000 WTNT42 KNHC 062050 TCDAT2 SUBTROPICAL STORM OTTO DISCUSSION NUMBER 3 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL172010 500 PM AST WED OCT 06 2010 SATELLITE IMAGERY AND DATA FROM AN AIR FORCE RESERVE RECONNAISSANCE UNIT AIRCRAFT INDICATE THAT THE SUBTROPICAL DEPRESSION HAS CONTINUED TO BECOME BETTER ORGANIZED AND HAS STRENGTHEN INTO SUBTROPICAL STORM OTTO. CONVECTIVE BANDING NOW WRAPS ABOUT 75 PERCENT AROUND THE BROAD INNER CORE WIND FIELD. MAXIMUM 1500-FT FLIGHT-LEVEL WINDS OBSERVED HAVE BEEN 63 KT IN THE SOUTHWESTERN QUADRANT...WHICH CORRESPONDS TO ABOUT 50 KT AT THE SURFACE. THIS VALUE IS CLOSE TO THE 48 KT CREDIBLE SFMR WINDS OBSERVED IN THE SAME AREA...SO THE INITIAL INTENSITY IS SET AT 50 KT. VISIBLE AND WATER VAPOR SATELLITE IMAGERY INDICATE THAT A SMALL UPPER-LEVEL COLD LOW...LOCATED PREVIOUSLY IN THE SOUTHEAST QUADRANT...HAS MOVED CYCLONICALLY AROUND THE LOW-LEVEL CIRCULATION AND IS NOW SITUATED ABOUT 30 NMI WEST OF THE LOW-LEVEL CENTER. THIS SUGGESTS THAT OTTO IS STILL A SUBTROPICAL CYCLONE...BUT THAT IT IS ALSO GRADUALLY ACQUIRING TROPICAL CHARACTERISTICS. THE INITIAL MOTION ESTIMATE IS 360/4. THERE ARE NO SIGNIFICANT CHANGES TO THE PREVIOUS FORECAST TRACK OR REASONING. OTTO IS MOVING NORTHWARD THROUGH A BROAD WEAKNESS IN A DEEP-LAYER RIDGE WHERE STEERING CURRENTS ARE WEAK. HOWEVER...A STRONG MID-TROPOSPHERIC TROUGH LOCATED OVER THE EASTERN U.S. IS FORECAST TO MOVE EASTWARD AND GRADUALLY NUDGE OTTO ON A NORTHEASTWARD TRACK DURING THE NEXT 24 HOURS...AND THEN RAPIDLY ACCELERATE THE CYCLONE TO THE EAST-NORTHEAST AHEAD OF A STRONG COLD FRONT BY 36 HOURS. BETWEEN 72 AND 96 HOURS...OTTO IS EXPECTED TO MERGE WITH THE COLD FRONT AND BECOME A STRONG EXTRATROPICAL LOW PRESSURE SYSTEM THAT MAY POSE A THREAT TO THE AZORES ISLANDS BY 120 HOURS. THE FORECAST TRACK IS JUST AN UPDATE OF THE PREVIOUS ADVISORY TRACK AND IS DOWN THE MIDDLE OF THE TIGHTLY PACKED NHC MODEL GUIDANCE. OTTO IS EXPECTED TO TRANSITION INTO A TROPICAL CYCLONE WITHIN THE NEXT 12 HOURS OR SO DUE TO A COMBINATION OF LIGHT VERTICAL WIND SHEAR...WARM SSTS OF AT LEAST 28C...AND THE EXPECTED DISSIPATION OF THE SMALL UPPER-LEVEL LOW. THE OFFICIAL INTENSITY FORECAST NOW CALLS FOR OTTO TO REACH HURRICANE STRENGTH BY 36 HOURS...AND THEN GRADUALLY WEAKEN AS THE WIND SHEAR INCREASES AHEAD OF THE AFOREMENTIONED STRONG UPPER-LEVEL TROUGH. BY 96 HOURS...OTTO IS EXPECTED TO TRANSITION INTO AN EXTRATROPICAL LOW. THE OFFICIAL INTENSITY FORECAST IS A COMPROMISE OF THE STRONGER SHIPS/LGEM MODELS AND THE WEAKER GFDL/HWRF MODELS. FORECAST POSITIONS AND MAX WINDS INITIAL 06/2100Z 23.2N 68.3W 50 KT 12HR VT 07/0600Z 23.8N 68.5W 55 KT 24HR VT 07/1800Z 24.5N 67.9W 60 KT 36HR VT 08/0600Z 25.5N 66.4W 65 KT 48HR VT 08/1800Z 26.8N 63.7W 65 KT 72HR VT 09/1800Z 30.3N 55.8W 55 KT 96HR VT 10/1800Z 34.5N 44.5W 40 KT...POST-TROP/EXTRATROP 120HR VT 11/1800Z 39.0N 31.0W 35 KT...POST-TROP/EXTRATROP $$ FORECASTER STEWART