Basically the "blob" of deep convection or thunderstorm activity is an indication of intense thunderstorms in the area. This is detected by using the Infrared Satellite which measures the temperature of the cloud tops. As thunderstorms grow stronger, the clouds rise higher and into cooler air, thus registering cooler on infrared satellite. The blob that has formed is thunderstorm activity associated with the tropical wave and accompanying low pressure area.

A good link to satellite images is at http://www.nhc.noaa.gov/satellite.shtml. Go to Caribbean and select infrared image for a still shot and infrared loop for a time lapse of the past several hours.