As North Texas braved perilous storms Sunday afternoon, cities received a severe-storm warning from the National Weather Service.

But Dallas officials, based on a technicality, decided not to sound the city's outdoor warning sirens, meant to alert those who are outside to seek shelter.

In fact, Rocky Vaz, director of Dallas' Office of Emergency Management, said he didn't know of any North Texas city that used its sirens — although some on social media reported hearing them in Rowlett. And while one woman drowned after her boat flipped on Eagle Mountain Lake northwest of Fort Worth, no injuries have been reported for people who were outdoors during the storm.

Still, the thunderstorm, which brought 70 mph winds and flash floods, have prompted some to wonder why the sirens didn't go off. Vaz said the weather service's warning didn't meet the criteria for setting off the sirens.

"There's a lot of thought behind the process and procedures," Vaz said.

One word in the guidelines all cities use made the difference.

An outdoor weather warning siren at Bradfield Park, at the corner of Castle Drive and Country Club Road in Garland. (DMN file photo)

To set off the sirens for a severe thunderstorm, impending hazards must include "sustained winds of 70 mph or greater." But the National Weather Service's warning said the storm included wind gusts of 70 mph.

Gusts are not sustained winds, Vaz said. And the radar had only picked up 64 mph winds, in Corinth, before the storm's strongest winds blew through Dallas.

The weather service has nothing to do with the cities' criteria, so meteorologists don't consider the difference in their interpretation of gusts and sustained winds. Meteorologist Monique Sellers said Monday that sustained winds persist for a period of time — but that the period of time is not defined. That means a sustained wind could last 3 seconds or 3 hours, she said.

"We just choose a certain wind speed. We're focusing on what is the maximum wind speed ... based on what we're seeing in the radar," Sellers said.

Vaz said he didn't yet know whether any policies would change after the storm.

"We will talk with the emergency managers in the region to look at if we need to revisit that," Vaz said.