Guam and the Northern Mariana Islands were preparing on Sunday for the arrival of Typhoon Bavi, a powerful storm that was forecast to make landfall in the U.S. Pacific territories a day later with the force of a Category 5 hurricane.

Bavi was a Category 4 tropical cyclone as it barreled west on Sunday afternoon. It was expected to bring destructive winds and heavy rainfall to a region that is still recovering from Typhoon Sinlaku, a storm that caused widespread damage in April.

Typhoon Bavi was forecast to pass between Guam and the Northern Mariana Islands, an archipelago that is home to about 43,000 people, on Monday morning local time. Guam’s Joint Information Center warned on Sunday that its path could change.

The authorities in Guam activated the highest level of a four-tier alert system, opened evacuation shelters, and urged residents to stay off the roads and out of the water. They also restricted access to U.S. military facilities on the island.

On Guam, which has a population of about 150,000 people, many residents spent the weekend preparing for the storm. Some covered the windows of their homes or businesses with plywood.

Marites Schwab, who lives in a village overlooking the island’s capital, Hagatna, said she had gone through her usual pre-typhoon routine of fueling up her cars with gasoline, collecting water in cans, and charging batteries and backup devices.

“Never perfect but I’m reasonably prepared,” she said. “I hope the power and water don’t go out.”

Adoracion Leomo said she had secured the window shutters of her home in a coastal village near Guam’s international airport. She hoped that would be enough, she said, but she wasn’t sure.

“I’m not sure what to expect,” she said. “I just need to say a prayer that God will shield us from a strong typhoon.”