000 WTPZ43 KNHC 231457 TCDEP3 TROPICAL STORM MIRIAM DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012 800 AM PDT SUN SEP 23 2012 SATELLITE IMAGERY INDICATES THAT THE CLOUD PATTERN OF MIRIAM CONTINUES TO INCREASE IN ORGANIZATION. A COMPACT CENTRAL DENSE OVERCAST HAS FORMED AND BECOME BETTER DEFINED DURING THE PAST 6-12 HOURS. THERE IS ALSO PLENTY OF COLD-TOPPED CONVECTION ASSOCIATED WITH THE CYCLONE...AND BANDING HAS BEEN INCREASING. DVORAK T-NUMBERS HAVE INCREASED TO 3.5 FROM TAFB AND SAB...AND UW-CMSS ADT CI VALUES ARE AT 3.6. BASED ON THESE DATA...THE INITIAL INTENSITY IS RAISED TO 55 KT. THE INITIAL MOTION ESTIMATE IS 305/07. THE CYCLONE IS FORECAST TO MAINTAIN THIS GENERAL MOTION DURING THE NEXT DAY OR SO AS IT TURNS AROUND THE SOUTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE LOCATED OVER NORTHERN MEXICO. BY 48 HOURS...MIRIAM SHOULD ENCOUNTER A WEAKNESS ALONG 115W AS A LONGWAVE TROUGH DEVELOPS OVER THE WESTERN UNITED STATES. THIS PATTERN SHOULD RESULT IN A GRADUAL NORTHWARD TURN AND A SLOWING OF THE FORWARD MOTION DURING THE 3 TO 5 DAY PERIOD. THERE REMAINS A NOTABLE SPLIT IN THE GUIDANCE AFTER 72 HOURS...PARTIALLY DUE TO THE DEGREE OF INTERACTION OF MIRIAM WITH A CUT-OFF LOW FORECAST TO DEVELOP WEST OF THE CYCLONE IN ABOUT 4 DAYS. THE NHC FORECAST HAS SHIFTED TOWARD THE EAST...FOLLOWING THE MULTI-MODEL CONSENSUS TVCE. THE OFFICIAL FORECAST ALSO REMAINS ON THE RIGHT SIDE OF THE GUIDANCE ENVELOPE...BUT NOT AS FAR EAST AS THE ECMWF/GFS SOLUTIONS. MIRIAM SHOULD BE MOVING THROUGH AN ENVIRONMENT OF WARM SSTS...A VERY MOIST MID-TROPOSPHERE...AND LOW SHEAR DURING THE NEXT DAY OR SO. THE SHIPS RAPID INTENSIFICATION INDEX RESPONDS TO THESE FAVORABLE PARAMETERS BY SHOWING A 75% CHANCE OF A 30 KT INCREASE IN STRENGTH DURING THE NEXT 24 HOURS. BY 72 HOURS...MIRIAM SHOULD BE NORTH OF THE UPPER-LEVEL RIDGE AXIS AND EXPERIENCE AN INCREASE IN WESTERLY SHEAR ALONG WITH GRADUALLY DECREASING SSTS. WEAKENING IS EXPECTED AT THAT TIME...BUT THE RATE OF WEAKENING SEEMS DEPENDENT ON THE EXACT TRACK OF THE CYCLONE AS WELL AS ITS POTENTIAL INTERACTION WITH THE AFORMENTIONED CUT-OFF LOW. A TRACK FARTHER EAST COULD KEEP MIRIAM OVER WARMER WATER LONGER AND AWAY FROM EVEN STRONGER SHEAR CONDITIONS ASSOCIATED WITH THE CUT-OFF LOW. THE NHC INTENSITY FORECAST IS INCREASED A BIT IN THE SHORT TERM RELATIVE TO THE PREVIOUS ONE BUT IS CLOSE TO THE INTENSITY CONSENSUS AIDS. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 15.6N 109.6W 55 KT 65 MPH 12H 24/0000Z 16.5N 110.8W 70 KT 80 MPH 24H 24/1200Z 17.6N 112.4W 85 KT 100 MPH 36H 25/0000Z 18.5N 113.7W 85 KT 100 MPH 48H 25/1200Z 19.0N 114.5W 80 KT 90 MPH 72H 26/1200Z 20.2N 115.5W 70 KT 80 MPH 96H 27/1200Z 21.4N 116.0W 55 KT 65 MPH 120H 28/1200Z 22.5N 116.0W 40 KT 45 MPH $$ FORECASTER KIMBERLAIN