Dallas Deluge Part of USA’s Summer of Floods

August 23, 2022

Bob Henson for Yale Climate Connections:

Yet another urban center has been seemingly laser-targeted with extraordinary cloudbursts in this strange U.S. summer of drought and flood. A series of “training” storms – well predicted by forecast models two days in advance – dropped torrential rain across the Dallas-Fort Worth metroplex from Sunday evening into midday Monday, August 20-21. At least one death was reported as of early Tuesday.

As discussed in a Saturday post at this site, the ingredients included a weak east-to-west frontal zone that straddled the DFW area, upper-level impulses traversing the front from west to east, and huge volumes of moisture drawn into northern Texas from the remnants of Potential Tropical Cyclone 4.

At Dallas-Fort Worth International Airport, the metro area’s main climate observing site, the total of 9.19 inches from 3 p.m. CDT Sunday to 3 p.m. Monday was the second heaviest on record for any 24-hour span, topped only by 9.57″ on September 4-5, 1932.

By itself, Monday was the metro area’s wettest day ever recorded in August, with 5.66″ swamping the 4.28″ observed on August 28, 1946. Official recordkeeping for the DFW area began in 1898.

Two other local airports notched similar totals from Sunday to Monday, according to Matt Moreland (National Weather Service/Southern Region Headquarters):

Fort Worth Meacham Field:  9.56

Dallas Love Field:  9.14

Even heavier totals were observed just east and south of downtown Dallas, on par with amounts one might expect in such a short period only about once every 1,000 years. The highest reported as of Monday night was 15.16″ at a gauge within the Dallas Area Flood Alert System located at White Rock Creek and Scyene Road. At least two CoCoRaHSstations, which typically report once each morning, recorded more than a foot of rain for the entire event, including 12.42″ near Mesquite and 12.31″ just northeast of downtown Dallas.

Mathew Cappuci in Washington Post:

What happened in the Dallas area came after the city and 29 percent of the state were gripped in a top-tier “exceptional” drought that impacted crops and drove water shortages. Some farmers were forced to thin their herds in a process called “culling,” according to the U.S. Drought Monitor. DFW International Airport was 11.11 inches behind for rainfall since Jan. 1.

Then Monday became the airport’s wettest calendar day on record.

The extreme rainfall in Dallas was a “1,000-year rain event,” an episode of flooding that has just a 0.1 percent probability of happening in any given year. It joins the company of 1,000-year rain events that struck Kentucky, St. Louis, eastern Illinois and Death Valley, Calif., since the end of July — all of which were experiencing abnormally dry conditions or in a severe drought beforehand.

Droughts can often make flooding worse. Droughts kill plants and leaves the ground bare, reducing soil absorption. They also harden top soils, which makes it easier for water to run off. The extremely dry ground, combined with the rapid rainfall, can trigger widespread flooding.

While no single weather event is caused by mankind’s influence on the atmosphere, the weather facing the nation bears the fingerprint of a warming world. While it seems contradictory, both drought and flooding are closely tied to human-driven warming and are altering our environment and how we interact with it.

We are witnessing firsthand the effects of ordinary weather events — a product of chaotic randomness and natural variability — supercharged by climate change.

Yale Climate Connections again:

The string of exceptional rains and inundations in the United States this summer has gained notice from the UK-based Guardian, which dubbed 2022 “America’s summer of floods.”

The recent event most akin to the DFW gully-washer was the record rainfall in St. Louis, Missouri, on July 26. The city smashed its rainfall records for any 24-hour period (9.07″, far outpacing the old record of 7.02″) and for spans of one, two, and three calendar days. Much like the DFW event, training storms shifted from west to east along a weak stationary front, with impulses of energy from an upper-level trough to the north and rich moisture flowing in from the south. As with DFW, the heaviest rains across an entire region happened to be focused in and near a major metro area. In both cases, these were urban flash floods that hit highways and motorists the hardest, without causing catastrophic structural flooding despite the magnitude of the quick-hitting rains. Two lives were lost in the St. Louis area.

Far more destruction occurred in eastern Kentucky, when torrential rains along the same stationary front that affected St. Louis – this time in a rugged, mountainous landscape ­­­– led to devastating floods in late July. At least 39 deaths were reported, which appears to be the largest toll from a U.S. flash flood unrelated to a tropical cyclone since the late 20th century. The widespread damage from the Kentucky floods, which ripped through historic downtowns and rural homes alike, is still being assessed.

WashPost again:

It’s well-established that a warmer world is a wetter world. That’s due to something called the Clausius-Clapeyron relation. For every degree Fahrenheit the air temperature warms, the air can hold about 4 percent more water. That’s leading to higher humidity and heat indexes — which can be taxing on the human body — but is also manifesting in precipitation extremes.

It’s not noticeable in the day-to-day, but let’s consider that we take a storm in preindustrial times and copy it into today’s environment. With about 1.8 degrees of warming since preindustrial times, the air would have a 7 to 8 percent greater capacity to store and transport moisture.

In a water-loaded environment like a thunderstorm complex or tropical system, you might think that would mean 7 or 8 percent more rainfall. But that’s where things get murky. Because an air mass is being constantly replenished and fed into these storms, that can quickly lead to a 10 or 20 percent increase in precipitation totals.

We’re seeing this quite prominently in rainfall rates, meaning the wetter atmosphere is leading to heavier instantaneous downpours. Dallas, for example, saw its highest one-hour total on record between 1 and 2 a.m. on Monday, with 3.01 inches coming down. Records at DFW International extend back to 1953, but seven of the top 10 wettest one-hour totals have occurred in the 2000s.

There’s already been a 24 percent spike in the frequency of top 1 percent rainfall events in Texas since the dawn of the 20th century. That trend is echoed across the country and world.

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