Flash floods are increasingly dangerous and expensive in the Northeast. These charts and maps show why.

Here are four things to know about more extreme precipitation.

A woman navigated the flooded intersection of Ferry Street and Centre Street in Malden as a heavy rain fell in August 2023.Jessica Rinaldi/Globe Staff

Flash floods are becoming more severe across the Northeast as climate change worsens, scientists have found. That’s taking an increasing financial toll on communities hit by expensive flood damage.

Between early July and mid-September 2023, Massachusetts experienced between 18 and 25 inches of rainfall, several times the normal amount. Extreme and sudden rainfall can prompt flash flooding.

Here’s how flash floods are affecting the Northeast:

The number of extreme precipitation days is increasing.

The planet has already warmed roughly 1 degree Celsius since the preindustrial era. (Even though the planet has warmed 1.5 degrees over the past 12 months, eclipsing a critical threshold scientists have long warned against, the Earth’s climate is measured in long-term averages taken over decades.)

Precipitation in the Northeast has increased on average in every season of the year, scientists have found.

Days with extreme precipitation events, where rain or snowfall totals have substantially exceeded what is normal, have increased by 60 percent in the Northeast, according to the National Climate Assessment. That’s the largest increase of any region in the nation.

Extreme precipitation events are more frequent now than in the 1950s.USDA Forest Service, Drexel University, NOAA NCEI, and CISESS NC

Days with extreme precipitation are likely to increase as climate change worsens.

Climate scientists say the planet is on track to exceed 1.5 degrees Celsius of warming for a sustained period sometime in 2030s. Strong policies that cut greenhouse gas emissions could still limit global warming to about 2 degrees.

As climate change worsens over the coming decades, the number of days with extreme precipitation will continue to increase, climate models suggest, depending on how much warming occurs.

As flash flooding has increased in Mass., the state’s disaster agency has begun responding to more localized ‘gap disasters’

Disaster response is typically financed in Massachusetts either by one-time appropriations by state lawmakers or federal disaster aid. But as flooding becomes more frequent, the number of localized disasters is increasing, and many of these events don’t qualify for federal aid.

The number of times that the Massachusetts Emergency Management Agency has responded to a disaster with regional emergency operations teams indicates the rise in these gap disasters.

Many properties in the Northeast are not covered by flood insurance.

A fairly low number of property owners maintain flood insurance in the Northeast. Typically between only 1 to 10 percent of homes in most counties in the region are covered by a National Flood Insurance Program policy.

Those figures don’t capture flood insurance purchased in the private market. But the proportion of homeowners who maintain private flood insurance also tends to be low in New England, said Shannon Hulst, a floodplain specialist at Barnstable County’s Cape Cod Cooperative Extension.

“Regular homeowners insurance doesn’t cover flooding,” Hulst said. “That misunderstanding is a big reason why most people don’t have it.”

People who carry mortgages in high-flood risk areas are required to have flood insurance. Even outside of high-risk areas, many lenders require it for a mortgage. But for people who own their home outright or rent, the typical homeowners and renters insurance policy usually does not include flood damage.

Federal flood maps are also outdated for today’s climate reality. The maps that determine high-risk areas do not factor in flooding caused by intense, localized rainfall events that are becoming more common in the Northeast due to climate change.