20,000 year storm Harvey could happen again

HOUSTON - Several folks have been asking us about how this Harvey event stacks up historically. No doubt, it was a historic and awful event and many of us we still be recovering for months to come.

Some hydrology experts say it was a 20,000 year storm for Houston.

From that statement, those who aren't familiar with the language of this statistical classification (and rightly so) will assume it means a flood like this happens only once every 20,000 years. That's incorrect.

A 20,000 year storm means there's a 1/20,000 percent chance each year of this happening. Or, in other terms, a 0.005% chance for a Harvey-type flood in any given year. We could see one next year. Or, we may not see one for 80 years ... or 800 years. The, "20,000" number doesn't have anything to do with when we're next likely to see a storm of this caliber.

If it was only a 1,000 year storm, it means there's a 0.1% chance that it could happen again in any given year. A 500 year storm means there's a 0.2% chance. Here's the problem: While there's a 99.8% chance a 500 year storm won't hit in any one given year, Houston saw two of these between the Memorial Day flooding and Tax Day floods, in two consecutive years. It can happen. It will happen again.

On a positive note, if you accept the classification that Harvey was a, "20,000 year storm", there's a  99.995% chance we won't see this happen again next year, or for any other years in our lifetimes. The only problem is that if any statistical chance is greater than 0%, it means we could. Let's hope we don't.

-Brooks
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