Tropical Storm Imelda enters the history books as one of the top five wettest tropical cyclones to ever strike the lower 48 states, with a maximum rainfall total of 43.39 inches. On Friday morning, floodwaters continued to block roads, damage homes and cause gridlock in the Houston metro area and especially in the vicinity of Beaumont and Port Arthur, where new flood warnings were issued for additional rainfall of up to four inches.
That this storm comes just two years after Hurricane Harvey dumped an almost unimaginable 60.58 inches of rain on the same general area is no accident. In addition, other major rain events in Southeast Texas in the past five years have caused extensive disruptions and damage.
Recent studies show that slow-moving tropical cyclones in the United States are becoming more frequent, and increased ocean heat content is supercharging the rainfall potential of such storms, making them more formidable rain producers than they otherwise would be.
For example, a study published in the journal Earth’s Future in 2018 found that Hurricane Harvey’s gargantuan rainfall totals were directly related to record high ocean heat content in the western Gulf of Mexico. The oceans are absorbing the vast majority of extra heat from human-caused greenhouse gas emissions, with temperatures increasing in the process.
This is translating into additional water vapor, which storms tap into as fuel and then wring out like a wet sponge. If their forward speed slows to a crawl, as Harvey and Imelda did as tropical depressions, they can produce rainfall totals measured in feet rather than inches. As Tropical Storm Imelda formed off the Texas coast, it drew in moisture from gulf waters that were between 1.8 and 3.6 degrees above average for this time of year.
The 2018 study found that the energy released into the atmosphere from Harvey’s rainfall was equivalent to the amount of energy removed from the ocean in the storm’s wake. In other words, the study found that the amount of heat stored in the ocean is directly related to how much rain a storm can produce.
“Record high ocean heat values not only increased the fuel available to sustain and intensify Harvey, but also increased its flooding rains on land,” the study said. “Harvey could not have produced so much rain without human-induced climate change.”
Two other studies found that climate change increased Hurricane Harvey’s rainfall by 20 to 35 percent.
In addition, many areas in the United States have seen increases in heavy-precipitation events overall, including Texas, as laid out in the 2018 Fourth National Climate Assessment.
From a meteorological standpoint, Hurricane Harvey and Tropical Storm Imelda both illustrate how copious amounts of rainfall can result when tropical cyclones, even in a relatively weak state as judged by their maximum sustained winds, interact with smaller-scale boundaries in the atmosphere.
This can act to focus the heaviest rainfall on specific locations for extended periods of time, like a stuck fire hose.
On Wednesday night into Thursday morning, a cluster of thunderstorms formed and re-formed over the same area southwest of Beaumont, turning Interstate 10 into an inland ocean and dumping more than two feet of rain in 24 hours. Such rain would not have happened without the circulation from the remnants of Imelda, but smaller-scale boundaries helped focus the rains in this area as opposed to downtown Houston, for example.
Southeast Texas has become America’s ground zero for flood risk and extreme rainfall
Southeast Texas has turned into one of the top hot spots in the country for heavy-rainfall events, which is especially problematic considering decades of land use decisions in the sprawling Houston metro area. These decisions have favored development in ways that make the region more susceptible to flash flooding.
The 43.39 inches from Imelda in North Fork Taylors Bayou, 15 miles southwest of Beaumont, Tex., ranks as the seventh-highest from a tropical weather system in U.S. history and the fourth-highest within the contiguous United States and the state of Texas. Thirty-three of those 43-plus inches fell in just 12 hours on Thursday, and the maximum rainfall rate reached 11 inches per hour.
This latest deluge came just two years after Harvey, which produced the most rain ever recorded from a tropical weather system anywhere in the United States.
Imelda’s rainfall footprint was not as extreme as Harvey’s, but it was larger than those of Tropical Storm Allison in 2001 and Tropical Storm Claudette in 1979 -- two of the other top-tier tropical rain events in Texas weather records.
George Bush Intercontinental Airport, just north of downtown Houston, recorded 2.08 inches of rain in just 27 minutes late Thursday morning and 9.21 inches through the evening, making for its wettest September day on record and its fifth-wettest day in any month.
Including Imelda, five exceptional rain events have occurred in the region in the last five years:
- In May 2015, the Houston area received 5 to 11 inches of rain in under 10 hours.
- In April 2016, “a flood event of excessive proportions” unloaded 5 to 17 inches on Houston in less than 24 hours.
- In late May 2016, parts of Southeast Texas between Houston and Austin saw 18 inches of rain in 24 hours.
- In August 2017, Harvey hit.
Since 1970, Houston’s average yearly rainfall has climbed between four and eight inches, depending on the location analyzed and how the data is parsed. Of the top 100 rainiest days since 1970 in Houston, 54 have occurred since 2000. These top-tier rainy days are twice as common in the 2000-2017 period, compared with the 1970-1999 interval.
Massive Texas rains fit into a bigger picture across the U.S.
What’s happened in Texas isn’t unique to that state.
Russ Schumacher, a meteorology professor at Colorado State University, found that rainfall events with totals of greater than 23.6 inches in 72 hours have become unusually common during the past four years.
From 2001 to 2015, Schumacher found, there were only two such events east of the Rocky Mountains: The 2002 San Antonio Flood and Tropical Storm Debby in 2012.
Yet each of the past four years has had one such event: A heavy-rain event in Louisiana in 2016, Hurricane Harvey in 2017, Hurricane Florence in 2018, and now Tropical Storm Imelda. Two of these events hit Southeast Texas, and three out of the four took place along the Gulf Coast.
Three of the top 10 wettest tropical weather systems (Imelda, Florence and Harvey) on record have now struck the United States in the last 25 months, and five states have set new tropical rainfall records. In addition to Imelda and Harvey in Texas:
- The remnants of Hurricane Barry set a new Arkansas state rainfall record for a tropical storm, dumping 16.59 inches on Dierks, Ark., which is about midway between Dallas and Memphis and 120 miles southwest of Little Rock. This preliminary report topped Arkansas’ previous record rain of 13.91 inches set in 1989 from the remnants of Tropical Storm Allison.
- Hurricane Lane bombarded Hawaii’s Big Island with 52.02 inches of rain in August 2018, becoming Hawaii’s rainiest tropical storm and the second-wettest tropical weather system on record in the United States, trailing only Harvey.
- When Hurricane Florence slammed into the Carolinas in September 2018, it dispensed 35.93 inches in North Carolina and 23.63 inches in South Carolina, both state records. Florence’s rainfall in North Carolina was the most for any tropical weather system north of Florida along the East Coast on record, and the fourth-most for any state.
Matthew Cappucci contributed to this article.