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Tropical Depression ELEVEN Forecast Discussion Number 2
2013-09-29 10:48:14| National Hurricane Center (Atlantic)
Issued at 500 AM AST SUN SEP 29 2013 000 WTNT41 KNHC 290847 TCDAT1 TROPICAL DEPRESSION ELEVEN DISCUSSION NUMBER 2 NWS NATIONAL HURRICANE CENTER MIAMI FL AL112013 500 AM AST SUN SEP 29 2013 NIGHTTIME VISIBLE IMAGERY INDICATES THE DEPRESSION HAS A ROBUST LOW-LEVEL CIRCULATION...BUT THE ASSOCIATED DEEP CONVECTION IS SHEARED WELL TO THE NORTH AND NORTHEAST OF THE CENTER. SOME WEAK CURVED BAND FEATURES HAVE DEVELOPED RECENTLY IN THE EASTERN SEMICIRCLE...SUGGESTING THAT THE SYSTEM IS TRYING TO GET BETTER ORGANIZED. FOR NOW...HOWEVER...AN INTENSITY OF 30 KT IS BEING MAINTAINED BASED ON A SATELLITE INTENSITY ESTIMATE OF 30 KT FROM TAFB AND UW-CIMSS ADT. THE INITIAL MOTION ESTIMATE IS 040/08 KT. THERE IS NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. THE SMALL DEPRESSION IS EXPECTED TO MOVE NORTHEASTWARD AND SLOW DOWN AS IT MOVES AROUND THE NORTH SIDE OF A WEAKENING MID-LEVEL RIDGE WHILE A SHORTWAVE TROUGH TO THE NORTH GRADUALLY LIFTS OUT. THIS SHOULD RESULT IN VERY WEAK STEERING FLOW FOR THE NEXT COUPLE OF DAYS. A NEW RIDGE IS THEN FORECAST BY THE MODELS TO BUILD IN AND BECOME HIGHLY AMPLIFIED TO THE WEST AND NORTH OF THE CYCLONE BY 48 HOURS...WHICH SHOULD ACT TO TURN THE CYCLONE WESTWARD THROUGH 72 HOURS...COMPLETING A SMALL CLOCKWISE LOOP IN THE PROCESS. AFTER THAT...A SECOND AND STRONGER MID-/UPPER-LEVEL TROUGH WILL MOVE EASTWARD FROM THE UNITED STATES...AND ACT TO LIFT OUT THE CYCLONE AND ACCELERATE IT TO THE NORTHEAST ON DAYS 4 AND 5. THE NEW NHC FORECAST TRACK IS ALMOST IDENTICAL TO THE PREVIOUS TRACK...AND LIES CLOSE TO A BLEND OF THE GFS AND ECMWF MODELS. ALTHOUGH THE GFS-BASED SHIPS INTENSITY MODEL IS ONLY INDICATING 9 KT OF SOUTH-SOUTHWESTERLY VERTICAL SHEAR ACROSS THE DEPRESSION...WHICH IS SIGNIFICANTLY LOWER THAN IT WAS ON THE PREVIOUS ADVISORY... SATELLITE IMAGERY AND THE ASSOCIATED CONVECTIVE CLOUD PATTERN OF THE DEPRESSION SUGGESTS THAT THE SHEAR IS CONSIDERABLY STRONGER... LIKELY DUE TO THE PRESENCE OF A MID-/UPPER-LEVEL LOW LOCATED ONLY ABOUT 300 NMI TO THE NORTHWEST OF THE CYCLONE. THAT LOW IS FORECAST BY MOST OF THE MODELS...ESPECIALLY THE GFS AND ECMWF...TO MOVE IN TANDEM WITH THE CYCLONE OVER THE NEXT 72 HOURS OR SO AS IT MAKES A CLOCKWISE LOOP. AS A RESULT...THIS SYSTEM WILL LIKELY BE PLAGUED BY AT LEAST MODERATE SHEAR FOR THE NEXT 2 TO 3 DAYS...WHICH SHOULD PREVENT ANY SIGNIFICANT STRENGTHENING FROM OCCURRING. THE NHC INTENSITY FORECAST IS IDENTICAL TO THE PREVIOUS PACKAGE AND REMAINS CLOSE TO THE INTENSITY CONSENSUS MODEL...ICON. HOWEVER...IF THE AFOREMENTIONED UPPER-LEVEL SHOULD DISSIPATE OR MOVE AWAY FROM THE CYCLONE SOONER THAN CURRENTLY EXPECTED...THEN THE SYSTEM COULD BE SOMEWHAT STRONGER THAN CURRENTLY FORECAST ON DAYS 4 AND 5. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 25.6N 49.7W 30 KT 35 MPH 12H 29/1800Z 26.4N 48.8W 35 KT 40 MPH 24H 30/0600Z 27.0N 47.6W 40 KT 45 MPH 36H 30/1800Z 26.8N 46.8W 40 KT 45 MPH 48H 01/0600Z 26.4N 46.6W 40 KT 45 MPH 72H 02/0600Z 26.3N 47.8W 35 KT 40 MPH 96H 03/0600Z 27.7N 48.6W 35 KT 40 MPH 120H 04/0600Z 30.0N 47.0W 30 KT 35 MPH $$ FORECASTER STEWART
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