633 
 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