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Tropical Depression SEVEN Forecast Discussion Number 1
2013-09-04 22:58:08| National Hurricane Center (Atlantic)
Issued at 500 PM AST WED SEP 04 2013 000 WTNT42 KNHC 042057 TCDAT2 TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013 500 PM AST WED SEP 04 2013 AIRCRAFT AND SATELLITE DATA FROM THIS AFTERNOON INDICATE THAT THE LOW PRESSURE SYSTEM SOUTH OF PUERTO RICO HAS JUST ENOUGH OF A CIRCULATION AND ORGANIZED CONVECTION TO BE CONSIDERED A TROPICAL DEPRESSION. THE INITIAL INTENSITY IS SET TO 30 KT BASED ON A 41 KT FLIGHT-LEVEL WIND FROM THE AIR FORCE PLANE. THE ENVIRONMENT NEAR THE DEPRESSION APPEARS TO BE CONDUCIVE FOR STRENGTHENING DURING THE NEXT DAY OR SO WHILE IT REMAINS FAR ENOUGH AWAY FROM HISPANIOLA. ONLY GRADUAL INTENSIFICATION IS FORECAST DUE TO THE SOMEWHAT DISORGANIZED INITIAL STATE OF THE DEPRESSION. AFTER THAT TIME...A COMBINATION OF INCREASING VERTICAL WIND SHEAR AND POSSIBLE LAND INTERACTION COULD SLOW THE DEVELOPMENT OF THE CYCLONE. THE NHC FORECAST IS ON THE LOW SIDE OF THE GUIDANCE ENVELOPE...AND IS CLOSER TO THE SHIPS AND LGEM MODELS. THE WIND SPEED PREDICTION ALSO FITS THE GENERAL IDEA OF THE GLOBAL MODELS...WHICH SUGGEST THAT THE CYCLONE WILL STRUGGLE AFTER IT LEAVES THE CARIBBEAN SEA. SOME INTENSIFICATION...PERHAPS DUE TO BAROCLINIC PROCESSES...IS POSSIBLE AT LONG RANGE AS THE CYCLONE INTERACTS WITH A MID-LATITUDE TROUGH. THE DEPRESSION APPEARS TO BE MOVING ROUGHLY 305/8 AROUND THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL RIDGE OVER THE CENTRAL ATLANTIC OCEAN. A LARGE TROUGH OVER THE WESTERN ATLANTIC OCEAN IS EXPECTED TO CAUSE THE RIDGE TO BREAK ALONG ABOUT 70W...WHICH SHOULD CAUSE THE CYCLONE TO TURN NORTHWARD IN TWO OR THREE DAYS. ONE COMPLICATING FACTOR IS THE LARGER DISTURBANCE LOCATED A FEW HUNDRED MILES NORTHEAST OF THE DEPRESSION. THE INTERACTION OF THESE TWO FEATURES MAKES THIS TRACK FORECAST MORE UNCERTAIN THAN WOULD BE SUGGESTED BY THE OVERALL MODEL SPREAD. THE NHC FORECAST IS NOT TOO FAR FROM THE MODEL CONSENSUS EARLY ON...AND THEN IS A BIT SLOWER THAN THE CONSENSUS AFTER THAT TIME...PUTTING LESS WEIGHT ON THE NORTHWARD GFDL/HWRF SOLUTIONS. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 16.5N 66.2W 30 KT 35 MPH 12H 05/0600Z 17.4N 67.1W 35 KT 40 MPH 24H 05/1800Z 18.6N 68.1W 40 KT 45 MPH 36H 06/0600Z 19.7N 68.8W 40 KT 45 MPH 48H 06/1800Z 20.9N 69.2W 40 KT 45 MPH 72H 07/1800Z 23.1N 69.4W 45 KT 50 MPH 96H 08/1800Z 25.3N 68.9W 45 KT 50 MPH 120H 09/1800Z 29.0N 65.0W 50 KT 60 MPH $$ FORECASTER BLAKE
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