000
WTNT44 KNHC 290843
TCDAT4
TROPICAL STORM MELISSA DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL142007
500 AM EDT SAT SEP 29 2007
 
DEEP CONVECTION HAS BEEN ON THE INCREASE DURING THE PAST SEVERAL
HOURS.  0319Z MICROWAVE IMAGERY INDICATED THAT THE CIRCULATION
CENTER WAS UNDER THE WESTERN PORTION OF THE CONVECTIVE CANOPY...BUT
SUBSEQUENT GOES IMAGES HINT THAT THE CONVECTION IS WRAPPING MORE
AROUND THE CENTER.  SUBJECTIVE DVORAK INTENSITY ESTIMATES AT 06Z
WERE A CONSENSUS 30 KT...BUT OBJECTIVE ESTIMATES ARE HIGHER AND
PROVIDE THE BASIS FOR UPGRADING THE SYSTEM TO A TROPICAL STORM WITH
35 KT MAXIMUM WINDS.
 
IT IS CLEAR THAT THE DEPRESSION IS BARELY MOVING...BUT THE EXACT
INITIAL MOTION IS STILL UNCERTAIN...ESTIMATED TO BE 270/3.  THE
CYCLONE IS EMBEDDED WITHIN VERY WEAK STEERING CURRENTS...LACKING A
SUBTROPICAL RIDGE TO ITS NORTH DUE TO A DEEP LAYER LOW PRESSURE
SYSTEM OVER THE NORTHEASTERN ATLANTIC.  AS THAT LOW PROCEEDS
EASTWARD...ALL OF THE MODELS DEPICT SOME RIDGING TO GRADUALLY BUILD
BACK IN OVER THE TROPICAL EASTERN ATLANTIC...BUT THEY DO NOT AGREE
ON HOW THE TRACK OF THE DEPRESSION WILL RESPOND.  THE HWRF PROVIDES
THE NORTHERNMOST SOLUTION...TAKING THE CYCLONE IMMEDIATELY
NORTHWESTWARD...WHILE THE ECMWF AND GFS ENSEMBLE MEAN ARE THE
SOUTHERN EXTREMES.  IN GENERAL...THE MODELS SEEM TO HAVE SHIFTED
AGAIN TO THE LEFT...AND SO HAS THE OFFICIAL FORECAST...WHICH CALLS
FOR THE CYCLONE TO MOVE IN A GENERAL WEST-NORTHWESTWARD DIRECTION
THROUGHOUT THE FIVE-DAY PERIOD.
 
THE DEPRESSION CURRENTLY RESIDES IN A RELATIVELY WEAK-SHEAR
ENVIRONMENT JUST TO THE SOUTH OF A BELT OF STRONG UPPER-LEVEL
WESTERLIES.  THE SHEAR WILL PROBABLY REMAIN WEAK ENOUGH TO ALLOW FOR
A LITTLE STRENGTHENING DURING THE NEXT 24-36 HOURS...BUT THEN IT
WILL BE ON THE INCREASE...ESPECIALLY BEYOND 48 HOURS WHEN
SIGNIFICANT WEAKENING IS EXPECTED.  THE OFFICIAL FORECAST PEAKS AT
40 KT...SLIGHTLY HIGHER THAN ALL OF THE OBJECTIVE GUIDANCE...BUT
CALLS FOR THE DEPRESSION TO DISSIPATE IN LESS THAN FIVE DAYS IN THE
HOSTILE ENVIRONMENT THAT LIES AHEAD LATER IN THE FORECAST PERIOD.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/0900Z 14.1N  27.4W    35 KT
 12HR VT     29/1800Z 14.4N  28.7W    35 KT
 24HR VT     30/0600Z 14.8N  30.5W    40 KT
 36HR VT     30/1800Z 15.5N  32.7W    40 KT
 48HR VT     01/0600Z 16.2N  34.8W    35 KT
 72HR VT     02/0600Z 17.5N  38.5W    30 KT
 96HR VT     03/0600Z 19.0N  42.0W    25 KT...DISSIPATING
120HR VT     04/0600Z 21.0N  45.0W    20 KT...REMNANT LOW
 
$$
FORECASTER KNABB