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| #445295 (Received by flhurricane at: 4:43 AM 28.Jul.2011) TCDAT4 TROPICAL STORM DON DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL042011 400 AM CDT THU JUL 28 2011 DON HAS NOT CHANGED APPRECIABLY DURING THE PAST SEVERAL HOURS... MAINTAINING A SMALL AREA OF CONVECTION NEAR THE CENTER. THERE IS NOT MUCH ORGANIZATION IN THE INFRARED CLOUD PATTERN...WHICH IS CONFIRMED BY A TRMM MICROWAVE PASS AT 0608Z. ASCAT DATA FROM 0332Z SUPPORTED AN INITIAL INTENSITY OF AROUND 35 KT...WITH SATELLITE CLASSIFICATIONS AT OR BELOW THAT VALUE. THE ASCAT DATA CONFIRMED THE EARLIER AIRCRAFT MEASUREMENTS THAT DON IS A RATHER SMALL TROPICAL CYCLONE. THE NEXT AIRCRAFT MISSION IS SCHEDULED FOR 1200Z THIS MORNING. THE INTENSITY FORECAST IS TRICKY. WHILE THE LARGE-SCALE ENVIRONMENT DOES NOT APPEAR TO HAVE MUCH SHEAR...THE LOW- AND MID-LEVEL CENTERS ARE NOT WELL-ALIGNED INITIALLY...WHICH WAS APPARENT ON A 2316Z SSMIS MICROWAVE IMAGE. THE GFS/ECMWF MODELS HAVE A REASONABLE REPRESENTATION OF THIS INITIAL STRUCTURE AND KEEP THE CENTERS FROM VERTICALLY STACKING...POSSIBLY WHY THESE MODELS DO NOT SHOW MUCH INTENSIFICATION. WHILE THE STATISTICALLY MODELS STILL FORECAST STEADY STRENGTHENING...THE HWRF/GFDL MODELS FORECAST A SLOW DECLINE. ANOTHER FACTOR TO CONSIDER IS THAT THE SIZE OF DON MAKES IT SUSCEPTIBLE TO RAPID CHANGES IN STRENGTH...BOTH UP AND DOWN...THAT ARE NEARLY IMPOSSIBLE TO FORECAST. GIVEN THE REDUCED ORGANIZATION...THE NHC INTENSITY FORECAST IS LOWERED SOMEWHAT FROM THE PREVIOUS FORECAST...BUT IS STILL ON THE UPPER END OF THE GUIDANCE ENVELOPE NEAR A COMPOSITE OF THE SHIPS/LGEM MODELS. BEST ESTIMATE OF INITIAL MOTION IS 300/9... WITH OVERNIGHT MICROWAVE IMAGES SUGGESTING THE SMALL CENTER HAS BEEN MOVING ERRATICALLY. DON IS MOVING AROUND THE SOUTHERN PERIPHERY OF A RIDGE OVER THE SOUTHERN UNITED STATES. THIS RIDGE IS FORECAST TO BUILD SLIGHTLY DURING THE NEXT DAY OR TWO AS A SERIES OF WEAK IMPULSES MOVE AWAY FROM THE NORTHERN GULF COAST. THIS SYNOPTIC PATTERN SHOULD STEER THE CYCLONE GENERALLY TO THE WEST-NORTHWEST AND ACCELERATE IT. THE TRACK GUIDANCE HAS SHIFTED A BIT TO THE WEST OVERNIGHT WITH LESS SPREAD THAN EARLIER. THIS SHIFT TO THE LEFT SEEMS REASONABLE WITH THE BUILDING RIDGE AND THE NHC FORECAST WILL BE MOVED IN THAT DIRECTION...BUT STILL LIES TO THE RIGHT OF THE MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 23.0N 88.7W 35 KT 40 MPH 12H 28/1800Z 23.9N 90.3W 40 KT 45 MPH 24H 29/0600Z 25.1N 92.5W 45 KT 50 MPH 36H 29/1800Z 26.3N 94.7W 50 KT 60 MPH 48H 30/0600Z 27.3N 97.1W 50 KT 60 MPH 72H 31/0600Z 29.0N 101.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 01/0600Z...DISSIPATED $$ FORECASTER BLAKE/BROWN |
