What at first appeared on radar as a relatively modest line of storms moving into Connecticut, was suddenly supercharged into a deadly flash flood — the same weather pattern responsible for Vermont’s two rounds of devastating flooding in July.

Stagnant, ‘training’ storms

Like trains on a track, Sunday’s thunderstorms kept multiplying over the same region, inundating parts of Fairfield and New Haven counties with flooding rains. A region that normally sees 3.5 inches of rain on average for the entire month of August was pounded with 13 to 16 inches of flooding rains, most of which fell within six to seven hours, state officials said.

Meteorologists said the deadly confluence of a very slow-moving system, an already extremely moist summer weather pattern and these “training” rainstorms wreaked havoc on the region, stretching from Fairfield to the Lower Naugatuck Valley, including Oxford and Monroe. Training storms develop when thunderstorms repeatedly move over the same area, generating from the outflow of previous storms, over and over again. Each storm is following along a path, lining up over the same region one after another like train cars on a track, except the track is not moving.

“Deep tropical moisture was in place, and there was no shortage of water vapor to work with,” said Ryan Hanrahan, chief meteorologist for NBC Connecticut. “A very narrow band of convergence developed, and thunderstorms kept redeveloping over this convergence zone.” Examples of convergence zones are fronts and boundaries like sea and land breezes. They act as a trigger for thunderstorms and in the Connecticut and Vermont flooding events, that zone of convergence barely moved.

The storms were “able to back build at times” and the hardest-hit town of Oxford, where two people died, swept away by floodwaters, was “in the bull’s-eye,” said James Tomasini, a meteorologist with the National Weather Service in New York.

Research by climate scientists Jennifer Francis, Stephen Vavrus and Michael Mann has found that climate change has resulted in these stagnant patterns that allow storm systems to stall out, increasing the likelihood of flooding events both on a small scale (like in Connecticut) and on a larger scale (like with Hurricane Harvey in Houston in 2017).

Meteorologists said the Connecticut flooding was unrelated to Hurricane Ernesto, which on Monday was over the open Atlantic Ocean.

Forecast models missed the mark

The intensity of last weekend’s storms caught forecasters by surprise. Even the morning of massive flooding, models predicted only 6 inches of rain.

“These events fly under the radar. We need to do a better job at looking at these localized, very high impact events,” Hanrahan said.

Today’s forecast models, although sophisticated, still lack the finer resolution to accurately capture and pinpoint small-scale variations in terrain, weather patterns, and local atmospheric conditions.

But despite the models miscalculating the dire nature of the approaching storms, warnings from the National Weather Service were issued well ahead of time, meteorologists said. The first flash flood warning of the day was issued at 9:26 a.m. on Sunday, according to James Tomasini, a meteorologist with the NWS in New York. The urgency of the alerts increased throughout the day and eventually a Flash Flood Emergency was issued by mid-afternoon, warning residents to take swift action to protect themselves from rapidly rising floodwaters.

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1-in-100 years now every 20 years

A warming climate, scientists say, has increased the frequency and intensity of flooding in the Northeast due to heavier bursts of rain.

The traditional “1-in-100-year” rainfall event — or, a storm with a 1 percent chance of happening each year — now occurs about every 20 years in Connecticut, according to an analysis conducted by Jeremy Porter, head of climate implications research for First Street Foundation, a nonprofit that quantifies financial climate risk.

In other words, the same type of extreme rainstorm that was once a rare occurrence in Connecticut is five times more likely now.

Extreme precipitation has increased by about 60 percent in the Northeast since the 1950s, according to the Fifth National Climate Assessment. Flooding events are expected to become “flashier” across the US as the climate warms, according to a 2023 federal meteorological assessment of extreme weather trends.

The driving force behind this shift in weather patterns has been our warming planet, say meteorologists and other scientists. According to Shel Winkley, a meteorologist with Climate Central, when the atmosphere warms by just 1 degree Fahrenheit, there is 4 percent more water vapor in the atmosphere, meaning more water is now available during these heavy rain events.

High ocean temperatures don’t help, as they add further moisture into the atmosphere. That abundant moisture is often injected into storm systems such as the one Connecticut experienced Sunday. Warming ocean currents off Connecticut are 10 times more likely due to climate change, according to the Climate Central Climate Shift Index.

University of Connecticut researchers project that annual precipitation in the state could increase by as much as 8.5 percent by the middle of the century compared to the 1970s if there is no action to reduce emissions.

James O’Donnell, executive director of the Connecticut Institute for Resilience and Climate Adaptation, said limited state data on precipitation and global climate models that are difficult to scale down make it challenging to predict where extreme flash flooding could occur.

But even if scientists can’t pinpoint exactly where intense flooding will worsen in a warmer climate “It’s likely that extreme precipitation events will increase in frequency,” O’Donnell said.

“These events — 10 inches in a day, or 12 inches in a day — we should plan for that everywhere in Connecticut,” he said.

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‘No infrastructure can sustain it’

State officials and climate experts say Connecticut’s roads, bridges, and stormwater drainage system, much of it aging and built for a climate that no longer exists, like that of Vermont, cannot withstand such extreme rainstorms.

Rain totaling 13 to 16 inches, roughly 6 inches an hour, quickly swelled into dangerous rushing floodwaters that coursed through the town of Oxford, about 25 miles northwest of New Haven.

“It was a catastrophic amount of rain — no infrastructure could sustain it,” said state Senator Tony Hwang, who represents Newtown and Bethel, two towns also hard hit by the severe flooding.

Much of Connecticut’s climate adaptation efforts have been focused on protecting communities from storm surges as sea levels rise, particularly after devastating flooding from Superstorm Sandy in 2012.

“It’s not just sea level rise and coastal storms, but also the potential for inland flooding that we need to think about,” said Christine Kirchhoff, who worked on Connecticut’s climate resilience planning efforts.

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The challenge, she said, is that adapting to a changing climate is “a moving target” so long as fossil fuel emissions are added to the earth’s atmosphere. “What we adapt to today will be different from what we need to adapt to tomorrow,” Kirchhoff said.

Michael Jastremski, watershed conservation director for the Housatonic Valley Association in western Connecticut, said that there are thousands of culverts in the state that carry stormwater and streams beneath roads, yet many of them are too small.

The association surveys culverts and advocates for larger storm infrastructure. In an initial assessment of culverts in the Housatonic River watershed, he said, about 18 percent of the culverts they’ve surveyed would fail in a flood that occurs on average once every 25 years, meaning that water would overtop the road that the culvert is intended to protect.

“This could’ve been a regional event, and that’s what keeps me up at night,” Jastremski said. “When this is happening in 20 towns or 30 towns instead of eight or 10.”

Meteorologist Chris Gloninger is a senior scientist in climate and risk communication at the Woods Hole Group. Follow him at @ChrisGloninger.

Erin Douglas can be reached at erin.douglas@globe.com. Follow her @erinmdouglas23.