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TROPICAL DEPRESSION EIGHTEEN DISCUSSION NUMBER   3
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT FRI OCT 20 2000
 
TROPICAL DEPRESSION EIGHTEEN IS ALMOST A TROPICAL STORM.  SHIP VRWK5
REPORTED 33 KT ABOUT 105 NM ESE OF THE CENTER AT 06Z...AND SATELLITE
INTENSITY ESTIMATES FROM TAFB AND SAB ARE 35 KT.  ON THE OTHER
HAND...THE CONVECTION IS LOOKING A LITTLE RAGGED OVER THE PAST FEW
HOURS...AND IT IS NOT CERTAIN WHERE THE CENTER IS IN RELATION TO THE
CONVECTION.  ALSO...THE INTENSITY ESTIMATE FROM AFWA IS 25 KT.  THE
CYCLONE WILL REMAIN A DEPRESSION FOR NOW.  FIRST-LIGHT VISIBLE
IMAGERY SHOULD RESOLVE THE WHEREABOUTS OF THE CENTER.

THE INITIAL POSITION COMPROMISES BETWEEN MORE NORTHERLY FIXES OF
TAFB...SAB...AND AFWA...AND A MORE SOUTHERLY POSITION FROM A 0119Z
SSM/I OVERPASS.  THE INITIAL MOTION IS 035/11.  SATELLITE IMAGERY
INDICATES THAT A DEEP LAYER TROUGH...INCLUDING THE REMAINS OF
MICHAEL...COVERS THE NORTHWEST ATLANTIC.  LARGE-SCALE MODELS AGREE
THAT THIS TROUGH WILL BY-PASS THE CYCLONE...LEAVING IT TO BE PICKED
UP BY A SECOND TROUGH DEVELOPING IN 36 TO 72 HR.  THIS PATTERN
SHOULD PRODUCE A GENERALLY NORTHEASTWARD TRACK THROUGH 36 TO 48
HR...WITH SOME TURN TO THE LEFT THEREAFTER.  THE OFFICIAL FORECAST
IS SHIFTED LEFT OF THE PREVIOUS TRACK...PARTICULARLY AFTER 48 HR. 
THIS IS A COMPROMISE BETWEEN THE AVN AND UKMET ON THE LEFT...AND THE
PREVIOUS FORECAST AND THE GFDL ON THE RIGHT.

THE INTENSITY FORECAST IS TRICKY.  THE ENVIRONMENT IS CURRENTLY
FAVORABLE...WITH AN UPPER LEVEL RIDGE OVER THE CYCLONE AND A JET
STREAM TO THE NORTHWEST PROVIDING OUTFLOW.  HOWEVER...LARGE-SCALE
MODELS SUGGEST THAT STRONG SOUTHWESTERLY WINDS WILL DEVELOP OVER THE
SYSTEM IN ABOUT 24 HR.  ADDITIONALLY...SATELLITE IMAGERY INDICATES A
COLD FRONT IS APPROACHING THE CYCLONE.  THIS COMBINATION SHOULD STOP
STRENGTHENING...AND THIS IS REFLECTED IN THE INTENSITY FORECAST.  AT
ABOUT 48 HR...THE CYCLONE MAY COME UNDER THE RIGHT REAR QUADRANT OF
A JET STREAM TO THE EAST OF A SHARP...POSSIBLY NEGATIVELY TILTED
UPPER TROUGH.  THIS BRINGS UP THE POSSIBILITY OF BAROCLINICALLY-
INDUCED STRENGTHENING AS HAPPENED WITH MICHAEL.  THE FORECAST WILL
NOT YET REFLECT THIS...AS IT IS UNCLEAR WHETHER THE FORCING FROM THE
TROUGH WILL OVERCOME THE STRONG VERTICAL SHEAR.  IN ANY CASE...THE
CYCLONE SHOULD LOSE TROPICAL CHARACTERISTICS BY 72 HR OVER THE COLD
WATERS OF THE NORTH ATLANTIC.

THE FORECAST WIND RADII HAVE BEEN ADJUSTED OVER THE SOUTHEAST
QUADRANT BASED ON THE SHIP REPORT AND THE EXPECTED MOTION.
 
FORECASTER BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     20/0900Z 29.8N  57.6W    30 KTS
12HR VT     20/1800Z 31.1N  56.5W    40 KTS
24HR VT     21/0600Z 32.9N  54.3W    45 KTS
36HR VT     21/1800Z 34.9N  51.7W    45 KTS
48HR VT     22/0600Z 37.5N  49.5W    45 KTS
72HR VT     23/0600Z 45.0N  44.0W    45 KTS...EXTRATROPICAL
 
 
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