The torrential rainstorm in Middle Tennessee on Saturday was localized, hyper-intense and largely unexpected. Cascades of rain inundated the region of Waverly, Tenn., located about 60 miles west of Nashville. The unrelenting storminess caused at least 21 fatalities and 40 people confirmed missing as of Monday morning.

High rainfall rates prompted the National Weather Service at Nashville to issue a flash flood emergency — a flood alert for the most dire situations — around 9 a.m. Saturday. Two and a half hours later, as rainfall totals catapulted into the double digits, the office tweeted that a “catastrophic situation” was unfolding.

Radars estimated that 21 inches of rain fell over the community in a single day; nine inches fell in just three hours. A rain gauge measured 17 inches of rain in 24 hours, which will set a new daily record for the state if confirmed.

“[T]he chance of getting over 17 inches of rain in 24 hours in any year at Waverly, TN is much more rare than 1 in 1,000,” wrote Geoffrey Bonnin, a hydrologist retired from the National Oceanic and Atmospheric Administration (NOAA), in a Facebook message to the Capital Weather Gang. He said that amount of rain is so rare that NOAA doesn’t have sufficient historical data to quantify it any further.

Rivers also rose out of their banks to historic levels. The river level at Piney River near Vernon reached 31.8 feet — shattering the previous record set in 2019 by almost 12 feet.

How exactly did so much rainwater pour into such a small area?

A stalled complex of intense thunderstorms initiated along a decaying frontal boundary draped across central Tennessee. Humidity levels throughout the atmosphere reached record levels for Aug. 21. There was also a weak spark in the upper-level flow, which arrived at just the right time — when the atmosphere was most unstable — to turn storms into highly efficient rainmakers.

The boundary was just a whiff of a stationary front, shown in the technical graphic below. A transient wind shift, it was only present on analysis maps for a day.

Meteorologists at the NOAA Weather Prediction Center in College Park, Md., first became concerned about flash flood potential at 2:20 a.m. on Saturday.

They noted that a narrow, fast stream of air in the lower levels of the atmosphere, called a low-level jet, would lift extremely humid and unstable (buoyant) air along the frontal boundary. Once the low-level jet and the weak front met, rain rates of two to three inches per hour were generated.

Steering winds (above the altitude of the low-level jet) were aligned from northwest to southeast. This direction ensured that cells triggered along the front would ride parallel to and along the front, repeatedly, one cell after another. This is typically called cell “training” or “backbuilding.”

The spatially small complex of thunderstorms can be seen in the graphic below, outlined in brown. It lay along the stationary front and was oriented in the direction of the steering winds (black wind barbs).

The elongated thunderstorm complex ended up being nearly stationary, with new cells erupting on the northwest side of the complex, moving through the system and dissipating on the southeast flank.

The high-resolution, short-term forecast models used by forecasters struggled to predict the flash flood event, significantly underestimating the magnitude of rainfall. Models predicted amounts in the three-to-four-inch range. These models did, however, identify the location of the convective rain band, and noted that it would persist for many hours.

Unfortunately, it is often the case that weather modeling either underestimates extreme rainfall, or, in other situations, fails to accurately localize its position. At times, it essentially boils down to a “nowcasting” exercise: Basically, once a training complex of storms has erupted, weather radar, satellite and subsequent model runs are used to assess its persistence and intensity.

Rainstorms have become more common and intense over land since the 1950s, according to observational data. In its latest report, the Intergovernmental Panel on Climate Change indicated that “human-induced climate change is likely the main driver.”

While global warming does not trigger an individual thunderstorm complex on a local scale, it does generate a larger storm environment that makes these extreme rain events more likely. The reason behind this intensification is increased atmospheric humidity, with water vapor evaporating in great quantity from warmer oceans.

Extreme daily precipitation events are projected to intensify at the global scale by about 7 percent for each 1-degree Celsius of global warming.