Unmanned Aerial Vehicles might provide a useful method for the deployment of sensors and collection od tornado data.
The nose could be fitted with sensors and imaging equipment. A jettisonable pod attached to the fuselage could be used to drop other sensors into the path of the tornado without the need for roads or endangering personnel.

If by "UAV" you mean a full MIL-SPEC system like Predator, then sure it might prove useful. That is, if a declassified civilian version becomes available.

If you're talking about a hobbyist grade R/C aircraft though, I don't believe that's very practical. Unlike Predator and other military UAVs, R/C planes use simple command controls, and rely totally on a human on location to fly the plane. Operating R/C planes can be pretty tricky during ideal weather. The pilot MUST be able to see the plane at all times, and be close enough so that the low power radio signals reach the plane. Otherwise the plane will crash. I seriously doubt that any R/C pilot in his right mind would run around outside during a tornado!

Go to http://www.tornadovideos.net/projects
Scroll down a little. It has been used successfully, and there is even a video somewhere on the site of it deploying probes into the inflow of a tornado

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