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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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