As flash floods rage, the tropical Atlantic stays mellow » Yale Climate Connections

During most of July, the polar jet stream has retreated into southern Canada, leaving upper winds light across most of the United States. As a result, thunderstorm complexes can erupt based on small-scale weather triggers, or they might congeal around the remnant circulations of the previous day’s storms. This kind of a spin-up is called a mesoscale convective vortex, and one was involved in the Kerr County disaster, which also drew on the four-day-old remnants of Tropical Storm Barry.

Such patterns are typical of midsummer in the United States. But when the inevitable thunderstorm complexes do take shape, they’re now drawing on an atmosphere whose average moisture content has been increased by long-term climate change. As the planet warms, more water evaporates from drought-stricken landscapes, worsening the drought impacts, as well as from oceans, adding to the airborne moisture supply that’s available to flow into storms where they occur.

Multiple rounds of the U.S. National Climate Assessment have confirmed that the wettest 1% of days are getting even wetter, especially toward the Midwest and Northeast (see graphic below). With the help of new, more fine-grained datasets and models, scientists are now probing sub-daily rainfall (amounts that fall over less than 24 hours, the kind that trigger flash floods), and they’re finding that the rainfall-intensifying effects of climate change are even greater as you zoom in to shorter time periods.

A study released in June examined hourly data from 332 U.S. rain gauges and found upward trends of 20% to 40% in short-period rainfall from 1949 to 2020, with the increases especially sharp since 2000. Whether a flash flood occurs depends on other factors, of course, including exactly where intense rainfall overlaps with watersheds and how development has affected where the water flows, but the hydrologic dice are clearly getting loaded in favor of flash floods.

Among other deadly U.S. flash floods in recent days:

• Torrential rains swamped parts of the greater New York City area on Monday, July 14, extending into central New Jersey. Two people died when their vehicle was swept away by floodwaters in Plainfield, NJ. A CoCoRaHS observer in Westfield, NJ, measured 6.24” in the 24-hour period ending Tuesday morning. Amounts in New York City reporting stations ranged from 1.02” (JFK airport) to 2.64” (Central Park). These were well short of a July record for daily rainfall, but the 2.07” measured at Central Park from 6:51 to 7:51 p.m. was the second-heaviest total in hourly data for the site going back to 1943, surpassed only by the 3.15” during Hurricane Ida, and once again enough to swamp multiple subway stations.

• Three people drowned on July 8 when the Riudoso River tore through Ruidoso in central New Mexico, setting the highest crest on record (20 feet), five feet above the prior record in data going back to 1998. In an increasingly common phenomenon, landscape scarring from fires in 2024 exacerbated the floods, according to an NWS forecaster quoted in the New Mexico Daily Lobo. At least 50 swift-water rescues were conducted.

• Flash flooding was also associated with Tropical Storm Chantal as it moved into easternmost South Carolina on July 6 and spread heavy rains of 10″ or more across parts of the Piedmont of central North Carolina. At Durham, the Eno River crested at an all-time high of 25.63 feet, and dozens of water rescues were carried out. At least six deaths were reported from Chantal-related flooding across the state.

Flash flooding will be possible over the next several days across a broad band from the Upper Midwest into the Ohio Valley, as rich moisture continues to flow toward a weak frontal zone.

All quiet in the tropical Atlantic this weekend

The disturbance known as Invest 93L moved inland on Thursday in southern Louisiana after hugging the upper Gulf Coast for several days as it trekked westward. Though it never managed to organize into a tropical storm (despite amped-up videos on TikTok warning of an imminent hurricane in Houston), and its heaviest rain stayed largely offshore, 93L managed to dump widespread rainfall totals of 1 to 3 inches in south-central and southeast Louisiana, with a few isolated pockets of heavier amounts.

No tropical development is expected in the Atlantic over at least the next seven days, according to the  Tropical Weather Outlook issued by the National Hurricane Center at 2 p.m. Friday, July 17. Only three tropical storms have formed in the Atlantic so far this year. Over the period 1991-2020, the average date of the fourth tropical storm is August 15, and the average date of the first hurricane is August 11, so climatology reminds us that the great bulk of hurricane season still lies ahead of us.

Jeff Masters contributed to this post.