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| #458518 (Received by flhurricane at: 11:00 PM 10.Sep.2011) TCDAT4 TROPICAL STORM MARIA DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL142011 1100 PM AST SAT SEP 10 2011 REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT AND SURFACE OBSERVATIONS FROM THE LEEWARD ISLANDS SHOW THAT THE WIND CIRCULATION OF MARIA HAS BECOME BETTER DEFINED DURING THE PAST SEVERAL HOURS...ALTHOUGH THE SOUTHWEST SIDE REMAINS WEAK. THE AIRCRAFT REPORTED MAXIMUM 850 MB FLIGHT-LEVEL WINDS OF 59 KT AND ESTIMATED THE CENTRAL PRESSURE NEAR 1005 MB. THERE WERE NUMEROUS 40-45 KT WIND ESTIMATES FROM THE SFMR THAT LOOKED RELIABLE...SO THE INITIAL INTENSITY IS INCREASED TO 45 KT. OVERALL...THE SYSTEM NOW HAS THE APPEARANCE OF A SHEARED TROPICAL CYCLONE WITH STRONG CONVECTION NEAR AND TO THE NORTHEAST OF THE LOW-LEVEL CENTER. THE INITIAL MOTION IS 315/9...A LITTLE SLOWER THAN EARLIER. OTHER THAN THAT...THE TRACK FORECAST REASONING HAS CHANGED LITTLE SINCE THE LAST ADVISORY. FOR THE NEXT COUPLE OF DAYS...A COMBINATION OF THE SUBTROPICAL RIDGE TO THE NORTH AND AN UPPER-LEVEL LOW NEAR 25N64W SHOULD STEER MARIA GENERALLY NORTHWESTWARD. AFTER THAT...A DEEP-LAYER WESTERLY TROUGH ADVANCING THROUGH THE EASTERN UNITED STATES SHOULD WEAKEN THE RIDGE AND ALLOW MARIA TO RECURVE INTO THE WESTERLIES. THERE IS SOME SPREAD IN THE TRACK GUIDANCE. THE UKMET AND ECMWF SHOW A MORE WESTERLY MOTION DURING THE FIRST 72 HOURS AND ARE SLOWER TO RECURVE. THE GFS SHOWS A MORE NORTHWARD MOTION AND IS THE FASTEST TO MOVE THE CYCLONE INTO THE WESTERLIES. THE NEW TRACK FORECAST IS SHIFTED A LITTLE TO THE EAST OF THE PREVIOUS FORECAST...BUT LIES A LITTLE TO THE LEFT OF THE CENTER OF THE GUIDANCE ENVELOPE THROUGH 72 HOURS. THE 96 AND 120 HOUR POINTS COMPROMISE IN SPEED BETWEEN THE FASTER GFS AND THE SLOWER ECMWF/UKMET. MARIA IS CURRENTLY EXPERIENCING 15-20 KT OF SOUTHWESTERLY VERTICAL WIND SHEAR DUE TO THE INTERACTION WITH THE UPPER-LEVEL LOW. THE DYNAMICAL MODELS ARE IN POOR AGREEMENT ON HOW MUCH SHEAR THE STORM SHOULD CONTINUE TO EXPERIENCE DURING THE FORECAST PERIOD. THE GFS...NOGAPS...AND CANADIAN MODELS FORECAST THE SHEAR TO DECREASE SIGNIFICANTLY BY 72 HOURS...WHILE THE UKMET AND ECMWF FORECAST A MORE HOSTILE ENVIRONMENT AND A WEAKER MARIA. THE MAJORITY OF THE INTENSITY FORECAST GUIDANCE CALLS FOR MARIA TO BE A HURRICANE BY 72 HOURS. DUE TO THE UNCERTAINTIES IN HOW MARIA WILL INTERACT WITH THE UPPER-LEVEL LOW...THE NEW INTENSITY FORECAST WILL NOT CALL FOR MARIA TO BECOME A HURRICANE AT THIS TIME. INSTEAD...THE INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS FORECAST WITH SLOW INTENSIFICATION DURING THE 24-96 HOUR PERIOD. THIS FORECAST IS AT THE LOWER EDGE OF THE INTENSITY GUIDANCE. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 18.5N 62.9W 45 KT 50 MPH 12H 11/1200Z 19.6N 64.0W 45 KT 50 MPH 24H 12/0000Z 21.0N 65.4W 45 KT 50 MPH 36H 12/1200Z 22.3N 66.8W 50 KT 60 MPH 48H 13/0000Z 23.3N 68.1W 50 KT 60 MPH 72H 14/0000Z 26.0N 69.5W 55 KT 65 MPH 96H 15/0000Z 32.0N 69.0W 60 KT 70 MPH 120H 16/0000Z 39.0N 63.5W 60 KT 70 MPH $$ FORECASTER BEVEN |
