Do you know what it's called, and whether it'll be shown on BBC America or something else (maybe PBS) here in the States?
Googled "bbc supervolcano" and the second hit was the BBC blurb on the show:
Supervolcano
The beauty of America's Yellowstone National Park masks one of the rarest and most destructive forces on Earth - a supervolcano.
A new two-part BBC factual drama asks: 'What if Yellowstone erupted?'
Parts 1 & 2: 13-14 March, 9pm BBC One
No listing on network or timing for US broadcast there.
Here's another hit from that same google search:
Yellowstone Supervolcano Drama on BBC this Month
By James Wray Mar 4, 2005, 08:55 GMT
which has more detail on the program. It does state that it is co-produced by BBC and Discovery (among others) so I guess that means it's going to be on Discovery.