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TROPICAL STORM CHANTAL DISCUSSION NUMBER  15
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT SUN AUG 19 2001

AIR FORCE RECON FOUND 62 KT FLIGHT-LEVEL WINDS AT 1500 FT IN THE 
SOUTHEAST QUADRANT AROUND 0615Z...WITH A DROP IN PRESSURE DOWN TO 
997 MB.  SATELLITE INTENSITY ESTIMATES RANGE FROM 55 KT FROM TAFB TO 
45 TO 50 KT FROM BOTH SAB AND AFWA.  HOWEVER...THE SATELLITE FIX 
POSITIONS WERE NEAR THE WESTERN EDGE OR OUTSIDE THE DEEP 
CONVECTION...WHEREAS THE RECON POSITIONS PLACE THE CENTER FURTHER 
INTO THE DEEP CONVECTION.  GIVEN THE CONTINUED INCREASE IN DEEP 
CONVECTION SINCE 06Z...THE INTENSITY WAS INCREASED TO 55 KT.  
OUTFLOW REMAINS GOOD TO THE NORTH AND EAST...AND IS IMPROVING 
ELSEWHERE AS THE SOUTHWESTERLY SHEAR HAS DECREASED.

THE INITIAL MOTION ESTIMATE IS 275/10...AND THAT MAY BE GENEROUS.  
THE CENTER WAS RELOATCED A LITTLE FURTHER SOUTH BASED ON THE PAST 4 
RECON POSITIONS.  EARLIER SATELLITE POSITION ESTIMATES WERE WELL TO 
THE NORTHEAST...BUT HAVE COME CLOSER TO THE LOW-LEVEL RECON FIXES OF 
LATE.  OWING TO THE MUCH SLOWER SPEED OF MOVEMENT...THE OFFICIAL 
FORECAST TRACK SPEED WAS ALSO DECREASED AND ADJUSTED SLIGHTLY TO THE 
RIGHT OF THE PREVIOUS TRACK.  HOWEVER...THE OFFICAL TRCAK IS FASTER 
THAN MOST OF THE DYNAMICAL MODELS...SOME OF WHICH BRING THE SPEED 
DOWN TO LESS THAN 8 KT BY 48 TO 72 HOURS.  00Z UPPER-AIR DATA FROM 
MERIDA INDICATES 24-HOUR HEIGHT FALLS OF 20 TO 50 METERS FROM 700 
THROUGH 150 MB...WHICH INDICATES THE WESTERN PORTION OF THE MID- AND 
UPPER-LEVEL RIDGE HAS ERODED SIGNIFICANTLY AS A TROUGH HAS DEVELOPED 
OVER THE CENTRAL AND WESTERN GULF OF MEXICO. MOST OF THE GLOBAL 
MODELS CONTINUE TO GRADUALLY ERODE THE WESTERN PERIPHERY OF THE 
SUBTROPICAL RIDGE...WHICH MAY ALLOW CHANTAL TO GAIN MORE LATITUDE 
THAN THE OFFICIAL FORECAST IS INDICATING.  NOTE...WHEN TROPICAL 
CYCLONES RAPIDLY DECREASE IN FORWARD SPEED...THAT IMPLIES A SHARP 
CHANGE IN THE STEERING FLOW AND OFTENTIMES IS FOLLOWED BY BRIEF 
ERRATIC MOTION.

NOW THAT THE LOW-LEVEL CENTER IS EMBEDDED FURTHER IN THE DEEP 
CONVECTION...STEADY INTENSIFCATION SHOULD OCCUR.  BASED ON THE 
HEIGHT FALLS AT MERIDA...THE UPPER-LEVEL SHEAR SHOULD ALSO DECREASE 
AND THE POLEWARD OUTFLOW SHOULD BE ENHANCED.  IF RECON FINDS LOWER 
PRESSURE AND A DEVELOPING EYEWALL AT THE NEXT FIX TIME AROUND 
12Z...THEN THE INTENSITY FORECAST WILL HAVE TO BE BE INCREASED AND 
WE COULD BE DEALING WITH SIGNIFICANT HURRICANE IN 36 TO 48 HOURS.  
THE SHIPS INTENSITY MODEL WAS FOLLOWED CLOSELY UNTIL LANDFALL ALONG 
THE YUCATAN COAST IN 48 TO 60 HOURS.

FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     19/0900Z 15.5N  78.6W    55 KTS
12HR VT     19/1800Z 16.0N  80.2W    60 KTS
24HR VT     20/0600Z 16.7N  82.4W    65 KTS
36HR VT     20/1800Z 17.6N  84.4W    70 KTS
48HR VT     21/0600Z 18.9N  86.3W    80 KTS
72HR VT     22/0600Z 21.5N  89.0W    65 KTS...INLAND
 
 
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