ZCZC MIATCDAT1 ALL TTAA00 KNHC DDHHMM TROPICAL DEPRESSION SIX DISCUSSION NUMBER 3 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT SUN JUL 20 2003 THE DEEP CONVECTION ASSOCIATED WITH TROPICAL DEPRESSION SIX HAS CONTINUED TO WEAKEN DURING THE PAST 6 HOURS...BUT THE WIND FIELD AND OVERALL CIRCULATION REMAIN FAIRLY IMPRESSIVE. IN FACT... CELL MOTIONS ON THE EAST SIDE OF THE DEPRESSION HAVE BEEN NOTED AS HIGH AS 40-45 KT DURING THE PAST FEW HOURS. THEREFORE...THE INTENSITY IS KEPT AT 30 KT BASED ON THIS INFORMATION AND A DVORAK SATELLITE INTENSITY ESTIMATE OF T2.0...OR 30 KT FROM TAFB. THE INITIAL MOTION IS A FASTER 280/19. THE DEPRESSION REMAINS EMBEDDED IN DEEP EASTERLY STEERING FLOW ON THE SOUTH SIDE OF A STRONG SUBTROPICAL RIDGE. THE RIDGE IS EXPECTED TO CHANGE LITTLE AND THIS IS NOTED IN VIRTUALLY NO CHANGE IN THE 700 AND 500 MB HEIGHTS AT SAN JUAN DURING THE PAST 48 HOURS. THEREFORE...THE CYCLONE IS EXPECTED TO TRACK THROUGH THE CENTRAL TO NORTHERN LESSER ANTILLES IN 48 TO 60 HOURS AND CONTINUE ON A WEST-NORTHWEST TRACK JUST SOUTH OF PUERTO RICO IN 72 HOURS. BEYOND THAT...THE FORECAST BECOMES LITTLE MORE COMPLICATED. A STRONG MID- TO UPPER-LEVEL TROUGH CURRENTLY OVER THE SOUTHEAST U.S. AND NORTHEAST GULF OF MEXICO IS FORECAST TO WEAKEN AND LIFT OUT TO THE NORTHEAST BY DAY 5. HOWEVER...THE AMOUNT AND EXACT TIMING OF THE TROUGH WEAKENING REMAINS IN QUESTION. ALL OF THE GLOBAL MODELS INDICATE THAT SOME OF THE TROUGH WILL REMAIN BEHIND AND HELP TO MAINTAIN A WEAKNESS IN THE SUBTROPICAL RIDGE ACROSS FLORIDA AND MOST OF THE GULF OF MEXICO. IF THIS PATTERN EVOLVES AS THE MODELS ARE SUGGESTING...THEN THE CYCLONE COULD POSE A SIGNIFICANT THREAT TO HISPANIOLA AND CUBA IN DAYS 4 AND 5. THE OFFICIAL FORECAST TRACK IS CLOSE TO THE PREVIOUS TRACK AND THE THREE BAM MODELS. THE GFS AND NOGAPS MODELS WERE DISCOUNTED SINCE THEY ESSENTIALLY WEAKEN THE CYCLONE INTO AN OPEN WAVE AND RACE IT RAPIDLY WESTWARD THROUGH DAY 5. OUT OF RESPECT FOR THE UKMET MODEL...THE DAY 5 POSITION WAS NUDGED SLIGHTLY NORTHWARD FROM THE PREVIOUS TRACK...BUT NOT NEARLY AS AS FAR NORTH AS THE NORTHERN BAHAMAS. THE PREVIOUS WARM UNSTABLE SOUTHERLY INFLOW HAS NOW BEEN SHUNTED OFF TO THE EAST INTO A WEAK DISTURBANCE NEAR 10N 35W. THE COOLER AND MORE STABLE NORTHEASTERLY TRADE INFLOW IS LIKELY THE MAIN CAUSE OF THE RECENT DOWNWARD TREND IN CONVECTIVE DEVELOPMENT. HOWEVER...THIS DOWNWARD TREND SHOULD BE SHORT-LIVED SINCE THE CYCLONE IS APPROACHING 50W LONGITUDE WHERE THE WATER IS WARMER. THE SHIPS INTENSITY MODEL INDICATE THE VERTICAL SHEAR SHOULD DECREASE TO LESS THAN 10 KT BY 36 HOURS...WHICH SHOULD FAVOR AT LEAST STEADY INTENSIFICATION. AS SUCH...THE CYCLONE IS EXPECTED TO BE NEAR HURRICANE STRENGTH WHEN IT MOVES THROUGH THE LESSER ANTILLES. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 20/0900Z 12.9N 48.3W 30 KT 12HR VT 20/1800Z 13.3N 51.1W 35 KT 24HR VT 21/0600Z 13.8N 54.3W 40 KT 36HR VT 21/1800Z 14.4N 57.6W 45 KT 48HR VT 22/0600Z 15.1N 60.7W 55 KT 72HR VT 23/0600Z 16.7N 66.6W 70 KT 96HR VT 24/0600Z 18.5N 72.5W 70 KT 120HR VT 25/0600Z 21.5N 78.5W 70 KT NNNN
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Page last modified: Monday, 07-Feb-2005 16:49:56 UTC