000 WTPZ43 KNHC 232112 TCDEP3 TROPICAL STORM MIRIAM DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012 200 PM PDT SUN SEP 23 2012 MIRIAM APPEARS TO BE GRADUALLY STRENGTHENING. THE CYCLONE IS MAINTAINING A CENTRAL DENSE OVERCAST WITH COLD CLOUD TOP TEMPEATURES. A 1554 UTC SSMIS PASS SHOWED A DEVELOPING MID-LEVEL EYE...AND A CLOSED LOW-LEVEL RING OF CONVECTION WAS EVIDENT IN AN EARLIER WINDSAT PASS. IN ADDITION...TIGHTLY COILED BANDS DOMINATE THE OUTER CIRCULATION OF THE CYCLONE. DVORAK T-NUMBERS ARE AT 3.5 AND 4.0 FROM TAFB AND SAB...RESPECTIVELY...AND UW-CIMSS ADT CI VALUES ARE AT 4.4. BASED ON A BLEND OF THE ABOVE DATA...THE INITIAL INTENSITY ESTIMATE IS RAISED TO 60 KT. SATELLITE AND MICROWAVE DATA SUGGEST THAT THE HEADING HAS BEEN SLIGHTLY TO THE LEFT OF THE PREVIOUS ESTIMATE. A LONGER-TERM AVERAGE OF FIXES SUGGESTS AN INITIAL MOTION ESTIMATE OF 300/08. GLOBAL MODELS SHOW A CONTINUING WEST-NORTHWEST TO NORTHWEST MOTION AS MIRIAM ROUNDS THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL RIDGE OVER NORTHERN MEXICO DURING THE NEXT 36 HOURS. A GRADUAL TURN TOWARD THE NORTH-NORTHWEST AND NORTH ARE EXPECTED FROM 48-96 HOURS AS MIRIAM MOVES INTO A WEAKNESS ALONG 115W...CREATED BY A SHORTWAVE TROUGH PASSING THROUGH THE WESTERN UNITED STATES. THE LATTER PART OF THE FORECAST PARTIALLY HINGES ON THE DEGREE OF INTERACTION WITH A MID- TO UPPER-LEVEL TROUGH FORECAST TO FORM WEST OR NORTHWEST OF MIRIAM. THE SPREAD IN THE GUIDANCE IS STILL RATHER SIGNIFICANT... ESPECIALLY BEYOND 96 HOURS...BUT LITTLE CHANGE IS NOTED BETWEEN THIS CYCLE AND THE PREVIOUS ONE. THE NHC TRACK FORECAST IS ESSENTIALLY AN UPDATE OF THE PREVIOUS ONE...AND IS ON THE RIGHT SIDE OF THE GUIDANCE ENVELOPE...CLOSEST TO THE ECMWF/GFS SOLUTIONS. MIRIAM IS EXPECTED TO REMAIN IN A FAVORABLE ENVIROMENT FOR STRENGTHENING DURING THE NEXT 24 HOURS...AND THE RATE OF INTENSIFICATION COULD POTENTIALLY BE RAPID AS INDICATED BY THE SHIPS RI INDEX. IN 2-3 DAYS...MIRIAM SHOULD CROSS THE UPPER-LEVEL RIDGE AXIS AND SHOULD ALSO INTERACT WITH A SHORTWAVE TROUGH DIVING SOUTHEASTWARD TOWARD THE CYCLONE. AN INCREASE IN SHEAR RELATED TO THE ABOVE FACTORS AND GRADUALLY DECREASING SSTS SHOULD PROMOTE WEAKENING. THE WEAKENING SHOULD CONTINUE DURING THE 3 TO 5 DAY PERIOD...WITH EVEN THE POSSIBILITY OF MIRIAM DECOUPLING ENTIRELY AS DEPICTED BY ECMWF. A CRITICAL ELEMENT OF THE INTENSITY FORECAST COULD BE THE LONGITUDE WHERE MIRIAM TURNS NORTHWARD...SINCE THE EXACT TRACK OF THE CYCLONE WITH RESPECT TO A STRONG GRADIENT IN SSTS COULD DETERMINE HOW QUICKLY THE CYCLONE WEAKENS. THE NHC INTENSITY FORECAST IS BASICALLY AN UPDATE OF THE PREVIOUS AND IS IN GOOD AGREEMENT WITH THE LATEST INTENSITY CONSENSUS AIDS. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 16.1N 110.5W 60 KT 70 MPH 12H 24/0600Z 16.8N 111.8W 75 KT 85 MPH 24H 24/1800Z 17.9N 113.4W 85 KT 100 MPH 36H 25/0600Z 18.7N 114.5W 85 KT 100 MPH 48H 25/1800Z 19.4N 115.2W 80 KT 90 MPH 72H 26/1800Z 20.7N 115.9W 60 KT 70 MPH 96H 27/1800Z 22.3N 116.0W 45 KT 50 MPH 120H 28/1800Z 23.5N 116.0W 35 KT 40 MPH $$ FORECASTER KIMBERLAIN