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HURRICANE GERT DISCUSSION NUMBER  20
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT THU SEP 16 1999
 
AN AIR FORCE RESERVE HURRICANE HUNTER HAS JUST REACHED GERT
REPORTING A CENTRAL PRESSURE OF 941 MB AND 119 KT FLIGHT LEVEL WINDS
IN THE NORTHEAST QUADRANT. AN EYEWALL DROPSONDE REPORTED 141 KT
ABOVE THE SURFACE. BASED ON THIS...AND A SLIGHTLY DEGRADED SATELLITE
SIGNATURE...THE MAXIMUM WINDS ARE DECREASED TO 125 KT. THE AIRCRAFT
ALSO REPORTED CONCENTRIC EYEWALLS WITH DIAMETERS OF 18 AND 30 NM.

THE INITIAL MOTION IS NOW 285/9...A LITTLE TO THE RIGHT OF THE
PREVIOUS ADVISORY. ALL MODEL GUIDANCE IS INDICATING A TURN TO THE
NORTH OVER THE NEXT THREE DAYS. WHILE THE UNANIMITY IS HARD TO
DISAGREE WITH...WATER VAPOR IMAGERY SHOWS NORTHERLY FLOW ABOUT 400
NM NORTH OF GERT. THIS SUGGESTS THE TURN MAY BE SLOWER THAN THE
MODELS INDICATE. THE FORECAST TACK GOES ALONG WITH THE TURN AND IS
SHIFTED NORTH FROM THE PREVIOUS PACKAGE...BUT IS ALONG THE SOUTHERN
EDGE OF THE MODEL GUIDANCE.
 
GERT CONTINUES TO HAVE IMPRESSIVE CONVECTION AND GOOD OUTFLOW. THE
SHIPS MODEL WEAKENS GERT THROUGH THE FORECAST PERIOD DUE TO
INCREASING VERTICAL SHEAR. A LOOK AT THE 200 MB WIND FIELDS IN THE
AVN AND NOGAPS MODELS DOES NOT OBVIOUSLY SHOW SUCH SHEAR. IT LOOKS
LIKE THAT CONCENTRIC EYEWALL CYCLES MAY BE MORE IMPORTANT IN
INTENSITY CHANGES THAN SHEAR. THE INTENSITY FORECAST CALLS FOR A
SLIGHT WEAKENING SIMILAR TO THE LAST FORECAST IN CASE SOME INCREASE
IN SHEAR DOES OCCUR. HOWEVER...THIS IS A LOW CONFIDENCE FORECAST...
AND THE MAXIMUM WINDS MAY VERY WELL OSCILLATE IN THE 115-130 KT
RANGE DURING THE PERIOD.

BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     16/0900Z 18.1N  52.2W   125 KTS
12HR VT     16/1800Z 18.5N  53.5W   120 KTS
24HR VT     17/0600Z 19.4N  55.2W   120 KTS
36HR VT     17/1800Z 20.8N  56.9W   115 KTS
48HR VT     18/0600Z 22.5N  58.5W   115 KTS
72HR VT     19/0600Z 26.5N  60.5W   115 KTS
 
 
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