Hurricane MIRIAM

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HURRICANE MIRIAM DISCUSSION NUMBER  16
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132012
200 PM PDT TUE SEP 25 2012
 
THE CLOUD PATTERN OF MIRIAM HAS DETERIORATED SIGNIFICANTLY DURING
THE PAST FEW HOURS. THE COVERAGE AND INTENSITY OF DEEP CONVECTION
HAS BEEN DECREASING...WHILE THE CENTRAL FEATURES OF THE CYCLONE
HAVE BECOME LESS DISTINCT.  UPPER-LEVEL WESTERLY FLOW ASSOCIATED
WITH A DEVELOPING MID- TO UPPER-LEVEL LOW SEEN IN WATER VAPOR
IMAGERY NEAR 26N111W IS ALREADY IMPINGING ON MIRIAM...WITH SHIPS
AND UW-CIMSS SHEAR ANALYSES CURRENTLY DIAGNOSING 20 KT OF
SOUTHWESTERLY SHEAR. DVORAK T-NUMBERS ARE AT 4.0 AND 4.5 FROM SAB
AND TAFB...WHILE ADT CI VALUES ARE RUNNING A BIT HIGHER.  THE
INITIAL INTENSITY ESTIMATE IS LOWERED TO 75 KT IN AGREEMENT WITH
THESE DATA.
 
THE ABOVEMENTIONED UPPER-LEVEL TROUGH APPROACHING MIRIAM WILL
PRODUCE PERSISTENT MODERATE TO STRONG SOUTHWESTERLY SHEAR OVER THE
CYCLONE AS IT MOVES OVER GRADUALLY DECREASING SSTS.  AS A RESULT...
STEADY WEAKENING IS EXPECTED DURING THE NEXT 48 HOURS.  AS THE SHEAR
BECOMES MORE EXTREME BY DAY 3...THE GFS AND ECMWF DEPICT MIRIAM
DECOUPLING AND LIKELY DISSIPATING BY DAY 5.  THE NHC INTENSITY
FORECAST IS REDUCED AGAIN COMPARED TO THE PREVIOUS ONE IN BEST
AGREEMENT WITH THE SHIPS AND LGEM OUTPUT.  GIVEN THE GOOD MODEL
AGREEMENT...REMNANT LOW STATUS IS NOW INDICATED SOONER...ON DAY 3.
 
THE INITIAL MOTION ESTIMATE IS 300/04.  MIRIAM IS ABOUT TO TURN
TOWARD THE NORTHWEST AROUND THE WESTERN EDGE OF A MID-LEVEL RIDGE
EXTENDING FROM THE GULF OF MEXICO.  A SLOW NORTH-NORTHWESTWARD
MOTION SHOULD COMMENCE BY TOMORROW AND CONTINUE FOR THE NEXT COUPLE
OF DAYS AS THE CYCLONE MOVES INTO A WEAKNESS ALONG 115W.  THE MODEL
GUIDANCE HAS SHIFTED WESTWARD THIS CYCLE IN THE SHORT TERM...
PARTIALLY IN RESPONSE TO THE MORE WESTERLY INITIAL MOTION THAT HAS
PERSISTED LONGER THAN EXPECTED.  SINCE IT APPEARS THAT MIRIAM WILL
BECOME A SHALLOW VORTEX SOONER THAN PREVIOUSLY EXPECTED...THE NHC
TRACK HAS BEEN SHIFTED SOMEWHAT TO THE WEST OF THE PREVIOUS ONE.
  
FORECAST POSITIONS AND MAX WINDS
 
INIT  25/2100Z 18.9N 114.9W   75 KT  85 MPH
 12H  26/0600Z 19.6N 115.1W   65 KT  75 MPH
 24H  26/1800Z 20.7N 115.4W   55 KT  65 MPH
 36H  27/0600Z 21.9N 115.8W   45 KT  50 MPH
 48H  27/1800Z 23.0N 116.0W   40 KT  45 MPH
 72H  28/1800Z 24.2N 116.1W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  29/1800Z 25.5N 116.1W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  30/1800Z...DISSIPATED
 
$$
FORECASTER KIMBERLAIN
 
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