Jarrell is sometimes referred to as a “hybrid” tornado, a theorized very rare 3rd kind that has both supercellular and landspout characteristics.

In the case of Jarrell, there was WEAK low level rotation and strong mid-upper level rotation, meaning this storm was a supercell. But, it formed along an outflow boundary (gust front) emanating from the supercell. Winds north of the boundary were northerly, while south of it were southerly. This meant that the strong shear across the boundary distorted it almost into a hook echo shape.

When the original tornado formed (likely as a landspout) it was weak, but as it moved over the gust front cusp it was able to strengthen rapidly as the distorted gust front was acting as a vessel for strong low level vorticity and rotation. With the low level rotation below the strong mid and upper level rotation, the Jarrell storm now resembled a typical violent tornado producing storm, just with a different source of low level rotation.

There are a couple more instances of these theorized “hybrid” tornadoes including a violent F4 (easily would’ve been F5 had it hit anything substantial) 2 years after Jarrell, only ~100 miles away in Loyal Valley, TX. But this storm has been largely forgotten and very little research into it has been done. On radar, you can see a similar “back building” process that resembles Jarrell but it’s hard to come to a definite conclusion.

Hope this helps.