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Tropical Storm MIRIAM Forecast Discussion Number 8

Issued at 200 PM PDT SUN SEP 23 2012

000
WTPZ43 KNHC 232112
TCDEP3
 
TROPICAL STORM MIRIAM DISCUSSION NUMBER   8
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132012
200 PM PDT SUN SEP 23 2012
 
MIRIAM APPEARS TO BE GRADUALLY STRENGTHENING.  THE CYCLONE IS
MAINTAINING A CENTRAL DENSE OVERCAST WITH COLD CLOUD TOP
TEMPEATURES. A 1554 UTC SSMIS PASS SHOWED A DEVELOPING MID-LEVEL
EYE...AND A CLOSED LOW-LEVEL RING OF CONVECTION WAS EVIDENT IN AN
EARLIER WINDSAT PASS.  IN ADDITION...TIGHTLY COILED BANDS DOMINATE
THE OUTER CIRCULATION OF THE CYCLONE.  DVORAK T-NUMBERS ARE AT 3.5
AND 4.0 FROM TAFB AND SAB...RESPECTIVELY...AND UW-CIMSS ADT CI
VALUES ARE AT 4.4.  BASED ON A BLEND OF THE ABOVE DATA...THE
INITIAL INTENSITY ESTIMATE IS RAISED TO 60 KT.
 
SATELLITE AND MICROWAVE DATA SUGGEST THAT THE HEADING HAS BEEN
SLIGHTLY TO THE LEFT OF THE PREVIOUS ESTIMATE. A LONGER-TERM
AVERAGE OF FIXES SUGGESTS AN INITIAL MOTION ESTIMATE OF 300/08.
GLOBAL MODELS SHOW A CONTINUING WEST-NORTHWEST TO NORTHWEST MOTION
AS MIRIAM ROUNDS THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL RIDGE
OVER NORTHERN MEXICO DURING THE NEXT 36 HOURS. A GRADUAL TURN
TOWARD THE NORTH-NORTHWEST AND NORTH ARE EXPECTED FROM 48-96 HOURS
AS MIRIAM MOVES INTO A WEAKNESS ALONG 115W...CREATED BY A SHORTWAVE
TROUGH PASSING THROUGH THE WESTERN UNITED STATES. THE LATTER PART
OF THE FORECAST PARTIALLY HINGES ON THE DEGREE OF INTERACTION WITH
A MID- TO UPPER-LEVEL TROUGH FORECAST TO FORM WEST OR NORTHWEST OF
MIRIAM. THE SPREAD IN THE GUIDANCE IS STILL RATHER SIGNIFICANT...
ESPECIALLY BEYOND 96 HOURS...BUT LITTLE CHANGE IS NOTED BETWEEN
THIS CYCLE AND THE PREVIOUS ONE. THE NHC TRACK FORECAST IS
ESSENTIALLY AN UPDATE OF THE PREVIOUS ONE...AND IS ON THE RIGHT
SIDE OF THE GUIDANCE ENVELOPE...CLOSEST TO THE ECMWF/GFS SOLUTIONS.
 
MIRIAM IS EXPECTED TO REMAIN IN A FAVORABLE ENVIROMENT FOR
STRENGTHENING DURING THE NEXT 24 HOURS...AND THE RATE OF
INTENSIFICATION COULD POTENTIALLY BE RAPID AS INDICATED BY THE
SHIPS RI INDEX.  IN 2-3 DAYS...MIRIAM SHOULD CROSS THE UPPER-LEVEL
RIDGE AXIS AND SHOULD ALSO INTERACT WITH A SHORTWAVE TROUGH DIVING
SOUTHEASTWARD TOWARD THE CYCLONE.  AN INCREASE IN SHEAR RELATED TO
THE ABOVE FACTORS AND GRADUALLY DECREASING SSTS SHOULD PROMOTE
WEAKENING.  THE WEAKENING SHOULD CONTINUE DURING THE 3 TO 5 DAY
PERIOD...WITH EVEN THE POSSIBILITY OF MIRIAM DECOUPLING ENTIRELY AS
DEPICTED BY ECMWF. A CRITICAL ELEMENT OF THE INTENSITY FORECAST
COULD BE THE LONGITUDE WHERE MIRIAM TURNS NORTHWARD...SINCE THE
EXACT TRACK OF THE CYCLONE WITH RESPECT TO A STRONG GRADIENT IN
SSTS COULD DETERMINE HOW QUICKLY THE CYCLONE WEAKENS.  THE NHC
INTENSITY FORECAST IS BASICALLY AN UPDATE OF THE PREVIOUS AND IS IN
GOOD AGREEMENT WITH THE LATEST INTENSITY CONSENSUS AIDS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  23/2100Z 16.1N 110.5W   60 KT  70 MPH
 12H  24/0600Z 16.8N 111.8W   75 KT  85 MPH
 24H  24/1800Z 17.9N 113.4W   85 KT 100 MPH
 36H  25/0600Z 18.7N 114.5W   85 KT 100 MPH
 48H  25/1800Z 19.4N 115.2W   80 KT  90 MPH
 72H  26/1800Z 20.7N 115.9W   60 KT  70 MPH
 96H  27/1800Z 22.3N 116.0W   45 KT  50 MPH
120H  28/1800Z 23.5N 116.0W   35 KT  40 MPH
 
$$
FORECASTER KIMBERLAIN
 

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