Madness at #AGU2018 – probability based tornado warnings

Do You Want More Complex Tornado Warnings??

Guest essay by Mike Smith, CCM

This is the story of a bad idea: probabilistic storm warnings. 

Under the current storm warning system, you are either in a storm warning or not. For example, in the radar illustration below, the red area is a tornado warning which is nested in a yellow severe thunderstorm warning (large hail or damaging winds).

So, if you are in the tornado warning, your weather radio or tornado siren goes off, or the TV cuts in to let you know. You immediately go to shelter. That’s it; that’s all a member of the public has to know.

Unfortunately, the meteorological research community has been in love with the idea of probabilistic weather forecasts (“20% chance of rain”) for decades and now wants to extend that concept into tornado and other storm warnings. In their minds, probabilities of flash floods and tornadoes (to name two) are “more scientific” than “deterministic” (yes or no) warnings.

What might a probabilistic tornado warning look like? Below is an example:

From a paper on probabilistic storm warnings
presented yesterday to an American Geophysical Union meeting

Instead of “Take Cover!” (or not) as with the current system, your local television station will show you a graphic similar to the above that will update as often as every one minute.

Instead of, “Go to the basement!,” you will learn you have a 70% chance of tornado in the next 15 minutes, a 40% of a tornado in the next 30 minutes and a 10% chance of a tornado in the next 40 minutes. And, the probabilities you are given will change at least every five minutes and, perhaps, as often as every one minute. Really.
This is a terrible idea that will cost lives.


Why is this a bad idea? Let me count the ways:

  • Surveys (here, among many others) show the public does not adequately understand what a “20% chance of rain” officially means (other than 80% is more than 20%). Those probabilities (and it rains far more often than a tornado occurs) have been around for a half-century.
  • If we have not been able to educate the public over a half-century as to the meaning of probability of precipitation, there is virtually no hope of educating them to learn that a 10% chance of a tornado in 30 minutes is actually high.
  • Unquestionably, you should go to the basement or take other shelter if there is a 40% chance (which is very high) of a tornado in the next 20 minutes. I confidently forecast this 40% number will cause mass confusion (deadly when dealing with life-threatening weather) and far fewer will take shelter than do now — and, that deaths and injuries will almost certainly rise.
  • With the colors and numerical probabilities constantly changing, they will be nearly impossible to convey on television and online in a meaningful way. In the above illustration, part of the area in the tornado’s path is green. Green is a color associated with safety, not danger.
  • Commercial radio and weather radio will not be able to convey this type of warning at all.
  • What do you do with tornado sirens?
  • These probabilities will be uncalibrated. Because tornadoes are so rare at a given location, there will be no way to know if a 55% chance of a tornado is actually meaningful or if it is just a number.

I first heard about this concept at a meeting in Norman, Oklahoma, five years ago this month. I, and others, raised these concerns then and since but those concerns have fallen on deaf ears. To give a simple example, I’ve mentioned that green is not a color that should be used in tornado warnings. As recently as yesterday (see above illustration), it still was.

What makes this even worse is that the existing tornado warnings continue to decline in accuracy — a topic we have discussed several times on this blog; if you want a recent example of a missed warning of a major tornado, go here.

via Watts Up With That?

https://ift.tt/2UErHzZ

December 12, 2018 at 06:54PM

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