The new flood has created yet another test for Mr. Reeves, the Republican who took office just over a month ago and has already faced a spell of destructive weather, including heavy rains and tornadoes, and a crisis inside the state’s prisons that escalated after a surge of violence and discord in recent weeks.

As the severity of the flood grew more evident on Saturday, Mr. Reeves joined local officials in beseeching residents in the potential flood zone to head out while they could still do so without the help of rescuers. “The water is coming,” Mike Word, the director of emergency operations for Rankin County, said in a televised briefing before the spillway was opened. “We have told you and told you and told you this. So please, heed our warnings.”

Before the worst of the flooding and the return of rainfall on Saturday night, officials were worried that residents would ignore the warnings because the weather had cleared briefly, with sunshine and temperatures in the 60s. “Do not let that lull you into a false sense of hope,” the governor warned.

But many in Mississippi have grown accustomed to nature’s wrath. “Floods, tornadoes, hurricanes,” said Wanda Webb, who drove north to Jackson from Pascagoula, on the Gulf Coast, to supervise a shelter opened by the American Red Cross. She joined the organization in 2005, after its volunteers brought her food and water when her home was destroyed by Hurricane Katrina.

“We’re here to meet the needs of these people the best way I know how,” Ms. Webb said, noting that more people might arrive at the shelter as the flooding spreads. “It is kind of rough, but it might get a bit rougher.”

By Sunday afternoon, just over 30 people had come to the shelter, which had been set up inside the Jackson Police Department’s officer academy. Rochelle Watson, 31, brought her four children on Saturday. For months, she said, her family had been itinerant, traveling from Oklahoma to California and Nevada before recently moving to Jackson, persuaded by a friend.

Ms. Watson said she had found a house that she liked and had hoped to rent, but it was damaged by the flood. “God blessed me with this beautiful home, and it ended up going in the water,” she said.