000
WTNT42 KNHC 310837
TCDAT2
 
TROPICAL STORM KATIA DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
500 AM AST WED AUG 31 2011

SATELLITE IMAGERY THIS MORNING INDICATES THAT KATIA IS GRADUALLY
BECOMING BETTER ORGANIZED.  THE PRIMARY CONVECTIVE BAND HAS WRAPPED
FARTHER AROUND THE CENTER...AND THE CLOUD TOPS IN THE BAND HAVE
COOLED.  SATELLITE INTENSITY ESTIMATES ARE 55 KT FROM SAB AND
TAFB...AND THAT IS THE INITIAL INTENSITY FOR THIS ADVISORY.  THE
CIRRUS OUTFLOW IS FAIR TO GOOD IN ALL DIRECTIONS.

THE INITIAL MOTION IS 285/18.  KATIA IS ON THE SOUTH SIDE OF A
DEEP-LAYER SUBTROPICAL RIDGE...WHILE WATER VAPOR IMAGERY SHOWS A
LARGE MID/UPPER-LEVEL LOW...A WEAKNESS IN THE RIDGE...CENTERED NEAR
30N54W AND MOVING WESTWARD.  THE TRACK GUIDANCE IS IN VERY GOOD
AGREEMENT THAT KATIA SHOULD MOVE GENERALLY WEST-NORTHWARD FOR THE
NEXT 48 HR OR SO....THEN GRADUALLY TURN TOWARD THE NORTHWEST AT A
SLOWER FORWARD SPEED AS IT APPROACHES THE LOW OR ITS REMNANTS.  THE
GUIDANCE ENVELOPE HAS SHIFTED A LITTLE TO THE SOUTH DURING THE
FIRST 36 HR...AND SHIFTED A LITTLE TO THE NORTH AFTER THAT TIME. 
THE NEW FORECAST TRACK MAKES SIMILAR SMALL ADJUSTMENTS COMPARED TO
THE PREVIOUS FORECAST AND LIES NEAR THE VARIOUS CONSENSUS MODELS.
 
A COUPLE OF ISSUES HAVE APPEARED REGARDING THE INTENSITY FORECAST. 
FIRST...WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER
DATA SHOW DRY AIR NOT FAR TO THE NORTH AND EAST OF THE CORE OF
KATIA.  THIS HAS NOT YET AFFECTED DEVELOPMENT...AND THE NEW
INTENSITY FORECAST KEEPS THE TREND OF THE PREVIOUS FORECAST FOR THE
FIRST 36 HR.  HOWEVER...THERE IS A POSSIBILITY THAT INTENSIFICATION
COULD BE SLOWER THAN FORECAST IF THE DRY AIR REACHES THE CENTER. 
SECOND...THE DYNAMICAL MODELS DISAGREE ON THE UPPER-LEVEL WINDS
NEAR KATIA FROM 72-120 HR.  THE GFS...NOGAPS... AND CANADIAN MODELS
SHOW THE UPPER-LEVEL LOW TO THE NORTHWEST WEAKENING TO THE POINT
WHERE THE STORM REMAINS IN LIGHT VERTICAL WIND SHEAR CONDITIONS. 
THE ECMWF AND UKMET KEEP A STRONG ENOUGH UPPER-LEVEL TROUGH FOR
KATIA TO EXPERIENCE SIGNIFICANT SOUTHWESTERLY SHEAR.  THE FORECAST
INTENSITIES HAVE BEEN LOWERED SLIGHTLY AFTER 48 HR DUE TO THE
POSSIBILITY OF SHEAR DURING THAT TIME.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  31/0900Z 13.9N  39.1W   55 KT  65 MPH
 12H  31/1800Z 14.3N  41.7W   65 KT  75 MPH
 24H  01/0600Z 15.0N  44.9W   75 KT  85 MPH
 36H  01/1800Z 15.8N  47.7W   85 KT 100 MPH
 48H  02/0600Z 16.8N  50.2W   90 KT 105 MPH
 72H  03/0600Z 19.0N  54.0W   95 KT 110 MPH
 96H  04/0600Z 21.0N  57.0W  100 KT 115 MPH
120H  05/0600Z 23.0N  60.0W  105 KT 120 MPH
 
$$
FORECASTER BEVEN