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Hurricane MIRIAM Forecast Discussion Number 13

Issued at 800 PM PDT MON SEP 24 2012

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
 

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