In the infamous April 3, 1974 Super Outbreak, which left 47 dead in Indiana, the longest-tracking tornado of all 148 of them was the violent F4 that tracked through the viewing area. There is strong evidence that this tornado was an EF5 on the new Enhanced Fujita Scale.
The F3 that tracked from near West Lebanon to Round Grove has been overshadowed by the Monticello to Rochester F4. The F3 first touched down at 3:30 p.m., while the F4 touched down at 3:45 p.m. Obviously, the second, following supercell become the dominant one.
Tornado tracks from the outbreak..........Track data all courtesy of NOAA.
Surface maps from NOAA archives.
The system that spawned the outbreak was very strong with extreme shear, impressive wind fields throughout the troposphere & a nice overlaying of surface CAPE & cold air aloft with the system, too.
There were a few lesser mentioned ingredients that led to the Monticello tornado being so long-lived.............that being:
1. An outflow boundary (from elevated MCS north of the warm front initally).
2. Triple point (where warm front, occluded front & cold front converge).
3. A dry line merging with the triple point (dew points dropped from 60s to 20s & 30s behind the front with rapidly clearing).
These three ingredients (when others present) are key in the strongest to most violent tornadoes.
I added the dryline, triple point & outflow boundaries & some surface temperatures in this late afternoon map of that day.
Warm front enhances surface shear & triple points do it too. Combine that with a dry line merging with the triple point & then the shear only enhanced by the rain-cooled outflow boundary merging with the warm front & you have a set-up for the type of massive, violent tornadoes you get in Kansas & Oklahoma.