553 
 WTNT44 KNHC 281502
 TCDAT4
  
 TROPICAL STORM DON DISCUSSION NUMBER   4
 NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042011
 1000 AM CDT THU JUL 28 2011
 
 THE CENTER OF DON REMAINS PARTLY EXPOSED ON THE NORTH SIDE OF A
 PERSISTENT AREA OF CONVECTION THIS MORNING.  AN AIR FORCE RESERVE
 HURRICANE HUNTER AIRCRAFT HAS TWICE EXTRAPOLATED A MINIMUM PRESSURE
 OF 1001 MB.  THE AIRCRAFT HAS ALSO REPORTED 850 MB FLIGHT-LEVEL
 WINDS OF 52 KT AND RELIABLE-LOOKING WINDS OF 35-40 KT FROM THE
 SFMR.  BASED ON THIS...THE INITIAL INTENSITY IS INCREASED TO 40 KT.
 
 THE INITIAL MOTION IS NOW 305/12.  DON IS TO THE SOUTH AND SOUTHWEST
 OF A DEEP-LAYER RIDGE OVER THE SOUTHEASTERN UNITED STATES...AND THE
 LARGE-SCALE MODELS FORECAST THIS FEATURE TO PERSIST FOR THE NEXT
 36-48 HR.  THIS SHOULD STEER DON GENERALLY NORTHWESTWARD TO WEST-
 NORTHWESTWARD UNTIL LANDFALL ON THE TEXAS COAST BETWEEN 36-48 HR. 
 WHILE THE TRACK GUIDANCE AGREES ON THIS SCENARIO...THERE HAS BEEN A
 SIGNIFICANT NORTHWARD SHIFT IN THE GUIDANCE SINCE THE PREVIOUS
 ADVISORY...WITH THE CENTER OF THE GUIDANCE ENVELOPE NOW NORTH OF
 CORPUS CHRISTI.  THE NEW TRACK FORECAST WILL ALSO SHIFT NORTHWARD
 BUT STAYS TO THE SOUTH OF THE CENTER OF THE NEW GUIDANCE ENVELOPE. 
 IT SHOULD BE NOTED THAT THE ECMWF AND UKMET FORECAST TRACKS ARE ON
 THE SOUTHERN EDGE OF THE GUIDANCE ENVELOPE TO THE SOUTH OF THE
 CURRENT FORECAST TRACK.
 
 ANALYSES FROM THE SHIPS MODEL AND FROM CIMSS AT THE UNIVERSITY OF
 WISCONSIN SHOW THAT DON IS EXPERIENCING 10-15 KT OF NORTHERLY
 VERTICAL WIND SHEAR.  LIGHT/MODERATE SHEAR IS NOW FORECAST TO
 CONTINUE UNTIL THE CYCLONE MAKES LANDFALL.  IN ADDITION...THE
 CYCLONE IS ON THE EDGE OF AN AREA OF DRY AIR IN THE WESTERN GULF OF
 MEXICO.  THESE FACTORS COULD AT LEAST SLOW INTENSIFICATION...AND
 NONE OF THE INTENSITY GUIDANCE CURRENTLY SHOWS DON BECOMING A
 HURRICANE BEFORE LANDFALL.  THE NEW INTENSITY FORECAST IS BASICALLY
 AN UPDATE OF THE PREVIOUS PACKAGE IN BEST AGREEMENT WITH THE SHIPS
 MODEL.
 
  
 FORECAST POSITIONS AND MAX WINDS
  
 INIT  28/1500Z 24.1N  90.1W   40 KT  45 MPH
  12H  29/0000Z 24.9N  91.5W   45 KT  50 MPH
  24H  29/1200Z 26.1N  93.8W   50 KT  60 MPH
  36H  30/0000Z 27.3N  96.0W   50 KT  60 MPH
  48H  30/1200Z 28.5N  98.2W   40 KT  45 MPH...INLAND
  72H  31/1200Z 30.0N 102.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
  96H  01/1200Z...DISSIPATED
  
 $$
 FORECASTER BEVEN
  
 

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