As Hurricane Harvey dropped anchor over Southeast Texas last week, Zello became the go-to app for rescuers working to save thousands of people trapped by floodwaters.

Within days of Harvey's arrival, the app saw a 20-fold increase in usage in Houston, according to Bill Moore, the Austin-based startup's chief executive.

As Hurricane Irma hurtles across the Caribbean toward the coast of Florida, Zello continues to boom in popularity. The free Internet "walkie-talkie" app — which relies on cellphone data plans or Wi-Fi and is designed to operate in places where signals are weak — became the top app on iTunes and Google Play Wednesday.

The latest influx began Tuesday and, at one point, Moore said, 120 people were registering for the app every second. In recent days, the app has also trended on Facebook and Twitter, offering another example of the pivotal role social media is playing in natural disasters.

"The heat map of where the registrations are occurring looks like the hurricane's forecast path," he added. "It's very dense at the tip among the Caribbean islands and then fans out across Florida."

Moore said so many people registered for the app Tuesday that Zello suffered some performance issues, which have since been resolved.

Forecasters are calling Irma "a potentially catastrophic Category 5" storm with sustained winds of 185 mph, according to the National Oceanic and Atmospheric Administration. Florida officials have begun evacuating portions of the eastern coastline, which is densely packed with cities, towns and suburbs.

The storm is the most powerful to threaten the East Coast in a decade, according to The Washington Post's Francisco Alvarado and Mark Berman. Interest in the storm is being reflected in app downloads, according to Adam Blacker, a research analyst for Apptopia, a company that tracks the mobile app economy.

"People are tracking the weather hardcore right now," Blacker said via email. "The top paid app right now is NOAA Pro Weather Alerts and the #3 paid app right now is Hurricane Tracker."

In Southeast Texas, also a densely populated area, Zello proved to be a useful tool for coordinating a chaotic rescue operation run by numerous groups filling in for overwhelmed public officials, many of them grass-roots volunteers like the Cajun Navy, a Louisiana group that helps with search-and-rescue efforts in time of need.

The app has more than 100,000 users worldwide, including in Hong Kong, where it is popular with taxi drivers. The app seems to thrive in locations where government services struggle to meet demand, such as Egypt, South Africa, Venezuela, and parts of the Middle East.

During Harvey, the app allowed victims and rescuers to post voice messages to specific channels, such as "the Cajun Navy" and "Harvey Animal Rescue."

In Houston, volunteers found another way to use the app. By monitoring Facebook, Twitter and Instagram, users were able to feed information to rescue boaters who then took that information and used Zello to coordinate rescues in flooded neighborhoods all over the region.

"With voice, someone can communicate a ton of information in a way that text does not," Moore told The Post last week. "In a few seconds of hearing your voice, I can guess what part of the country you're from, if you've been drinking, what mood you're in, whether you're afraid or in distress."

"For that reason, voice becomes great for solving problems, and it demands attention from both sides in a way that texting does not," he added. "Your brain is wired for voice."

Despite the app's booming popularity, Moore said the recent spike that arrived with Irma caught even him by surprise.

"It's just a lot of scale," he said. "People are hearing about the app and how generally useful it is during disasters. It's great to be part of the solution."