Comparing historical days is always a tough task, simply because you can’t always draw 1:1 parallels between days. However, the beginning of Autumn typically brings with it a secondary severe weather season on the Southern Plains. Typically, for chasers, this means at least 1-2 decent supercell chases and every 4-5 years it seems it means we get a good tornado day. One such day was last November, as our team scored huge on November 7, 2011 in Southwest and then Central Oklahoma, with tornado after tornado after tornado. There were literally wedges, ropes, stovepipes, and more. Myself, Josh, and Cody were within 100 yards of the Tipton EF4 tornado as it crossed the highway in front of us.

It was a surreal day, and there’s so many lessons to be learned. However, what I found interesting when comparing November 7, 2011 to the fall outbreak just a little over a decade prior, the October 9, 2001 outbreak — were both the similarities and differences that kept cropping up. Both days were similar in that they were cool season events, and both were similar in the sense that they involved big western US troughs (most tornado events are in the latter), but a casual look at the skew-t’s for each day reveals many similarities in the overall environment. Yet one was an epic tornado outbreak (for October as well) and the other was a significant tornado event. Why?

While at first blush the upper pattern looks largely similar with troughiness in the west — you see the storm track for the November 7th system was actually further south and west with cooler upper temperatures centered over Arizona instead of the Central Plains. This is likely why November 7 saw a line of storms build with just a single lone tornadic supercell producing tornadoes ahead of it in Oklahoma whereas October 9 had isolated supercells through sunset. The main energy for October 9 was north, which is where the line of storms formed on that day. It’s worth noting there were also tornadoes in Nebraska on 10/9/01.

The location of the upper system led to a much different look at 850mb, but with similarly good shear profiles. With a further east and north location on October 9, the 850mb layer was more SSW instead of due south like on November 7. This moist inflow on November 7 coupled with more energy resulted in more numerous storm development in the southern plains. This limited what would have otherwise been a large tornado outbreak.

Perhaps the most amazing coincidence is that the amount of Surface CAPE on both November 7 and October 9 was exactly the same. Both days had a similar look, with November 7th’s shear actually being somewhat stronger — however it is worth noting in both instances that the soundings were taken after the main action of the day had already passed and more importantly, a little further east of the tornado reports.

So the question is, why did October 9, 2001 produce more tornadoes than November 7, 2011?

I think the answer lies within the upper system’s location, which heavily impacted storm modes. The storm modes on October 9, 2001 were supercellular in Oklahoma, and the storms were isolated for the entire length of the event. On November 7, the Supercell which produced the tornadoes was ahead of the main convective band all day so the storm mode worked against tornado production.

So despite both days having pretty high tornado potential, November 7th failed to produce as many tornadoes simply because storms were not isolated supercells through the majority of the event whereas they were on October 9.

It’s just more proof forecasting a successful chase day doesn’t just involve finding the prettiest colors on a weather map.

We’ll be looking for signs our fall season is set to begin, but right now it appears that day is quite awhile away.