000 WTPZ43 KNHC 222034 TCDEP3 TROPICAL STORM MIRIAM DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012 200 PM PDT SAT SEP 22 2012 THE SATELLITE PRESETNATION OF MIRIAM HAS IMPROVED OVER THE PAST FEW HOURS. THERE ARE NOW SEVERAL CURVED CONVECTIVE BANDS FEEDING INTO AN AREA OF COOLING CLOUD TOPS NEAR THE CENTER. HOWEVER...AN ASCAT PASS FROM AROUND 1730 UTC SUGGESTED THAT MIRIAM WAS NOT AS STRONG AS EARLIER SATELLITE ESTIMATES INDICATED...SHOWING ONLY A COUPLE OF 30 KT WIND VECTORS SOUTHEAST OF THE CENTER. BASED ON THE IMPROVING SATELLITE PRESENTATION...THE INITIAL INTENSITY IS HELD AT 35 KT. MIRIAM APPEARS POISED FOR STRENGTHENING...WITH SOME SIGNS OF A DEVELOPING INNER CORE AND A TRACK OVER VERY WARM WATERS IN A LOW SHEAR ENVIRONMENT FOR THE NEXT 24 TO 36 HOURS. IN FACT...THE SHIPS RAPID INTENSIFICATION INDEX SUGGESTS THAN A 25-KT INCREASE IN THE NEXT 24 HOURS IS LIKELY AND A 30-KT INCREASE IS ABOUT A 50/50 PROPOSITION. THE OFFICIAL INTENSITY FORECAST HAS BEEN INCREASED IN THE SHORT RANGE AND IS A LITTLE HIGHER THAN THE SHIPS MODEL AND CLOSE TO THE HWRF THROUGH 48 HOURS. AFTER THAT TIME...MIRIAM WILL BE MOVING INTO COOLER WATERS AND EXPERIENCE AN INCREASE IN SOUTHWESTERLY SHEAR...WHICH SHOULD LEAD TO STEADY WEAKENING BY THE END OF THE PERIOD. THE AFOREMENTIONED ASCAT PASS AND VISIBLE IMAGERY SUGGEST THAT THE LOW-LEVEL CENTER HAS REFORMED TO THE NORTHEAST OF THE POSITION FROM THE LAST ADVISORY. DUE TO THE RELOCATION...THE INITIAL MOTION ESTIMATE IS A HIGHLY-UNCERTAIN 300/08. OVER THE NEXT COUPLE OF DAYS MIRIAM SHOULD MOVE NORTHWESTWARD ON THE SOUTHWEST SIDE OF A SUBTROPICAL RIDGE THAT EXTENDS FROM NORTHERN MEXICO INTO THE EASTERN PACIFIC. THROUGH 48 HOURS THE NEW NHC FORECAST IS ABOUT 40-50 N MI TO THE RIGHT OF THE OLD ONE DUE TO THE REPOSITIONING OF THE CENTER AND AN ADJUSTMENT HALFWAY TOWARD THE NEW TVCE MULTI- MODEL CONSENSUS. AT DAYS 3 THROUGH 5 AN AMPLIFYING MID/UPPER-LEVEL TROUGH MOVING INTO THE WESTERN UNITED STATES WILL WEAKEN THE RIDGE...BUT THE TRACK MODEL GUIDANCE BECOMES HIGHLY DIVERGENT AS TO HOW MUCH LATITUDE MIRIAM WILL GAIN. AT ONE EXTREME...THE GFDL AND GFS SHOW A MOTION EAST OF DUE NORTH AT THE END OF THE PERIOD AS MIRIAM INTERACTS WITH A SHORTWAVE TROUGH NEAR 30N/130W BY DAY 5. ON THE OTHER EXTREME THE ECMWF AND HWRF SHOW MIRIAM TURNING WESTWARD AND BYPASSING THE SHORTWAVE. SINCE MIRIAM IS EXPECTED TO BE A DEEP CYCLONE IT SHOULD GAIN AT LEAST SOME LATITUDE IN RESPONSE TO THE BREAK IN THE RIDGE. GIVEN THIS REASONING...AND A DESIRE TO MAINTAIN CONTINUITY DUE TO THE INCREASING MODEL SPREAD...THE NHC TRACK AT DAYS 3 THROUGH 5 SHOWS A SLOW NORTHWESTWARD MOTION. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 14.3N 107.8W 35 KT 40 MPH 12H 23/0600Z 14.8N 108.5W 45 KT 50 MPH 24H 23/1800Z 16.1N 110.2W 60 KT 70 MPH 36H 24/0600Z 17.2N 111.9W 75 KT 85 MPH 48H 24/1800Z 18.2N 113.3W 80 KT 90 MPH 72H 25/1800Z 19.3N 115.2W 75 KT 85 MPH 96H 26/1800Z 20.2N 116.1W 60 KT 70 MPH 120H 27/1800Z 21.5N 117.0W 50 KT 60 MPH $$ FORECASTER BRENNAN