New York Metro station flooding. July 2025
It Won’t Be The Last
2025 is following the upward trend in extreme precipitation events we highlighted in the 2023 CAC Climate Conference — our warmer world is primed for more destructive rain events. This year put the spotlight on the most dangerous consequence of extreme precipitation events — flash floods.
Floods affect more people globally than any other type of extreme weather event. In the U.S., floods are the most common and widespread weather-related disasters, and the National Weather Service rates flash floods as the country’s number one storm-related killer.
As with so many other weather-related phenomena, climate change is playing a role in changing the frequency and severity of flash floods.
Types of Floods
Broadly speaking, there are three types of floods — coastal, fluvial and pluvial.
Coastal floods include storm surge floods and tidal floods exacerbated by sea level rise, and are only indirectly connected to precipitation events.
Fluvial floods occur when rivers, streams or lakes overflow their banks. They are typically the result of excessive rain or snowmelt.
Pluvial floods occur when extremely intense rainfall overwhelms the capacity of the land to absorb or remove it, causing water to accumulate and flood the surface. There are two types of pluvial flooding: surface water floods and flash floods.
Surface water floods are common in urban areas, occurring when stormwater drainage systems are overwhelmed, causing water to overflow into streets and buildings. These floods tend to develop relatively slowly and predictably, allowing plenty of time for evacuation. Surface floods are usually shallow (less than a few feet deep) and seldom life-threatening, although the damage to property and infrastructure can be extensive and costly.
Flash Floods — like the events that plagued the U.S. this summer — are a special case of pluvial floods, defined as a flood which follows during or shortly after a heavy or extreme rainfall event. A flash flood happens very quickly — on a scale of minutes or hours. Flash floods are so difficult to predict that the National Weather Service only began issuing Flash Flood Warnings in 1971. As the NWS developed the national network of weather radars and better predictive models, flash flood warnings became more reliable.
Flash Flood Summer
In the U.S., summer has always been flash flood season. The weather is hotter, the jet stream weakens and moves north, and the overall movement of thunderstorms and other precipitation producing systems slows. Slow moving, nearly stationary thunderstorms are a result, potentially generating intense precipitation over an extended period of time and fueling flash floods. Unfortunately, the summer of 2025 has been exceptional for the number and severity of flash floods.
Notable 2025 flash flood disasters include:
- the Guadalupe River flood in Kerr County, Texas, a 1 in 1000 years event;
- record flooding in North Carolina from intense rainfall from Tropical Depression Chantal;
- in Chicago, 1 in 1000 and 1 in 500 years extreme rain events just 2 weeks apart;
- in New Mexico, the Rio Ruidoso flash flood triggered by a 1 in 1000 years rain event;
- New York City and SE New York state flooding powered by exceptional rainfall — one Metro Transit station logged over 2 inches of rain in just 30 minutes. Its previous one hour record was 1.72 inches.
Why Are Flash Floods So Dangerous?
Flash Floods are dangerous because they develop quickly and rapidly build up flood water. In a post-event analysis of July’s tragic Guadalupe River flood by station KSAT, Dr. Chris Combs of the University of Texas at San Antonio highlighted the often underestimated destructive power of flood waters: “Just a gallon of water is about eight pounds…We had, at some of the worst spots, about a million gallons per second of water flowing through those rivers… that works out to around 10 million pounds per second of water…” The resulting damage was devastating.
Events like the Guadalupe flood fall into the National Weather Service “Flash Flood Emergency” category. The NWS has only been issuing Flash Flood Emergency warnings since 2003, “… for the EXCEEDINGLY RARE situations when extremely heavy rain is leading to a severe threat to human life and CATASTROPHIC DAMAGE from a flash flood is happening or will happen soon. Typically, emergency officials are reporting LIFE-THREATENING water rises resulting in water rescues/evacuations.”
A Record Year
The National Weather Service issued almost 4,000 flash flood warnings by the end of July 2025, more than in any year since comparable records began in 1986. In the month of July, NWS issued 1,434 flash flood warnings — the second-highest July total in that 40 year period — and 17 flash flood emergencies. At this pace, 2025 is on track to double the national average for flash flood warnings in a single year.
