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Tropical Storm MIRIAM Forecast Discussion Number 7

Issued at 800 AM PDT SUN SEP 23 2012

000
WTPZ43 KNHC 231457
TCDEP3
 
TROPICAL STORM MIRIAM DISCUSSION NUMBER   7
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132012
800 AM PDT SUN SEP 23 2012
 
SATELLITE IMAGERY INDICATES THAT THE CLOUD PATTERN OF MIRIAM
CONTINUES TO INCREASE IN ORGANIZATION.  A COMPACT CENTRAL DENSE
OVERCAST HAS FORMED AND BECOME BETTER DEFINED DURING THE PAST 6-12
HOURS.  THERE IS ALSO PLENTY OF COLD-TOPPED CONVECTION ASSOCIATED
WITH THE CYCLONE...AND BANDING HAS BEEN INCREASING.  DVORAK
T-NUMBERS HAVE INCREASED TO 3.5 FROM TAFB AND SAB...AND UW-CMSS ADT
CI VALUES ARE AT 3.6.  BASED ON THESE DATA...THE INITIAL INTENSITY
IS RAISED TO 55 KT.
 
THE INITIAL MOTION ESTIMATE IS 305/07.  THE CYCLONE IS FORECAST TO
MAINTAIN THIS GENERAL MOTION DURING THE NEXT DAY OR SO AS IT TURNS
AROUND THE SOUTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE LOCATED
OVER NORTHERN MEXICO.  BY 48 HOURS...MIRIAM SHOULD ENCOUNTER A
WEAKNESS ALONG 115W AS A LONGWAVE TROUGH DEVELOPS OVER THE WESTERN
UNITED STATES.  THIS PATTERN SHOULD RESULT IN A GRADUAL NORTHWARD
TURN AND A SLOWING OF THE FORWARD MOTION DURING THE 3 TO 5 DAY
PERIOD.  THERE REMAINS A NOTABLE SPLIT IN THE GUIDANCE AFTER
72 HOURS...PARTIALLY DUE TO THE DEGREE OF INTERACTION OF MIRIAM
WITH A CUT-OFF LOW FORECAST TO DEVELOP WEST OF THE CYCLONE IN ABOUT
4 DAYS.  THE NHC FORECAST HAS SHIFTED TOWARD THE EAST...FOLLOWING
THE MULTI-MODEL CONSENSUS TVCE.  THE OFFICIAL FORECAST ALSO REMAINS
ON THE RIGHT SIDE OF THE GUIDANCE ENVELOPE...BUT NOT AS FAR EAST AS
THE ECMWF/GFS SOLUTIONS.
 
MIRIAM SHOULD BE MOVING THROUGH AN ENVIRONMENT OF WARM SSTS...A VERY
MOIST MID-TROPOSPHERE...AND LOW SHEAR DURING THE NEXT DAY OR SO. 
THE SHIPS RAPID INTENSIFICATION INDEX RESPONDS TO THESE FAVORABLE
PARAMETERS BY SHOWING A 75% CHANCE OF A 30 KT INCREASE IN STRENGTH
DURING THE NEXT 24 HOURS. BY 72 HOURS...MIRIAM SHOULD BE NORTH OF
THE UPPER-LEVEL RIDGE AXIS AND EXPERIENCE AN INCREASE IN WESTERLY
SHEAR ALONG WITH GRADUALLY DECREASING SSTS.  WEAKENING IS EXPECTED
AT THAT TIME...BUT THE RATE OF WEAKENING SEEMS DEPENDENT ON THE
EXACT TRACK OF THE CYCLONE AS WELL AS ITS POTENTIAL INTERACTION
WITH THE AFORMENTIONED CUT-OFF LOW. A TRACK FARTHER EAST COULD KEEP
MIRIAM OVER WARMER WATER LONGER AND AWAY FROM EVEN STRONGER SHEAR
CONDITIONS ASSOCIATED WITH THE CUT-OFF LOW. THE NHC INTENSITY
FORECAST IS INCREASED A BIT IN THE SHORT TERM RELATIVE TO THE
PREVIOUS ONE BUT IS CLOSE TO THE INTENSITY CONSENSUS AIDS.
  
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/1500Z 15.6N 109.6W   55 KT  65 MPH
 12H  24/0000Z 16.5N 110.8W   70 KT  80 MPH
 24H  24/1200Z 17.6N 112.4W   85 KT 100 MPH
 36H  25/0000Z 18.5N 113.7W   85 KT 100 MPH
 48H  25/1200Z 19.0N 114.5W   80 KT  90 MPH
 72H  26/1200Z 20.2N 115.5W   70 KT  80 MPH
 96H  27/1200Z 21.4N 116.0W   55 KT  65 MPH
120H  28/1200Z 22.5N 116.0W   40 KT  45 MPH
 
$$
FORECASTER KIMBERLAIN
 

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