Here’s what you need to know about flash flooding

Neighbors embrace as they greet each other in front of flood damage on Lancaster Street in Leominster, MA, on Sept. 12, 2023.Jessica Rinaldi/Globe Staff

In Leominster, residents on Tuesday were told to evacuate low-lying areas after a dam was threatened following torrential rains. Flash flooding in the area swamped cars, homes, and some businesses, as raging waters coursed through streets and yards. More rain is in the forecast for much of the state on Wednesday.

But what exactly is flash flooding, and what can you do to stay safe? Here’s what Kristina Dahl, principal climate scientist at the Union of Concerned Scientists, says you need to know. Her responses have been lightly edited for length and clarity.

What is flash flooding?

Flash flooding typically occurs when there’s too much rain in a short period of time for the ground to absorb. It usually happens within a few hours of a heavy rain event but can also result from a dam or levee breaking. What makes it dangerous is that flash flood waters can rise very quickly, which makes it difficult for people to accurately judge the threat and to get prepared.

How does flash flooding happen?

Heavy rain is the most common cause of flash flooding, but whether rain causes flooding depends on a number of additional factors. The state of the land surface, for example, is one important factor. If the land is covered with manmade materials like concrete, or if the soil is already too saturated to absorb water, flooding could happen. Engineered waterways and floodplains can also exacerbate the potential for flooding, including in rural areas. Where the rain falls, how much falls, the topography of the land, the local stormwater infrastructure, and the state of any rivers or streams in the area are also important factors.

An additional factor that has been increasingly in the mix is that old and deteriorating dam infrastructure can fail amidst excessive rainfall events and precipitate catastrophic flooding. Residents of some neighborhoods in Leominster, which has seen extreme flooding in the recent days, [were] put on alert about the potential for a dam failure there [Tuesday morning] and urged to evacuate.

How much rain does it take for flash flooding to happen?

Rainfall on the order of one inch in one hour, or two inches in six hours, is often cited as having the potential to cause flash flooding, but because flash flooding depends on so many factors, there’s no specific threshold for the amount of rain that can potentially cause flash flooding. Most places have the potential to experience flash flooding under the right conditions, so it’s a risk any time excessive rainfall is predicted.

What’s the climate change connection?

Climate change is undoubtedly contributing to growing instances of heavy rainfall events in many parts of the US and the world because our warmer atmosphere can hold more water. Whether that leads to flooding or flash flooding, though, depends on a variety of local factors. Some places in the US have experienced an increase in flood frequency in recent decades, while others have experienced no change or even a decrease.

How can you stay safe?

First and foremost, heed any local warnings from the National Weather Service and your local meteorologists. If you’re under a flash flood warning or watch, be sure to stay away from flood waters. Try to stay indoors and off the roads. About half of all flash flood-related deaths are related to cars, as floodwaters don’t have to be very deep to carry you or your car away. In the wake of a flash flood, be alert to any information about water contamination, as things like sewage and chemical waste can be present in floodwaters and affect drinking water systems.

In the long term, if you live in an area that has the potential to experience flash flooding — and most of us do — make sure you have an emergency plan and supplies in place for yourself and your loved ones. If you find yourself faced with water coming into your home, either get out or get to a higher floor or the roof. If you’re outdoors, try to get to higher ground. And wherever you are, if you’ve been ordered to evacuate, do so as quickly as possible.

After the flood, it’s important to be sure to remove flooded materials so there’s less likely a chance of black mold. Having a safety net like flood insurance can help. Many people falsely believe that their home insurance covers flood damages.

Wherever you are, it’s important to know and understand your flood risk.