Twin Tornadoes Are Rare, But Also Take Many Forms

Yesterday's giant twin tornadoes in Nebraska.  

Last night me, and a lot of other weather watchers, were hyperventilating over those twin tornadoes that wrecked the town of Pilger, Nebraska, killing a five year old kid and injuring several others.  

I, and many others, said they have never seen anything like it: Two strong tornadoes, shoulder to shoulder, roaring across the Plains.

But there have been a few similar incidents over the years.

However, it remains to be seen if there have ever been two tornadoes as strong as the Nebraska ones co-existing a mile or less apart.

The closest thing I could find to yesterday's twister: Twin funnels
Elkhard, Indiana, April, 1965  

The closest thing I could find to yesterday's Nebraska disaster was a tornado in Elkhart, Indiana on April 11, 1965.

The famous photo of the tornado shows two distinct, large, strong funnels. But it's unclear if that is two independent tornadoes, or a one multivortex tornado.  

Multivortex tornadoes are more common than two distinct tornadoes near each other.

Multi-vortex tornadoes are indeed one twister, but they contain mini tornadoes that swirl around inside the main circulation of the funnel.

The mini funnels help explain why you often seen tornado damage in which one house is destroyed while another just feet away has relatively minor damage.

Another example of twin tornadoes.  A view from Yankton, South
Dakota across the Missouri River to Nebraska shows these
two twisters in May, 2007. These storms were weaker than
the ones that hit Nebraska Monday.  

The mini-funnels inside a multi-vortex tornado focus and intensify the wind in small paths within the larger path of the parent tornado.

That's why the damage is so variable in many tornadoes.

In other cases, two tornadoes exist at the same time within one supercell thunderstorm. These giant storms often cycle tornadoes.

One forms, carves a path of destruction, then starts to weaken in favor of a new tornado nearby.

Usually, though, one of the tornadoes is weak, while one is strong. I wonder if yesterday's Nebraska tornadoes set some sort of record for two nearby tornadoes co-existing while maintaining their strength.

This looks like three tornadoes, but it's one multivortex tornado
The funnels are rotating around a main circulation within one twister.  

I'm sure the meteorologists will be studying this one really intensely.