Tropical Depression Nine - 10pm Sunday Discussion

By: National Hurricane Center
By: National Hurricane Center

TROPICAL DEPRESSION NINE DISCUSSION NUMBER 2
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013
1100 PM AST SUN SEP 08 2013

SATELLITE IMAGES SHOW THAT DEEP CONVECTION ASSOCIATED WITH THE DEPRESSION HAS INCREASED OVER THE WESTERN PORTION OF THE CIRCULATION THIS EVENING. RECENT ASCAT DATA WAS EXTREMELY HELPFUL IN LOCATING THE CENTER OF THE CYCLONE...WHICH IS STILL LOCATED NEAR THE EASTERN EDGE OF THE MAIN CONVECTIVE CLUSTER DUE TO SOME NORTHEASTERLY SHEAR. THE ASCAT DATA AND SUBJECTIVE DVORAK INTENSITY ESTIMATES FROM TAFB AND SAB SUGGEST THAT THE MAXIMUM WIND SPEED HAS INCREASED TO NEAR 30 KT.

THE DEPRESSION IS MOVING WESTWARD AT ABOUT 10 KT. THE CYCLONE IS FORECAST TO CONTINUE MOVING WESTWARD TO WEST-NORTHWESTWARD DURING THE NEXT DAY OR SO...AROUND THE SOUTH SIDE OF A DEEP-LAYER RIDGE OVER THE EASTERN ATLANTIC. AFTER THAT TIME...A MID- TO UPPER-LEVEL LOW...CURRENTLY WELL TO THE NORTHWEST OF THE DEPRESSION IS FORECAST TO DROP SOUTHWARD...CREATING A BREAK IN THE RIDGE ALONG 30W. THIS SHOULD CAUSE THE DEPRESSION TO TURN NORTH-NORTHWESTWARD IN 36 TO 48 HOURS AND THE SYSTEM SHOULD REMAIN ON THAT GENERAL HEADING THROUGH MUCH OF THE REMAINDER OF THE FORECAST PERIOD. THE TRACK GUIDANCE IS IN A LITTLE BETTER AGREEMENT DURING THE FIRST 72 HOURS...BUT AFTER THAT TIME...THERE IS A BIT MORE SPREAD THAN NOTED IN THE PREVIOUS CYCLE. THE GFS...HWRF...AND GFS ENSEMBLE MEAN KEEP THE CYCLONE ON A NORTH OR NORTH-NORTHWEST TRACK ALONG OR EAST OF 30W...WHILE THE ECMWF SHOWS A FASTER NORTHWESTWARD OR WEST-NORTHWESTWARD MOTION AT DAYS 4 AND 5. FOR NOW...THE OFFICIAL FORECAST SPLITS THESE DIFFERENCES AND REMAINS NEAR THE MULTI-MODEL CONSENSUS.

THE MODERATE NORTHEASTERLY SHEAR OVER THE DEPRESSION IS EXPECTED TO DECREASE WITHIN THE NEXT 12 TO 24 HOURS. DURING THAT TIME...THE DEPRESSION WILL BE IN A MOIST AIRMASS AND OVER WARM WATERS. THIS SHOULD ALLOW FOR STEADY STRENGTHENING...AND THE UPDATED NHC FORECAST SHOWS A FASTER RATE OF INTENSIFICATION DURING THE NEXT DAY OR SO. THE NHC FORECAST REMAINS MORE CONSERVATIVE THAN THE STATISTICAL GUIDANCE AND IS CLOSE TO THE INTENSITY CONSENSUS. LATE IN THE PERIOD... INCREASING SOUTHWESTERLY SHEAR AND COOLER WATERS ARE LIKELY TO CAUSE SOME WEAKENING.


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