000 WTPZ42 KNHC 200834 TCDEP2 HURRICANE IVO DISCUSSION NUMBER 8 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP122007 200 AM PDT THU SEP 20 2007 CONVENTIONAL SATELLITE IMAGERY SHOWS THAT IVO HAS MAINTAINED ITS DEEP CONVECTION AND UPPER-LEVEL OUTFLOW....AND RECENT MICROWAVE PASSES FROM BOTH SSM/I AND AMSU-B INDICATE AN EYE FEATURE. AN EARLIER QUIKSCAT PASS AT 0136 UTC HAD MAXIMUM WIND VECTORS OF AT LEAST 55 KT. THESE VECTORS WERE LIKELY ATTENUATED BY HEAVY RAIN...SO THE ACTUAL MAXIMUM WINDS WERE PROBABLY CLOSER TO 65 KT. DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN AT 4.0 OR 65 KT...WHICH IS USED FOR THE INITIAL INTENSITY. THE 34- AND 50-KT WIND RADII HAVE BEEN ADJUSTED BASED ON THE QUIKSCAT PASS. IVO IS MOVING A BIT TO THE RIGHT OF THE PREVIOUS TRACK...AND THE INITIAL MOTION IS NOW ESTIMATED AT 330/6. THE FORWARD MOTION HAS SLOWED AS THE HURRICANE MAKES ITS WAY AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE. A SLOW NORTHWARD MOTION IS ANTICIPATED FOR THE NEXT 36 HOURS OR SO. THEREAFTER A MID-LATITUDE TROUGH IS EXPECTED TO DIG SOUTHEASTWARD NEAR THE SOUTHERN CALIFORNIA-NORTHERN BAJA CALIFORNIA BORDER. THIS STEERING FEATURE SHOULD CREATE A MORE NORTH-NORTHEASTWARD TO NORTHEASTWARD MOTION IN THE LATER PERIODS. SOME OF THE DYNAMICAL MODELS SUCH AS THE GFS AND HWRF SUGGEST THAT IVO WILL WEAKEN AND TAKE A MORE NORTHWARD OR EVEN NORTHWESTWARD TRACK NEAR THE END OF THE PERIOD. THE OFFICIAL TRACK FORECAST IS CLOSER TO THE GFDL SCENARIO OF A STRONGER SYSTEM THAT WILL BE MORE RESPONSIVE TO THE TROUGH. THIS IS SIMILAR TO THE PREVIOUS OFFICIAL FORECAST. THE HURRICANE IS CURRENTLY IN A FAVORABLE ENVIRONMENT FOR ADDITIONAL STRENGTHENING...WITH WARM WATERS AND RELATIVELY WEAK SHEAR. BOTH OF THESE ENVIRONMENTAL CONDITIONS ARE EXPECTED TO CONTINUE TO PREVAIL FOR THE NEXT COUPLE OF DAYS. THE INTENSITY FORECAST IN THE LATER PERIODS IS HIGHLY UNCERTAIN. IF IVO TRACKS SLIGHTLY TO THE NORTH OF OUR FORECAST...STRONGER SHEAR COULD CAUSE THE CYCLONE TO WEAKEN SIGNIFICANTLY. THE OFFICIAL INTENSITY FORECAST IS A BIT ABOVE THE CONSENSUS TECHNIQUE...ICON...BUT A LITTLE BELOW THE GFDL. INTERESTS IN SOUTHERN BAJA CALIFORNIA SHOULD CLOSELY MONITOR THE PROGRESS OF IVO DURING THE NEXT FEW DAYS. ONE SHOULD NOT FOCUS ON THE EXACT TRACK AT 4 AND 5 DAYS BECAUSE THERE ARE LARGE UNCERTAINTIES AT THESE FORECAST PERIODS. FORECAST POSITIONS AND MAX WINDS INITIAL 20/0900Z 16.5N 112.5W 65 KT 12HR VT 20/1800Z 17.3N 113.0W 70 KT 24HR VT 21/0600Z 18.3N 113.3W 75 KT 36HR VT 21/1800Z 19.3N 113.1W 75 KT 48HR VT 22/0600Z 20.3N 112.7W 70 KT 72HR VT 23/0600Z 22.0N 111.5W 65 KT 96HR VT 24/0600Z 23.5N 110.5W 55 KT 120HR VT 25/0600Z 25.5N 109.5W 50 KT $$ FORECASTER MAINELLI/PASCH