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A tornado and flood are coming your way: Take cover or move higher?

Researchers say individuals should respond to the hazard most pressing at the time and prepare to move locations if necessary.

People look at cars abandoned on the flooded Major Deegan Expressway after a night of extremely heavy rain from the remnants of Hurricane Ida on Sept. 2, 2021, in the Bronx borough of New York City.
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As Hurricane Ida’s remnants spread over the Mid-Atlantic on Sept. 1, 2021, thousands of people in Bucks County, Pa., faced a seemingly impossible choice: take cover from a potential tornado or move higher to avoid flooding.

The situation was unusual but not unheard of. Ida’s powerful thunderstorms were loaded with both spin and moisture. Soon after a particularly intense thunderstorm began to rotate, the National Weather Service declared a rare tornado emergency for a swath of Pennsylvania and New Jersey.

The warning came with an urgent action item: “To protect your life, TAKE COVER NOW! Move to an interior room on the lowest floor of a sturdy building.”

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Almost simultaneously, the moisture-laden storm began to unload exceptionally heavy rain. Then, the Weather Service declared a dire flash flood emergency. At the bottom of this warning, an equally firm call to action was posted: “Move to higher ground now!”

Thunderstorms that produce simultaneous tornadoes and flash flooding, known as “dual hazard” or “TORFF” events (TORFF is short for tornado and flash flooding) to meteorologists, have long posed a dilemma: When flash flooding threatens, the worst place to be is below ground; when tornadoes strike, high ground is incredibly dangerous.

In the past two decades, thousands of such concurrent events have taken place across the United States, leaving many that are in the path of destruction uncertain on what they should do. While the Weather Service is improving their communication about compound hazards, researchers say individuals should respond to the hazard most pressing at the time and be prepared to quickly move locations if necessary.

How and where dual-hazard events happen

In 2013, a tragedy underscored the dangers of these double-impact events. As the El Reno tornado threatened populous suburbs of Oklahoma City on May 31, the local Weather Service office declared a strongly-worded tornado emergency:

“THIS IS AN EXTREMELY DANGEROUS AND LIFE THREATENING SITUATION. IF YOU CANNOT GET UNDERGROUND GO TO A STORM SHELTER OR AN INTERIOR ROOM OF A STURDY BUILDING NOW.”

A family of seven heeded this advice to hurry underground, sheltering in a storm drain. But the tornado was followed by exceptionally heavy rain, which led to a round of flash flooding. All seven were killed as swiftly moving water filled the drain. Flash floods resulted in more fatalities than tornadoes that evening.

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Like both tornadoes and heavy rain, TORFF events generally occur in environments with abundant moisture and powerful winds. But forecasting TORFF events can be difficult because wet, windy environments are associated with many potential weather hazards.

Erik Nielsen, an assistant professor at Texas A&M University who has been studying these events for nearly a decade, and Russ Schumacher, an associate professor at Colorado State University, conducted the first scientific study on TORFF events. Between 2008 and 2020, the pair documented locations that saw both a tornado warning and a flash flood warning within a half-hour. They found thousands of examples, spread from Hawaii to Florida and from California to Maine.

TORFF events are most common, according to Nielsen, when individual thunderstorm cells coalesce into larger storm complexes. They also occur frequently amid landfalling tropical storms and hurricanes. Occasionally, even individual supercells or single rotating thunderstorms are known to produce both tornadoes and flash flooding.

Forecasting TORFF events brings together numerous forecasting challenges at once, Nielsen said. Forecasters must predict the potential for a tornado, which is tricky by itself. At the same time, they must forecast the amount of precipitation and its potential to trigger flooding. Then they must predict the evolution of the storms themselves.

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“This is even before you get to the communication challenges of concurrent, collocated hazards with contradicting lifesaving calls to action,” Nielsen said.

Warning of dual-hazard events

Even after the 2013 tragedy, it took several years for TORFF events to gain significant attention. In fact, it wasn’t until 2017, when Hurricane Harvey hit, that TORFF events gained national awareness.

Katy Christian, a research associate with the University of Oklahoma’s Cooperative Institute for Severe and High-Impact Weather Research and Operations, wrote in an email that Harvey was the event that “catapulted TORFF events into the national spotlight.”

“During Harvey, you had embedded tornadoes within heavy tropical rainbands that were also producing widespread catastrophic flash flooding,” Christian wrote. “The conflicting response actions from these overlapping hazards ended up creating a new separate hazard of the public being unsure of what actions to take to protect themselves.”

Jen Henderson, who researchers TORFF events at the Texas Tech University Risk and Equity in Disasters Lab, said that the Weather Service’s warning process wasn’t set up to deal with concurrent hazards with conflicting action items.

“Our weather warning systems were designed to detect and warn for singular threats — they’re biased toward a more siloed and scientific classification of threats,” Henderson wrote in an email.

A tornado expert may be issuing a tornado warning, while someone else with heavy precipitation expertise may be managing the flood alerts. As a result, Henderson explained, the warnings for different types of hazards are sent by different people “who may not be aware of the other threats in the heat of the moment.”

But as the visibility of TORFF events continues to increase, the Weather Service is providing training to their forecasters on how to address the overlapping hazards.

Barb Boustead, a meteorological instructor with the Weather Service’s Warning Decision Training Division, wrote in an email that the Weather Service “is raising awareness about TORFF hazards among its forecasters and staff by providing training about these overlapping hazards.”

Boustead said the training includes tips to better communicate between different departments, such as “being in adjoining workstations and communicating directly with each other.”

“The training also encourages offices to use social media to communicate both hazards with equivalent weight, to make it clear that both threats are possible during an event,” Boustead said.

As a TORFF event is occurring, Weather Service forecasters have also learned a variety of ways to help the public decide what actions to take. According to Christian, these strategies can include strategically triggering wireless emergency alerts for whichever hazard appears more imminently pressing, trimming tornado warnings as soon as the twister has passed and editing warning action items during events so that overlapping headlines do not contradict one another.

Surviving dual-hazard events

When the forecast calls for the potential for both tornadoes and flooding, there are several things people can do to be prepared.

Well before a TORFF scenario develops, Christian advises that, “If you live in a flood-prone area, you should have a designated place where you can safely shelter from both the tornado and flash flood threat and arrive there before the storms even start.”

Should both flash flooding and tornadoes prove imminent, she continues that you should “take whatever safety measure corresponds to the most pressing hazard at that location and time. If you have to shelter from a tornado during an ongoing flash flood threat, if possible, you should shelter in a safe building or home that does not require driving afterward. Nearly half of flash flood fatalities are vehicle-related, so you want to avoid driving in flooded roadways if at all possible.”

“Further,” she adds, “because the flash flood threat often comes directly after the tornado threat, you should be prepared to quickly emerge from your tornado shelter and get in a position to protect yourself from flooding impacts.”

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