Interesting Weather Information

Tuesday, July 11, 2017

Tornado Warning 2017.07.07 by @NWSILN - No Tornado but a Good Warning Part I

What @NWSILN Saw on Radar that Lead to the Warning

The Grey Zone
In the grey zone of meteorological knowledge is that shadowy region between rotation in a thunderstorm and damage on the ground thousands of feet below when a tornado touches down.

Thanks to doppler radar we can clearly see rotation in thunderstorms. Thanks to intense research efforts we now know that the rotation for a tornado is mostly imported to the storm, drawn in with the inflow and tilted to nearly vertical by the updraft. As the inflow is stretched by upwards acceleration in the updraft the rotation rate increases. Spin-up is taking place.

Think of the ice skater.

VoilĂ ! We can now forecast and warn for tornadoes.

Not so fast.  Between cloud base and the ground, in the grey zone, something happens. What it is and how it works is masked by a fog created by an environment too dangerous to make observations, by the small scale of the funnel and by the capricious nature of tornado occurrence.

A Weak Link
We have a weak link in our cascade of knowledge between rotation in the storm and rotation in the environment leading to a tornado on the ground.

That weak link is one reason there are so many false alarms -  tornado warnings without a tornado.

Friday evening, 2017.07.07 was one of those false alarms. But not all false alarms are equal.

A number of tornado warnings have been issued over the years by @NWSILN that have made me cringe. The polite version of my initial comment would be, "Just what are they thinking?" You can probably guess what the gritty version contains.

The tornado warning of 2017.07.07 was not one of those. I was a good but an un-verified warning. Good, because all the elements were there for a tornado touchdown.

Radar Views from Friday Evening 2017.07.07
Take a look at KILN Super-Res Velocity at 5:25PM EDT (21:25 UTC) 2017.07.07. below.

Doppler radar can only measure the component of the wind directly towards or away from the radar which is along a radial like the long arrow.  It does not measure the true wind only the "radial" part of the wind.

At 5:25 PM EDT KILN saw a tight velocity couplet between Oxford and Germantown. Red indicates wind blowing away from the radar or positive velocity. Green indicates winds blowing towards the radar - negative velocity.

Here are some closeup screen grabs.

On the left is reflectivity - precipitation  rate, on the right radial velocity. Remember the true velocity is higher because radar is looking only at the wind component along the radar pulse radial path.

Velocity values peaked a bit higher about 10 minutes before these images.

This is a tight velocity couplet and could indicate enough spin-up for a tornado to touch down. In addition the couplet weakened and strengthened several times, indicating here was plenty of energy and rotation and the couplet was not yet dying. In this case I feel the warning was a good one.

Watch the video to see the velocity couplet form, move and dissipate.

Coming Soon:
Part II - The Meteorology of the Spin-up
Part III - 3D Radar Views

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