A photo, purportedly from the passage of Tropical Storm Isaac near Florida, is not. 

The cloud feature is a shelf cloud, a phenomenon commonly seen around typical thunderstorms.

It is produced when cold air rushing from a thunderstorm collides with warmer and more moist air in the surrounding environment.  

The warmer air is rapidly forced upward by the colder, more dense air.  It then condenses aloft into a spectacular shelf-like cloud formation.

Tropical systems have a warmer core and thunderstorms around the core of the storm rarely produce sufficiently cold air to produce a shelf cloud formation.  

And even if that were to happen, strong wind motion just off the surface would tear a developing shelf cloud to shreds before being able to reach an appearance like this.   

Around the periphery of the storm, it is possible.

Typically, clouds associated with most squalls around a tropical storm or hurricane appear to the human eye as ragged and rather unremarkable.

This particular photograph has been circulating the internet for several years.  I've seen it before.

A meteorologist at Tampa's Bay News 9 reports he's seen a different foreground photoshopped underneath the cloud in the past.

So, tell your friends: this picture of "Isaac" is a fake!

(Update: source of the photo found.  http://www.mooncreek.com/stock/pages/050709-dennis-094.html)