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Sirens silent during July 23 tornado because of warning system software error, no weather service alert

Chicago Tribune 6 days ago Suzanne Baker, Chicago Tribune

A software error failed to trigger emergency warning sirens like this one on 95th Street on July 23 when an EF-0 tornado moved through Naperville's southeast side, officials said.

© Suzanne Baker / Naperville Sun/Chicago Tribune/TNS A software error failed to trigger emergency warning sirens like this one on 95th Street on July 23 when an EF-0 tornado moved through Naperville's southeast side, officials said.

Residents in southern Naperville didn’t receive early warning of the potential for a tornado touchdown July 23 because the National Weather Service issued its alert after the storm had passed.

But the city’s outdoor warning sirens still should have activated when a weather service map of the tornado warning area was issued, a city emergency official said.

In a memo to City Manager Doug Krieger, Naperville Emergency Management Agency coordinator Dan Nelson said the sirens failed to sound because new software installed in April to manage the system wasn’t set up properly.

The vendor acknowledged the configuration error, corrected it and is creating systemwide changes across the nation to ensure the automated system operates properly going forward, Nelson said.

An investigation of storm damage by the National Weather Service confirmed an EF-0 tornado first hit at the Aurora-Naperville border near the northwest corner of the White Eagle Golf Club at 5:40 a.m. July 23 and traveled 4.5 miles southeast until 5:46 a.m., where it lifted near 119th Street and Book Road.

A tornado warning message wasn’t issued by the weather service until 5:47 a.m. as the storm headed toward Romeoville.

The agency’s Romeoville office is responsible for severe weather warnings issued in the Naperville area and directly informs the public via weather radio, cable and television overrides, and phone alerts using the Federal Emergency Management Agency’s wireless emergency alerts, Nelson said.

The agency also notifies the city through bulletins sent to the city’s 911 dispatch and emergency operations center.

Nelson said Naperville’s policy is to activate the outdoor warning sirens when the city is included in a weather service tornado warning.

Kevin Donofrio, science and operations officer for the National Weather Service, said a severe thunderstorm warning was issued at 5:29 a.m. mentioning the potential for damaging wind gusts. The tornado that passed through south Naperville was embedded in a line of thunderstorms and difficult to see, he said.

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“These tornadoes tend to develop really rapidly. It’s very difficult to detect on radar in the time for warnings to be issued prior to touchdown,” Donofrio said.

The decision to issue a tornado warning is based on multiple factors, he said, including whether there is an environment to support a tornado, a report of a tornado and a signature on radar.

Donofrio said the weather service typically looks for at least two of the factors before there is a decision to issue a warning, and on July 23 the environment was largely supportive for tornadoes.

“They weren’t happening all night, but we were aware of them. The radar signature was very, very subtle, and it’s kind of hard to tell, at least initially, when these are really small to see,” Donofrio said.

It was a much different storm than the one that spawned an EF-3 tornado that hit Naperville on June 20, 2021, he said. That had a radar signature that was hard to miss, he said.

Severe thunderstorms can pack as much punch as an EF-0 (winds of 40 to 72 mph) or EF-1 (winds of 73 to 112 mph). Donofrio said the storms on July 23 had a history of producing strong EF-type winds before reaching Naperville, which is why the severe thunderstorm warning was issued.

“I know many communities don’t alarm for those,” he said. “But it’s up to the community as to how they take our warnings and decide to activate their sirens.”

The challenge for forecasters is communicating the severity. “The wind is producing the damage, whether it’s a severe thunderstorm or tornado warning,” he said.

Software the city had installed in April should have automatically triggered the sirens regardless of the written bulletin because the geographic boundaries in the weather service’s tornado warning map included southern Naperville. The configuration error is what prevented that from happening, Nelson said.

Emergency dispatch personnel could have manually sounded the sirens if a weather service bulletin mentioned Naperville, he said.

Before April, the warning sirens could only be activated manually.

The failure of the sirens to go despite the map including Naperville is what caused city staff to launch an investigation into why, he said.

The Naperville Emergency Management Agency will continue to review the incident and investigate ways to ensure citizens have accurate and timely warnings before potential severe weather hits, Nelson said.

subaker@tribpub.com

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