I just found some archived livecam images from around the time of the wind gust from the anenometer station,
http://www.evsboca.com/ . They have been added to the account above. You can make out something of the clouds moving across particularly in the full size image. These clouds were slightly darker than everything else that was blowing through but perhaps not remarkably so. It may have been a shelf cloud but it is hard to tell from the image.
Guys were riding 6 miles north in Delray at the time. There also might have been guys out to the south in Pompano and Ft. Lauderdale. I imagine if they were hit by a 87 kt. gust, even if they were rigged for winds in the 40's the outcome would likely be grim. There is a private anenometer in Delray that stayed at around 45 kts. around the time of this event. Nothing remarkable showed up at the next anenometer about 10 miles to the south at Ft. Lauderdale Executive AP. I wonder if this might have been a microburst? There have been past indications of microbursts in this area at one, perhaps two other times.
NOTE: there was a lull in the wind record before the big gust spike. This happens sometimes as was the case on August 8, 2006. More at
http://fksa.org/viewtopic.php?t=2375 The wind eased off, one guy came in to rig a larger kite despite obvious storm conditions moving in and without any knowledge of what was on the radar. Others have done this in the past, ignoring what may be a good indication of much stronger winds to come. There isn't always a lull however, only sometimes before major gust spikes.
I have been looking in on the radar velocity images and ikitesurf realtime windspeeds off the Carolinas as Ernesto moves north. Sometimes the radar agrees with the realtime wind speeds and other times not. It does make you wonder.