000 WTPZ43 KNHC 230843 TCDEP3 TROPICAL STORM MIRIAM DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012 200 AM PDT SUN SEP 23 2012 MIRIAM HAS CONTINUED TO BECOME BETTER ORGANIZED SINCE THE PREVIOUS ADVISORY...ESPECIALLY DURING THE PAST COUPLE OF HOURS. A SMALL CDO FEATURE HAS DEVELOPED OVER THE CENTER AND CONVECTIVE BANDING HAS ALSO BECOME MORE PRONOUNCED. SATELLITE INTENSITY ESTIMATES ARE A CONSENSUS T3.0/45 KT FROM TAFB AND SAB...AND RECENT UW-CIMSS ADT VALUES WERE T3.5/55 KT. BASED ON THESE DATA...THE INITIAL INTENSITY IS INCREASED TO 45 KT. UPPER-LEVEL OUTFLOW IS GOOD IN ALL QUADRANTS...AND CONTINUES TO EXPAND AND BECOME MORE SYMMETRICAL. MICROWAVE SATELLITE IMAGERY OVER THE PAST 6 HOURS INDICATES THE CENTER HAS DEVELOPED A LITTLE FARTHER TO THE NORTHEAST OF THE PREVIOUS ADVISORY TRACK...AND THE INITIAL MOTION ESTIMATE IS NOW 310/07 KT. MIRIAM IS EXPECTED TO MOVE NORTHWESTWARD AROUND A DEEP-LAYER SUBTROPICAL RIDGE FOR THE NEXT 72 HOURS. BY DAYS 4 AND 5...HOWEVER...THE TRACK FORECAST BECOMES A LITTLE MORE UNCERTAIN DUE TO A LARGE SPREAD IN THE MODEL GUIDANCE. THE NEW 23/00Z ECMWF MODEL MADE A MAJOR TRACK SHIFT TO THE EAST AND IS NOW THE RIGHTMOST OF ALL THE MODEL GUIDANCE...WHICH IS MORE IN LINE WITH THE PREVIOUS GFS MODEL RUNS. IN CONTRAST...THE HWRF AND GFDL MODELS HAVE CONTINUED THEIR MORE WESTWARD MOTION IN THE LATTER PERIODS. GIVEN THAT MIRIAM IS EXPECTED TO BECOME A FAIRLY ROBUST HURRICANE BY 72 HOURS...THE OFFICIAL FORECAST TRACK HAS BEEN SHIFTED TO THE RIGHT OF THE PREVIOUS ADVISORY TRACK...AND FOLLOWS THE TREND OF THE GFS AND ECMWF MODELS SINCE BOTH OF THOSE MODELS DEVELOP A DEEPER VORTEX THAT IS LIFTED NORTHWARD ON DAYS 4 AND 5 BY A SHORTWAVE TROUGH FORECAST TO DIG SOUTHWARD TO THE WEST OF BAJA CALIFORNIA. ENVIRONMENTAL CONDITIONS ARE FORECAST TO BE QUITE FAVORABLE FOR ADDITIONAL STRENGTHENING TO OCCUR FOR AT LEAST THE NEXT 48 HOURS. IN FACT...WITH VERTICAL SHEAR VALUES ARE EXPECTED TO REMAIN BELOW 5 KT FOR THE NEXT 36 HOURS...AND IT WOULD NOT SURPRISE ME IF RAPID INTENSIFICATION OCCURS AT SOME POINT DURING THAT TIME PERIOD. DUE TO MIRIAM FORECAST TO BE FARTHER EAST AND OVER WARMER WATERS THAN PREVIOUS EXPECTED ON DAYS 4 AND 5...THE INTENSITY WAS NUDGED UPWARD SLIGHTLY AT THOSE LATTER TIME PERIODS...AND MIGHT NEED TO BE INCREASED ON SUBSEQUENT ADVISORIES IF A FURTHER EASTWARD SHIFT IN THE FORECAST IS WARRANTED. THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO BUT SLIGHTLY HIGHER THAN THE HWRF AND FSSE MODELS... BOTH OF WHICH ARE ABOVE THE REMAINING NHC INTENSITY GUIDANCE. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 15.3N 108.8W 45 KT 50 MPH 12H 23/1800Z 16.2N 110.1W 55 KT 65 MPH 24H 24/0600Z 17.2N 111.7W 65 KT 75 MPH 36H 24/1800Z 18.1N 113.2W 75 KT 85 MPH 48H 25/0600Z 18.7N 114.3W 85 KT 100 MPH 72H 26/0600Z 19.8N 115.7W 75 KT 85 MPH 96H 27/0600Z 20.8N 116.5W 65 KT 75 MPH 120H 28/0600Z 21.8N 117.0W 55 KT 65 MPH $$ FORECASTER STEWART