Is it time to rethink how we classify big weather events?

DENVER — It feels more frequent these days that Colorado is breaking weather record after weather record. From record heat days to record droughts to big rainstorms, Colorado and states across the country are experiencing extreme weather events more regularly.

“We’re absolutely seeing a new normal in the trends of the number of records we’re breaking,” said Paul Schlatter, a meteorologist with the National Weather Service office in Boulder.

While it’s difficult to say if an individual storm is more powerful due to climate change, there is no doubt, for scientists, that it is playing a factor in the overall frequency of these events.

Climate change results in a warming of the Earth. When that happens, the air can hold more moisture, so when it rains, there’s much more of it.

“The thing to consider with a warming climate is, you’re essentially just giving it a shot in the arm or a jolt of steroids, if you will, where those types of events will be more intense and more common,” Schlatter said. “Climate change is that steroids, where events will be more intense and more frequent as we go forward.”

100-year storms are storms that are so intense they only have a 1% chance of happening during any given year. That’s not to say that a storm of that size will only happen once a century, only that it has a low probability of happening.

To determine what constitutes a 100-year storm, scientists have used historical data to figure out annual rainfall in a particular area. Different regions of the state may also look very different for what a 100-year storm looks like.

Because some areas had better record-keeping and tracking, scientists are more confident about what a 100-year storm is in some places compared to others. However, researchers say the data is not perfect.

“We know that the way that we had come up with recurrence intervals and 100-year storms in the past could be improved. It was started in the 1950s, and it was started on extremely limited observations. The science has come a long way since then,” said Kelly Mahoney, a research scientist with the National Oceanic and Atmospheric Administration (NOAA).

However, the historical data for what constitutes a 100-year or 1,000-year storm did not account for climate change.

Now, NOAA is leading the research on how to update the characterization of these major storm events by factoring in the growing impact of climate change. Then, it will be up to policy makers to determine what to do with that information to make cities and states more prepared and resilient to these storms.

As record heat, storms become more common, is it time to rethink how we classify big weather events?

However, there is another factor to consider. Human changes to the geography also affect how big storms affect a particular area.

A century ago, there was much less infrastructure in Colorado than there is today. More open space was better for these big rain events because the soil was able to absorb the water.

“The rainfall came down, it soaked into the soil, it was absorbed, it was a slow reaction. There wasn’t rainfall pooling everywhere. Now that rainfall pours onto concrete, developed surfaces, there’s nowhere for it to go. It rushes off, it’s really fast moving, so then you get a flash flood,” Mahoney said.

So, infrastructure that was developed years or decades ago to withstand a 100-year storm might not have taken future development into consideration and now might only be able to withstand a 10 or 20-year storm with the changed landscape.

For now, NOAA researchers are hoping the new data will be useful to cities and states as they prepared for more frequent and more intense weather events.

“You hope for the best, but you absolutely need to plan for the worst. And unfortunately, the worst-case scenarios are becoming worse than we previously had planned on,” Mahoney said.

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