Fully 83% of the warnings to date were issued during high-intensity rainfall events lasting less than three hours, highlighting both the speed with which a flood situation can develop and the importance of rapid warning capability.
Climate Warming and Flash Floods
As we discussed in 2023 and 2024, climate warming has a direct impact on precipitation via the Clausius-Clapeyron thermodynamic relationship — the atmosphere can hold 7% more moisture for every additional Celsius degree. The effect of this exponential relationship is to increase the intensity of the most extreme precipitation events — the events most likely to generate flash floods. As Prof. Andrew Dessler (Atmospheric Science, Texas A&M) puts it “…climate change doesn’t cause rain events. Rather, the role of climate change is like steroids for the weather — it injects an extra dose of intensity into existing weather patterns.”
Atmospheric scientists at Berkeley Earth project that 2025 will be the 2nd or 3rd warmest since the mid-1800’s, continuing the upward trend in global climate warming. (2023 and 2024 are the warmest years). July 2025 was the 3rd warmest July on record, surpassed only by 2023 and 2024. So, this summer there was no shortage of atmospheric steroids to boost extreme precipitation.
Scientists at Climate Central analyzed changes over time in the total precipitation falling on the heaviest 1% of days. They found that as the climate warmed from 1958 to 2021, the most extreme precipitation days intensified in every major U.S. region, led by the Northeast (+60%) and Midwest (+45%).
UCLA climate scientist Daniel Swain sums it up: “Increasingly extreme downpours are one of the clearest meteorological manifestations of a warming atmosphere. This is already detectable in most places with adequate records (including the eastern U.S.); it will almost certainly become so elsewhere sooner vs later.”
Swain notes that most of the “very most extreme” precipitation events in any given location are usually associated with strong, long-range, atypical moisture transport from elsewhere. In an interview with CNN, he points out that during much of the 2025 summer, atmospheric conditions over the U.S. funneled humid air north from the unusually warm Gulf and the western Atlantic, including the Gulf Stream.
Pluvial/Urban flooding
A notable impact of the extreme rain events boosted by climate warming is an increased risk of pluvial flooding (urban surface flooding when drainage infrastructure is overwhelmed) and flash flooding on fast-responding watersheds with limited capacity to absorb the intense rainfall.
In cases of pluvial flash flooding in urban settings or on fast-responding watersheds, the sheer intensity of rainfall is the most important factor. And torrential downpours lasting minutes to hours are clearly increasing in a warming climate.
The tragic Texas flash flood is an example of a flash flood on a fast-responding watershed. Factors leading to the catastrophic surface water flooding included a preceding drought, steep terrain with a thin layer of hard, dry soil covering underlying limestone, an abnormally hot Gulf of Mexico pumping moisture into the atmosphere, and a cluster of thunderstorms fueled by the remnants of Tropical Storm Barry. The thunderstorms dumped 2 to 4 inches of rain an hour on the unreceptive landscape, and the cluster of storms was moving very slowly, so it essentially stalled over the landscape. NOAA estimates 20 inches of rain fell in a few days. leading to widespread 1 in 100 year flooding and localized 1 in 1000 year flooding.
In an interview by The Grist concerning the factors contributing to the Texas flash flood, Brett Anderson, senior meteorologist at AccuWeather, predicted that “A lot of these places, 1 in 100 year floods may be becoming more like 1 in 50, even 1 in 10.” AccuWeather’s preliminary estimate puts the economic damage of the flooding at between $18 billion and $22 billion.
Flash flooding in Great Neck, Long Island, July 31st, 2025
Urban flash flooding is becoming more common, not only because extreme rainfall events are becoming more frequent, but also because cities are expanding, creating ever-larger impervious surfaces. Flooding is inevitable if the rate of rainfall exceeds the capacity of storm sewers and other drainage infrastructure.
In the case of New York City, the century-old sewer infrastructure was designed for a climate where peak rainfall is less than 1.75 inches per hour. On July 14th, 2025, the city received over over 2 inches in an hour. But that wasn’t a record. In the city’s first-ever Flash Flood Emergency in 2021 the remnants of Hurricane Ida dumped 2.64 inches, beating a previous record set in 1908! These extreme events are not unexpected. As Climate Central found, the Northeastern U.S. is ground zero for intensifying extreme rainfall.