000
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