When thunderstorms or other severe weather crop up, will the Air Traffic Control in the area proactively route traffic (horizontally or vertically) around the trouble area, or do they leave it up to the individual pilots to request an alternate route?

My intent was asking about [commercial aircraft](http://en.wikipedia.org/wiki/Commercial_aviation) flying [IFR](http://en.wikipedia.org/wiki/Instrument_flight_rules), however the information about [general aviation](http://en.wikipedia.org/wiki/General_aviation) and [VFR](http://en.wikipedia.org/wiki/Visual_flight_rules) is appreciated