Tornado intensity identification for BEGINNERS

[Joshua West]

Hi trackers, first post here so I'd like to start with a brief introduction.

My name is Josh and I'm a radio broadcaster from Auckland, New Zealand, with an amateur interest in weather forecasting. I use local weather station data to forecast the early morning weather for clubbers in downtown Auckland, estimating average temperatures and predicting the arrival time of rain using met service radar images and wind speed / direction.

All pretty basic stuff. So recently, Auckland has experienced a few tornadoes and waterspouts, and this prompted me to do some googling to try and learn what conditions create tornadoes, and as a result I have been able to accurately predict the last two tornado events in Auckland.

What I would like to do now is learn about how to identify the intensity of a tornado, both visually and by using weather data, so our listeners can figure out whether or not the tornado is likely to kill them, and what they should do about it. As New Zealanders, we have never experienced even a SMALL tornado, let alone the full destructive twisters that we read about or watch on television.

How can we tell how big a tornado is? What should we do if we see one coming? Should we stay in our houses or should we try to outrun it? Please note, New Zealand tornadoes are pathetically small in comparison to the ones in the U.S, and they can barely push cars around. Trampolines get thrown, and sometimes old roofing can get ripped off, the most severe damage we've ever had from a tornado was a hatchback through the roof of a shopping mall.
(Actually, one person died - link for reference: http://tvnz.co.nz/national-news/one-man-dead-tornado-wreaks-havoc-in-auckland-4152830)

In addition to this thread, I would also like to invite a tornado specialist to discuss this issue with me live on my radio show at 1200 NZST on Wednesday 22nd of Feb 2012. You can contact me via email, apologies for the link, hope I didn't break protocol Thanks for your time

Josh

 

[rdale]

If all they do is rip a few shingles off and knock over a trampoline, it wouldn't make much sense to get into your car and drive away. You'd actually be at more risk according to your description. It would be a better idea to shelter in the safest part of your house / basement.

 

[Jeff Duda]

Hey, Josh. Great to see you take some initiative and learn however you have to to be able to forecast a tornado. That's not an easy task, so kudos to you right off the bat.

There's a lot to cover from the questions you asked and I know there are many other knowledgeable members on this forum, so I will answer a portion of your questions and leave others to answer other questions or fill in the gaps.

Gauging the intensity of a tornado whilst it is in progress is generally rather qualitative rather than quantitative (unless you can get data immediately from some sensor that took a hit from it...such events are exceedingly rare). I would look at the ratio between the apparent width of the dust column or condensation funnel and the rate of rotation of the funnel/debris cloud at the ground. You can't look at just one or the other because the apparent width of a tornado is a function not only of its strength but also of the thermodynamic environment it's in. Even weak tornadoes that form in a very moist environment with high relative humidity (RH) can appear to be very large. Similarly, not every small tornado is weak. Google the Elie, Manitoba tornado from 2007. It was rated F5 despite having a rather narrow tube for most of its life. It's tough to tell just from the rate of rotation (aka, vorticity) of the tornado just how strong it is, but if you watch enough tornadoes in videos or ever get to see enough in your life, you'll be able to tell the difference between a weak tornado and a much stronger tornado. Having knowledge of the larger scale environment in which it forms can also help you make a more accurate guess as to its strength. Tornadoes that form in high CAPE, high shear environments are more likely to be strong than those that form in more marginal environments.