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Hurricane MIRIAM Forecast Discussion Number 15

Issued at 800 AM PDT TUE SEP 25 2012

000
WTPZ43 KNHC 251452
TCDEP3
 
HURRICANE MIRIAM DISCUSSION NUMBER  15
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132012
800 AM PDT TUE SEP 25 2012
 
THE OVERALL SATELLITE APPEARANCE OF MIRIAM HAS NOT CHANGED MUCH
DURING THE LAST 6 HOURS. THE CLOUD PATTERN REMAINS WELL
ORGANIZED...CONSISTING OF A CIRCULAR CENTRAL DENSE OVERCAST
COMPRISED OF PLENTY OF OF DEEP CONVECTION. SATELLITE DATA SUGGEST
THAT AN EYEWALL REPLACEMENT CYCLE IS STILL ONGOING...WITH A
FRAGMENT OF THE INNER EYEWALL LIKELY REMAINING AND AN OUTER EYEWALL
AT ABOUT 30 N MI RADIUS GRADUALLY CONTRACTING. DVORAK T- AND
CI-NUMBERS ARE 4.5 FROM TAFB AND SAB...BUT UW-CIMSS ADT CI VALUES
ARE AT 5.6. THE INITIAL INTENSITY IS LOWERED TO 85 KT...BASED ON A
BLEND OF THE ABOVE DATA.
 
THE FORWARD SPEED OF MIRIAM HAS BEEN SLOWING DURING THE LAST 12-24
HOURS.  A LONGER-TERM AVERAGE OF CENTER FIXES GIVES AN INITIAL
MOTION ESTIMATE OF 300/04. MIRIAM SHOULD TURN NORTH-NORTHWESTWARD
SOON AS IT MOVES AROUND THE WESTERN SIDE OF A MID-LEVEL RIDGE
CENTERED OVER THE GULF OF MEXICO. A SLOW NORTHWARD MOTION IS
EXPECTED BY 36 HOURS AS THE CYCLONE MOVES INTO A WEAKNESS ALONG
115W. A MID- TO UPPER-LEVEL CLOSED LOW FORMING OFFSHORE OF THE
SOUTHERN CALIFORNIA COAST IN 2-3 DAYS COULD ALLOW FOR A NORTH-
NORTHEAST MOTION BY DAY 3. THE NHC TRACK FORECAST IS VIRTUALLY
UNCHANGED FROM THE PREVIOUS ONE...CLOSE TO THE MULTI-MODEL
CONSENSUS TVCE THROUGH 36 HOURS AND IN BETWEEN THE ECMWF AND GFS AT
LATER TIMES.
 
THE EYEWALL REPLACEMENT CYCLE WILL NOT LIKELY COMPLETE ITSELF BEFORE
MIRIAM MOVES OVER MARGINAL SSTS AND INTO AN ENVIRONMENT OF
INCREASING WESTERLY SHEAR IN ABOUT 12-24 HOURS.  DURING THE NEXT
COUPLE OF DAYS...A PREVAILING MODERATE TO STRONG SOUTHWESTERLY
SHEAR AND SLOWLY DECREASING SSTS SHOULD RESULT IN A STEADY
WEAKENING OF THE CYCLONE.  A FURTHER INCREASE IN SOUTHWESTERLY
SHEAR ASSOCIATED WITH THE AFOREMENTIONED CLOSED LOW TO THE
NORTHWEST OF MIRIAM SHOULD OCCUR AFTER 72 HOURS.  RAPID WEAKENING
IS FORECAST AFTER THAT TIME AND MIRIAM COULD DECOUPLE ENTIRELY...AS
DEPICTED IN BOTH THE GFS/ECMWF MODEL SOLUTIONS.  THE NHC INTENSITY
FORECAST IS LOWER COMPARED TO THE PREVIOUS ONE...BUT NOT AS LOW AS
THE SHIPS... LGEM AND FSSE OUTPUT SUGGESTS.
  
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  25/1500Z 18.7N 114.5W   85 KT 100 MPH
 12H  26/0000Z 19.3N 114.7W   80 KT  90 MPH
 24H  26/1200Z 20.2N 115.0W   70 KT  80 MPH
 36H  27/0000Z 21.2N 115.2W   60 KT  70 MPH
 48H  27/1200Z 22.2N 115.4W   50 KT  60 MPH
 72H  28/1200Z 23.9N 115.3W   40 KT  45 MPH
 96H  29/1200Z 25.3N 115.2W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  30/1200Z 26.7N 115.1W   20 KT  25 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER KIMBERLAIN
 

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