000 WTPZ43 KNHC 242040 TCDEP3 HURRICANE MIRIAM DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012 200 PM PDT MON SEP 24 2012 THE INTENSIFICATION PHASE THAT MIRIAM HAS BEEN UNDERGOING HAS COME TO AN END...AS THERE HAS BEEN A DEGRADATION IN THE SATELLITE APPEARANCE OF THE CYCLONE IN THE LAST FEW HOURS. CLOUD TOP TEMPERATURES HAVE GRADUALLY BEEN WARMING AND THE EYE HAS BECOME LESS DISTINCT. A 1640 UTC TRMM IMAGE INDICATED THAT AN EYEWALL REPLACEMENT COULD BE UNDERWAY...WITH A SMALL INNER EYEWALL EVIDENT AND AN OUTER EYEWALL HAVING CONTRACTED SLIGHTLY TO A RADIUS OF 50 N MI. DVORAK T-NUMBERS HAVE LEVELED OFF AT 5.5 FROM BOTH SATELLITE AGENCIES...AND UW-CIMSS ADT CI VALUES HAVE DECREASED TO 5.6. THE INITIAL INTENSITY ESTIMATE IS HELD AT 105 KT...IN AGREEMENT WITH THESE DATA. THE EYE OF MIRIAM HAS WOBBLED A BIT TO THE LEFT DURING THE LAST FEW HOURS...AND HAS SLOWED IN FORWARD SPEED. A LONGER-TERM AVERAGE OF CENTER FIXES GIVES AN INITIAL MOTION ESTIMATE OF 305/08. MIRIAM SHOULD CONTINUE ON A SLOWER NORTHWESTWARD COURSE FOR THE NEXT DAY OR SO AS IT ROUNDS THE SOUTHWESTERN SIDE OF A MID-LEVEL RIDGE OVER NORTHERN MEXICO. AFTER THAT TIME...MIRIAM SHOULD MOVE INTO A WEAKNESS ALONG 115W AND TURN NORTHWARD FOR A COUPLE OF DAYS. THE INFLUENCE OF A MID- TO UPPER-LEVEL LOW FORMING OFFSHORE OF THE SOUTHERN CALIFORNIA COAST FROM DAYS 3-5 SHOULD PUSH THE TRACK OF MIRIAM NORTH-NORTHEASTWARD...OR EVEN NORTHEASTWARD...DURING THAT TIME. NO SUBSTANTIVE CHANGES WERE MADE TO THE PREVIOUS FORECAST...WHICH IS WEIGHTED HEAVILY ON A BLEND OF THE ECMWF AND GFS MODELS. AS IN PREVIOUS OFFICIAL FORECASTS...THIS IS NEAR THE RIGHT SIDE OF THE GUIDANCE ENVELOPE. IT IS UNCLEAR AS TO WHETHER THE CURRENT EYEWALL REPLACEMENT WILL CONTINUE. IN ANY EVENT...A COMPLETION OF THE EYEWALL REPLACEMENT CYCLE IS NOT LIKELY TO OCCUR BEFORE MIRIAM REACHES MARGINALLY WARM WATERS AND SOME INCREASE IN WESTERLY SHEAR IN ABOUT 24 HOURS. WEAKENING IS EXPECTED DURING THE INTERIM AND AFTERWARDS AS THE CYCLONE MOVES CLOSE TO A STRONG GRADIENT OF SSTS AND EXPERIENCES MODERATE WESTERLY SHEAR. A FURTHER INCREASE IN WESTERLY SHEAR ASSOCIATED THE ABOVEMENTIONED CLOSED MID- TO UPPER-LEVEL CYCLONE TO THE WEST OF MIRIAM WILL LIKELY OCCUR LATE IN THE PERIOD. THERE IS NOW A GENERAL AGREEMENT OF BOTH THE ECWMF AND GFS GUIDANCE THAT THIS WILL RESULT IN A DECOUPLING OF THE CYCLONE AND A RAPID WEAKENING. THE NHC INTENSITY FORECAST HAS BEEN REDUCED RELATIVE TO THE PREVIOUS ONE...REPRESENTING AN AVERAGE OF THE SHIPS/LGEM AND THE LATEST INTENSITY CONSENSUS AIDS. REMNANT LOW STATUS IN INDICATED ON DAY 5. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 18.1N 113.5W 105 KT 120 MPH 12H 25/0600Z 18.5N 114.2W 100 KT 115 MPH 24H 25/1800Z 19.3N 114.8W 90 KT 105 MPH 36H 26/0600Z 20.1N 115.0W 80 KT 90 MPH 48H 26/1800Z 21.1N 115.1W 70 KT 80 MPH 72H 27/1800Z 23.4N 115.2W 55 KT 65 MPH 96H 28/1800Z 25.5N 114.4W 40 KT 45 MPH 120H 29/1800Z 27.5N 113.5W 30 KT 35 MPH $$ FORECASTER KIMBERLAIN