Expert: 20-year storms will become more frequent.
Tropical Storm Henri dumped as much as 5 inches of rain on some Connecticut communities, and one expert says the state should prepare for storms like this to hit more often.
A storm that drops 4.8 inches over a 24-hour period had been thought of as a one-in-20-year event, but Guilin Wang of the University of Connecticut said those extreme weather events will get progressively more extreme as the century progresses.
“It's going to become more frequent,” she said. “So, previously, it was a one-in-20-year event, but by mid-century, it's going to become a one-in-10 year event, or one-in-five-year event. So we are going to see those extreme events more frequently.”
Wang, a professor of environmental engineering, said that this is not new information.
“Theoretically, we knew that we are going to be looking at more intense precipitation,” she said.
What’s new is the detail in global emissions data, allowing Wang and her colleagues to drill down to the state level and predict with some degree of confidence increased rainfall over the next 80 years in Connecticut.
What Wang found, and included in the Connecticut Climate Assessment Report produced by the Connecticut Institute for Resilience & Climate Adaptation, is that extreme weather events will be both more extreme and more frequent.
“All indices representing the frequency or intensity of heavy precipitation are projected to increase for both mid-century and late century,” that report says. “Extreme events of a given size are projected to occur more frequently in the future approximately threeto four-times as often during the mid-century and two- to three-times as often during the late-century.”
This means, as Wang explained, that a storm the size of which we might expect to see once every 20 years could happen as much as four times as often by the year 2050.
But a one-in-20-year storm does not mean those storms will happen every 20 years.
“The definition of onein-20-year events should be the probability of that happening in any given year is 5 percent,” Wang said. “You could have one-in-20-year events every year for three years. You can calculate that probability. There is a probability for that to happen, and it's not that low.”
At the same time, Wang expects fewer wet days — more frequent and heavier storms, but fewer days of lighter rainfall. Given this, the average daily rainfall in Connecticut should not increase significantly, even as storms get worse and arrive more often.
On average, Connecticut sees 0.4 inches of rain every day. That should increase only about 10 percent by the middle of the century.
“The days with light rain might become lighter, and the days with the heavy rain will become heavier,” she said.
Theoretically, Connecticut and the rest of the Northeast could struggle with drought, as well as flooding.
“We are going to see more flooding, because the rain is going to become more intense,” Wang said. “And we are going to see more frequent and more intense drought, because precipitation is going to become further apart.”
“We are looking at increased drought risk and increased flooding risk at the same time,” she said.
As an environmental engineer, the question for Wang becomes whether or not Connecticut communities can handle increasingly intense and increasingly common storms.
On average, the largest storms drop 2.8 inches of rain on Connecticut in any 24-hour period, but that doesn’t mean the infrastructure of any given city or town can handle that much rain.
Municipalities, Wang said, need to ask specific questions, like “is this dam going to fail? Or is this channel that we designed for 20-year events, will it get flooded?”
Though she acknowledged there is significant work being done to prepare communities statewide for major storm events, when asked if the state has the infrastructure to handle 20-year storms every five years, Wang said. “Probably not yet.”