Severe storms bring 1-in-1000-year rain event to Tampa Bay

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Jeff Berardelli is WFLA’s Chief Meteorologist and Climate Specialist

A squall line of severe storms blasted through the Bay Area late this afternoon — triggering a tornado warning, with confirmed damage in eastern Hernando County, and a rare flash flood warning in Plant City, which was particularly hard hit.

Doppler radar estimates that as much as 12 inches of rain hit the city in just a few hours, more specifically between 8-10 inches fell in just three hours.

That much rain, in that short period of time, meets the criteria for a one-in-1000-year rainfall.

So even in the Tampa Bay’s tropical, storm-laden climate, that much rain is very rare for such a short period of time. Max Defender 8 rainfall estimates are below.

The map below from NOAA shows the expected recurrence interval for the amount of rain that accumulated in a 3-hour window.

The scale tops off at 200+ years, but right below that map is another image with more detail, showing that eight inches in three hours has an expected recurrence of 1000 years. The “x” shows how much rain actually fell, which is above the 1000-year level.

Over the past couple of weeks there have been many of these so-called “1-in-1000 year” rainfall events across the nation. During summer there is a lot more moisture in the air, so flooding is more common. So let’s break down what a 1-in-1000 year rainfall really means.

Any given area in the US has statistics of rainfall of around 100 years – some less, some more. When you apply stats to the ~100 year rainfall record of that particular area, you can ascribe a value for how often a given amount rainfall should be measured in a given area. It varies by region.

The National Weather Service has a website where it lists the criteria for each point on the US map. In the case of three hours in Plant City, the criteria for a one-in-1000 recurrence are about eight inches. But that does not mean the event is only expected every 1000 years. It means that — statistically speaking — the chance that criteria are met is .001% in any given year in that specific location.

The issue is that we are limited by the historical record and these numbers need to be calculated and inferred from what data we have. Of course, a longer period of record would help to clarify just how rare the event really is.

With all these rare flood events happening now, many have questioned if they are becoming more common. It appears the answer is yes and that’s because the planet is now much warmer than it was 100 years ago. And a warmer atmosphere holds more water vapor.

The equation is very straightforward. For every two degrees Fahrenheit the air warms, 8% more moisture can be held. Global water vapor has indeed increased over the past century by about 7%. Near storms, where moisture is consolidated, the effect is significantly more amplified than 7%.

In many places, this means heavier downpours are falling. Nationwide, some cities have seen a 40% increase of rain in their heaviest rainfall events. The biggest impact has been in the Great Lakes and the Northeast.