Areas of rotation in leading edge of line of storms

[NancyM]

I was watching the radar on TV as a line of strong storms moved through St. Louis. They were watching several areas of rotation along the leading edge. The meteorologist said something about the likelihood of the rotations becoming tornadoes increased as the line became more "linear" North to South. What does the orientation of the line of storms have to do with formation of (I think pop-up) tornadoes? Or maybe I misheard.

These storms produced a couple of tornadoes shortly after crossing into IL. Someone on here chased these storms and posted about it.

 

[Alex Elmore]

You did not mishear. That is part of the "Three Ingredients Method" used by NWS forecasters to aid in forecasting mesovortex formation/tornadogenesis within a QLCS. One of the three ingredients is locating where the line orients itself roughly perpendicular to the 0-3km bulk shear vector. The greater the magnitude of the vector (higher shear), the more wiggle room the line has in being perpendicular to the vector. For the storms in St. Louis that evening, the shear vector was roughly oriented west to east, so as part of the line oriented itself more north to south, it became more perpendicular to the shear vector - increasing the probability of mesovortex formation/tornadogenesis.

However, as far as I know, this line of storms did not produce any tornadoes. The damage was consistent with straight-line winds. The portions of linear storm modes that are more prone to produce tornadoes are often associated with straight-line winds too.

 

[NancyM]

Mostly over my head for obvious reasons, but you've given me a rough idea of what the perpendicular business is about. I'll do some googling for terminology. Straight-line winds -- oops.

Thanks.

 

[Sarah B. Wood]

Nancy-
That is what happened here in Northern Indiana on August 10th of this year. We had a derecho/ QLCS come through the area and there were several areas of rotation along the leading edge that later lead to 3 confirmed tornadoes in the area.

 

[Drew Terril]

I think the difference with the 10 Aug event was the presence of bowing segments as well. IIRC, the line still had a NE-SW orientation, but was angled nicely in relation to the surface winds and the overall direction of motion. That's ultimately the big factor though; the orientation of the line in relation to shear vector, as Alex explained, as well as the direction of motion.