This is a re-post from Yale Climate Connections by Bob Henson and Jeff Masters
Torrents of rain that began before dawn on Tuesday, July 26, gave St. Louis, Missouri, its highest calendar-day total since records began in 1873. And the deadly event is just the latest example of a well-established trend of intensifying downpours in many places across the globe.
The official reporting site at Lambert International Airport received 8.6 inches of rain from midnight to 11 a.m. Central Standard Time on Tuesday. (Standard time is used year-round to separate calendar days for meteorological data purposes.) Another 0.46 inch had been recorded just before midnight CST on Monday, bringing the total for July 25-26 to 9.04 inches as of 11 a.m. CST Tuesday.
The predawn onset of the storms meant that many residents were startled awake by floodwaters or caught on highways during early commutes. Dozens of water rescues were carried out across the area. The storm closed Interstate 70 and many other key roads across the St. Louis area throughout the morning, according to the St. Louis Post-Dispatch. At least one death has been reported as of Tuesday evening, a person pulled from an inundated vehicle.
More heavy rain is possible early Wednesday, although the storms in that round are expected to keep moving rather than stalling out.
Below are several records set in Tuesday’s brief but intense deluge. Update (3:30 pm CDT Wednesday): The numbers below now reflect observations from St. Louis Lambert International Airport for Monday through Wednesday morning, July 25-27. We will provide another update if additional rain falls before midnight Wednesday night.
Figure 1. The 24-hour total of just over 9 inches in St. Louis would be expected to occur about once every 200 to 300 years if the climate were “stationary,” based on the NOAA Atlas 14 catalog of recurrence intervals. Atlas 14 is based on past observed rainfall, so it does not take ongoing climate change into account. Recurrence intervals are most likely getting shorter in many locations. (Image credit: NOAA Atlas 14)
Figure 2. The cold, high tops of an intense mesoscale convective system that tracked across the St. Louis metropolitan area on July 26, 2022, can be seen in this infrared satellite image from 0800Z (3 a.m. CDT) Tuesday, with a particularly intense core (dark red) stretching from west to east across the St. Louis area. (Image credit: RAMMB/CIRA/CSU)
The culprit behind the early-morning deluge was a mesoscale convective system — a large, highly organized complex of showers and thunderstorms — that moved across the area in a “training” fashion from west-northwest to east-southeast, with cells regenerating and tracking along a constant path while the complex itself moves much more slowly. The system included one band of intense rainfall passing directly over St. Louis, with a secondary band roughly 60-70 miles to the north. Some areas just northwest of St. Louis may have seen as much as 14 inches of rain.
Update (3:30 p.m. Wednesday): CoCoRaHS stations in the St. Louis area reported the following impressive one- and two-day totals through Tuesday and Wednesday morning, respectively.
2.2 miles NE of St. Peters: 12.34 in., 14.69 in.
3.9 miles NNW of O’Fallon: 10.58 in., 11.83 in.
1.2 miles ESE of Maryland Heights: 10.35 in., 11.87 in.
Figure 3. Hydrograph for Dardenne Creek near I-70 in St. Louis. In the seven hours beginning at 12:30 a.m. July 26, the creek rose 21 feet, and peaked just 1.36 feet below the all-time record set on September 15, 2008. (Image credit: NOAA)
Although the Midwest climate is naturally drought-prone, recent decades have trended more toward the wet side of the spectrum. “Annual precipitation in the Midwest has increased by 5% to 15% from the first half of the last century (1901–1960) compared to present day (1986–2015),” the Fourth National Climate Assessment observed in 2018.
There’s a well-established trend in many parts of the globe for the heaviest short-term rainfall events to become more intense, consistent with a human-warmed atmosphere allowing more moisture to evaporate from warmer oceans and to flow into rainmaking systems. Over the past 120 years, the Midwest has seen a 42% increase in the amount of rain falling in top-end short-term rainfall events (the heaviest 1% of events). That was larger than any other region of the country for the period 1901-2016, although it was surpassed by the Northeast in the period 1958-2016 (see tweet below, which includes a National Assessment analysis).
“Storm water management systems and other critical infrastructure in the Midwest are already experiencing impacts from changing precipitation patterns and elevated flood risks,” the National Climate Assessment, a sweeping government report on the impacts of climate change in the United States, noted in 2018.
As one of the 10 biggest U.S. cities between 1850 and 1960 — but only the 70th-largest as of 2020 — St. Louis joins other venerable but now-smaller U.S. cities in having to adapt major century-old infrastructure to the evolving climate of the 21st century. As noted in the National Climate Assessment, the Metropolitan Sewer District in St. Louis had embarked upon a $100 million “rainscaping” project designed to divert storm water runoff in the northern portion of the city of St. Louis and adjacent north St. Louis County, including areas where the July 26 storm tracked.
The stalled frontal boundary that brought St. Louis its record flooding will be draped over the Ohio River Valley through Friday, bringing an elevated threat of flash flooding across portions of Kentucky and West Virginia. On Tuesday evening, this area was been placed under a moderate-risk threat of excessive rain leading to flash flooding. The threat extended across a 48-hour period from Wednesday through Friday morning by the National Weather Service, which is predicting widespread rainfall amounts of 2-4 inches on Wednesday and 1-3 inches on Thursday. This area has already seen significant flash flooding during the past few weeks from extreme rainfall; however, soil moisture levels are currently near average.
This region, which is highly mountainous, is particularly prone to deadly flash floods.
Posted by Guest Author on Monday, 1 August, 2022
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