000 WTNT42 KNHC 150859 TCDAT2 HURRICANE JULIA DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL122010 500 AM AST WED SEP 15 2010 A RAPID STRENGTHENING EPISODE WAS NOTED A FEW HOURS AGO. THIS WAS MOSTLY UNEXPECTED...AND REMINDS US OF OUR LIMITED UNDERSTANDING OF TROPICAL CYCLONE INTENSITY CHANGE. JULIA IS STILL STRENGTHENING... BUT AT A SIGNIFICANTLY SLOWER RATE THAN WAS OBSERVED LAST EVENING AND EARLIER THIS MORNING. ALTHOUGH THE EYE HAS NOT WARMED MUCH...THE CONVECTIVE BANDING AROUND THE EYE HAS BECOME BETTER ORGANIZED. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB ARE 115 KT...AND THAT WILL BE THE INITIAL WIND SPEED FOR THIS ADVISORY... MAKING JULIA A CATEGORY FOUR HURRICANE. JULIA COULD STRENGTHEN A LITTLE MORE TODAY AS IT REMAINS OVER RELATIVELY WARM WATER AND IN A MODERATE WIND SHEAR ENVIRONMENT. HOWEVER...MUCH OF THE INTENSITY GUIDANCE SHOWS GRADUAL WEAKENING BEYOND 12 TO 24 HOURS AS JULIA MOVES OVER COOLER WATER AND THEN INTO STRONG NORTHERLY SHEAR ASSOCIATED WITH THE OUTFLOW OF HURRICANE IGOR. THE NHC OFFICIAL INTENSITY FORECAST IS A LITTLE HIGHER THAN THE PREVIOUS ADVISORY...AND IS ABOVE ALL OF THE MODELS FOR THE FIRST FEW DAYS AS NONE OF THE GUIDANCE ANTICIPATED THE OBSERVED STRENGTHENING OF JULIA. THE HURRICANE HAS NOW TURNED TO THE NORTHWEST...WITH A 12-HOUR INITIAL MOTION ESTIMATE OF 305/9. A CONTINUED NORTHWESTWARD MOTION IS EXPECTED FOR THE NEXT DAY OR SO AS JULIA NEARS A MID- TO UPPER-LEVEL TROUGH OVER THE EASTERN ATLANTIC. HOWEVER...A TURN BACK TO THE WEST-NORTHWEST IS FORECAST IN ABOUT 36 HOURS AS THE HURRICANE IS STEERED BETWEEN AN UPPER-LEVEL LOW AND A STRENGTHENING SUBTROPICAL RIDGE. JULIA IS EXPECTED TO APPROACH THE WESTERN PERIPHERY OF THE RIDGE IN 3 TO 4 DAYS ALLOWING THE SYSTEM TO TURN NORTHWARD AND POSSIBLY NORTH-NORTHEASTWARD BY DAY 5. THE OFFICIAL TRACK FORECAST IS ADJUSTED SLIGHTLY TO THE RIGHT AND IS A LITTLE FASTER THAN THE PREVIOUS ONE TO COME INTO BETTER AGREEMENT WITH THE NEW GUIDANCE SUITE. FORECAST POSITIONS AND MAX WINDS INITIAL 15/0900Z 17.3N 31.8W 115 KT 12HR VT 15/1800Z 18.6N 33.2W 120 KT 24HR VT 16/0600Z 20.4N 35.9W 120 KT 36HR VT 16/1800Z 22.2N 39.3W 105 KT 48HR VT 17/0600Z 23.4N 42.6W 95 KT 72HR VT 18/0600Z 26.3N 48.1W 80 KT 96HR VT 19/0600Z 30.0N 50.5W 65 KT 120HR VT 20/0600Z 34.0N 49.5W 50 KT $$ FORECASTER CANGIALOSI