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%.
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.