHERN CALIFORNIA AND NORTHERN BAJA CALIFORNIA. IN RESPONSE TO THIS FEATURE MOST OF THE DYNAMICAL TRACK MODELS TURN IVO NORTHEASTWARD OR...IN THE CASE OF THE GFDL MODEL... EAST-NORTHEASTWARD...IN 3 TO 5 DAYS. THE OFFICIAL FORECAST IS SHIFTED A LITTLE TO THE EAST OF THE PREVIOUS ONE AND IS CLOSE TO THE GUNA DYNAMICAL TRACK MODEL CONSENSUS. AS NOTED EARLIER...THE TRACK FORECAST AT 3-5 DAYS IS HIGHLY UNCERTAIN BECAUSE THERE IS A LARGE SPREAD IN THE CREDIBLE NUMERICAL GUIDANCE AT THAT TIME RANGE. FORECAST POSITIONS AND MAX WINDS INITIAL 19/0900Z 14.7N 110.0W 50 KT 12HR VT 19/1800Z 15.4N 111.2W 55 KT 24HR VT 20/0600Z 16.4N 112.5W 60 KT 36HR VT 20/1800Z 17.3N 113.4W 65 KT 48HR VT 21/0600Z 18.2N 113.8W 65 KT 72HR VT 22/0600Z 20.0N 113.5W 60 KT 96HR VT 23/0600Z 21.0N 112.5W 55 KT 120HR VT 24/0600Z 21.5N 112.0W 50 KT $$ FORECASTER PASCH