Not long ago, we knew where we had to worry about floods. People with businesses or buildings on flood plains or ocean coastlines knew they faced the possibility of flood, even if they hadn’t been inundated themselves. Now, though, all bets are off. Everywhere is at risk.

It used to be that people on the coast knew the danger from experience, since nearly every coastal city and town in the USA has been impacted by tropical storms or hurricanes over the past 60 years. For inland denizens, confident awareness of flood risk (or its happy absence) began with flood mapping by the Federal Emergency Management Agency in 1968. That knowledge developed over many years, but for decades everyone in a FEMA-defined flood area has known about the danger. 

Unfortunately being away from the coast or outside a FEMA zone no longer means you’re safe from flooding. Something really has changed: intense localized rainfall events have begun to occur with regular frequency, reaching a crescendo over the past year which is not likely to abate. These downpours have readily demonstrated that everyone is exposed to flood.  

We’ve experienced multiple “not-in-a-generation” floods caused by rain in just the past few months:

Sept 2023: Intense storm floods in metro New York City delivered 12.76 inches of rain over the month of September, three times greater than the historical average.  

Sept 2023: Intense concentrated rainfall in Leominster, Massachusetts threw down 9.5 inches in one day, and a monthly precipitation total also three times average.

August 2023: Numerous rainfalls over Detroit delivered more than three inches of rain in six hours or fewer. The worst of them dropped 7.36 inches.

July 2023: An intense storm flooded towns in New York and Vermont, including the state capitol of Vermont, with 6.8 inches over three days, of which 5.28 inches fell during a single day.  Once again, the monthly total was 300% of historical.

Other surprising and damaging rain events: 

• Pennsylvania in September: 4 inches in one day

• West Virginia in August: 4 inches in six hours

• Western Kentucky in July: 11 inches in one day

• Pennsylvania in July: 7 inches in one day

• Florida in April: 22.5 inches in one day, with 26 inches (more than two feet!) over four days in Ft Lauderdale – without a hurricane

• Eastern Kentucky in February: more than 4 inches in one day

• California in January and February: atmospheric river events 

These are inland storm rainfall events, not the more familiar tropical storms. Fortunately 2023 was a quiet year for tropical cyclones. Notable were Idalia, the only Gulf of Mexico hurricane of the year, and Hilary, an unusual Pacific storm. They were not particularly devastating, except to those not expecting that much rain.

Surprisingly, given the sheer number of inland floods, 2023 has so far been unexceptional in terms of total rainfall. Average US precipitation in the first nine months of the year has fallen into the middle third of NOAA’s 129-year record, but its concentration has been obvious. More rain is falling during storms, and less is spread out over the days between them.

reThinking Flood

In response to this development, we need to rethink the way we understand flood risk. We have typically and always imagined coastal and inland floods to be caused by water rising from a river or ocean, and from there, coming up to and into a building. But the floods listed above were caused by deluge rainfalls that came down into buildings as surface runoff. 

When small creeks overflow their banks from upstream rainfall, bridges and foundations can be washed away, spreading damage well beyond the stream, and over new routes. Developers and builders work hard to manage stream, creek, and storm-water runoff, but that work defines flood boundaries. The edge of every area of flood exposure includes assumptions about the integrity of water management, as well as the probability of the casual event. But mother nature doesn’t respect boundaries or assumptions, and she can wield probability viciously. Add the increased intensity of rainfall caused by the warming atmosphere, and sober estimates of flood probability can be rendered meaningless.   

Flood damage to a culvert. Photo: Ed Haas

The 2023 downpours may seem unusual, but climate science tells us that a warmer atmosphere holds more moisture. That water must eventually rain down upon us, and since warmer air unloads its moisture much more quickly when it meets a cold front, it will do so with greater intensity. 

Mean US temperatures from January through September were 1.9^oF above average creating an increase in frequency and severity due to climate change. Therefore, we need to consider overall atmospheric temperature when thinking about flood risk, and rethink where to worry about floods. 

For brokers, this means educating your customers about their flood risk and ensuring they are adequately protected with insurance, even if they are not in an area that has historically been at risk of flooding.