The infamous Tri-State Tornado of 1925 holds the record for the longest track by a single tornado, spanning at least 219 miles across Southeast Missouri, Southern Illinois, and into Southern Indiana.
That record might be in jeopardy, however, as yesterday's tornado outbreak across the South may have produced a similar long-lived tornado. Preliminary reports suggest that the twister that decimated Tuscaloosa may have stayed on the ground for over 200 miles from Mississippi through Alabama and Georgia before ending in Tennessee. If this tornado did stay on the ground continuously for that long, it could approach, or surpass, the record held by the Tri-State Tornado.
The death toll from the 1925 twister was over 600 (not counting deaths from other tornadoes during the same day). Hopefully that number won't be surpassed by yesterday's disaster, although the reports from Alabama and elsewhere are not looking good. This is clearly a once-in-a-generation kind of disaster.
If there's a silver lining (and it's tiny), meteorologists now have lots of radar and video imagery of the storms which may help them understand these worst-case outbreaks a little better. The Tri-State Tornado has long been a puzzle for meteorologists trying to figure out how a single tornadic storm can hold together for so long across varied terrain. Future research into the Tuscaloosa tornado may shed some light on how these freak storms are created.