000 WTPZ43 KNHC 250237 TCDEP3 HURRICANE MIRIAM DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL EP132012 800 PM PDT MON SEP 24 2012 MIRIAM HAS WEAKENED OVER THE LAST SEVERAL HOURS. THE EYE HAS BECOME OVERCAST AND CLOUD TOPS HAVE WARMED NEAR THE CENTER RECENTLY. MICROWAVE IMAGES INDICATE THAT THE INNER EYEWALL...WHICH WAS EVIDENT IN A TRMM PASS AROUND 17Z...HAS COLLAPSED. IN ADDITION... THE LATEST SSMI OVERPASS SHOWED AN ASYMMETRIC STRUCTURE...WHICH IS LIKELY THE RESULT OF SOUTHWESTERLY SHEAR. SATELLITE INTENSITY ESTIMATES HAVE DECREASED THIS CYCLE...AND THE INITIAL WIND SPEED IS LOWERED TO 90 KT BASED ON A BLEND OF THE DVORAK T- AND CI-NUMBERS FROM TAFB...SAB...AND UW-CIMSS. A GRADUAL WEAKENING IS FORECAST DURING THE NEXT 24 TO 36 HOURS AS SOUTHWESTERLY SHEAR...ASSOCIATED WITH A TROUGH TO THE NORTH-NORTHWEST OF MIRIAM...INCREASES A LITTLE. LATER IN THE WEEK...THE GLOBAL MODELS SHOW A LARGE MID- TO UPPER-LEVEL LOW DEVELOPING NEAR 30N 130W. THIS LOW WILL CAUSE A FURTHER INCREASE IN SOUTHWESTERLY SHEAR. IN FACT...VERTICAL CROSS SECTIONS FROM THE GFS AND ECMWF GLOBAL MODELS SHOW A TITLED VORTEX THAT IS BEING AFFECTED BY A DEEP LAYER OF DRY AIR AND WESTERLY FLOW IN 4 TO 5 DAYS. THE OFFICIAL FORECAST IS A LITTLE LOWER THAN THE PREVIOUS ONE...BUT SLIGHTLY ABOVE THE CONSENSUS AIDS. MIRIAM CONTINUES MOVING NORTHWESTWARD BUT AT A SLIGHTLY SLOWER FORWARD SPEED. A GRADUAL TURN TO THE NORTH IS EXPECTED DURING THE NEXT 48 TO 72 HOURS AS MIRIAM MOVES TOWARD A TROUGH OVER THE WESTERN UNITED STATES. BEYOND THAT TIME...A NORTH-NORTHEASTWARD MOTION IS FORECAST AS MIRIAM BECOMES EMBEDDED IN THE FLOW TO THE EAST OF THE AFOREMENTIONED MID- TO UPPER-LEVEL LOW. THE OFFICIAL TRACK FORECAST IS FAIRLY CLOSE TO THE PREVIOUS ONE AND REMAINS BETWEEN THE GFS AND ECMWF MODELS. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 18.5N 113.9W 90 KT 105 MPH 12H 25/1200Z 19.0N 114.5W 85 KT 100 MPH 24H 26/0000Z 19.8N 114.9W 80 KT 90 MPH 36H 26/1200Z 20.7N 115.1W 75 KT 85 MPH 48H 27/0000Z 21.8N 115.2W 65 KT 75 MPH 72H 28/0000Z 23.7N 115.4W 50 KT 60 MPH 96H 29/0000Z 25.5N 115.0W 40 KT 45 MPH 120H 30/0000Z 27.0N 114.5W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ FORECASTER CANGIALOSI