A new study has taken a closer look at what happens when high rain fall combines with a storm surge, a departure from how scientists have typically analyzed the two main drivers of flooding, and they have found that the flood risk across the United States has been greatly underestimated.

Disaster experts have analyzed storm surges and high rainfall to define flood zones and generate preparedness plans, but a study published in the journal Nature Climate Change suggests that experts aren’t combining the two to examine their effects when they occur in the same place at the same time — and in the last 100 years, the threat of these types of events has been dramatically increasing, according to a TIME report.

New York City has been particularly vulnerable, experiencing a large increase in so-called “compound events” in which a storm surge combined with heavy rainfall. This is due to weather patterns that favor both conditions.

In an even more worrying finding, the scientists found that these compound events double the odds of a flood in the city.

Typically, an extreme storm surge would be necessary to cause flooding, or a period of very heavy rainfall. But now, scientists know that combining these two events — even when neither is extreme — could cause large damages than either of them on their own, no matter how extreme.

What is your risk of experiencing these compound events? It varies depending on what city you live in, but cities that are most likely to see hurricanes — i.e., cities on the East and Gulf Coasts — are more likely than cities on the West Coast.

The research focused on storm surges and precipitation, but scientists noted that long-term rises in sea level due to climate change represents a bigger driver of greater flood risk. A recent study suggested that sea levels may rise by 10 feet over the next century.