Hurricane Hilary is set to pummel Mexico’s northern Pacific coast Sunday morning before the storm hurdles north toward Southern California.

The National Hurricane Center warned of “catastrophic and life-threatening” flooding expected across Mexico’s Baja California peninsula and large swaths of southwestern United States through Monday.

The hurricane — which was downgraded to a Category 3 storm on Saturday, meaning winds of roughly 125 mph, after losing some steam overnight — has already started to dump rain on Mexico, with flash flood alerts already in effect ahead of the storm’s center, according to officials.

Mexican cities are expected to take the brunt of the storm, which is set to make landfall south of the port city of Ensenada on the Baja peninsula Sunday around 6 a.m. local time, forecasters said.

The hilly border town of Tijuana is anticipating devastation. Prone to landslides and flooding, its shacks perched on cliffs and thousands of migrants living under tarps on the streets are especially at risk.

“We are a vulnerable city, being on one of the most visited borders in the world and because of our landscape,” said Tijuana Mayor Montserrat Caballero Ramirez.

Mexico issued a tropical storm warning for mainland parts of the country and put 18,000 soldiers on alert.

“If your home is safe, be prepared to stay in it,” said Salvador Cervantes, head of civil protection in Baja California.

“If your home is not safe or is in a risk zone, request assistance to move to a temporary shelter or to a relative’s home in a safe area,” said Cervantes, the San Diego Union-Tribune reported.

As Hilary churns toward the Golden State, it is expected to weaken to a tropical storm, the first since Sept. 25, 1939 to make landfall in Southern California, the National Weather Service said. The storm is also predicted to be the wettest tropical cyclone in state history and was the catalyst for the first-ever Tropical Storm Watch ever issued for California, according to the state.

Nevada and Arizona are also in for record-setting deluges. 

Officials predict said the storm could dump 3 to 6 inches in some areas, with a whopping 10 inches possible in isolated parts of California and Nevada.

“The places most likely to be impacted by the flooding are going to be Western Arizona, Southern California and Southern Nevada,” said Jordan Overton, senior weather producer and meteorologist at Fox Weather. “Death Valley is a place that’s going to be impacted. Some of these desserts could easily get a year’s worth of rain from Hurricane Hilary.”

The heaviest rainfall is expected east of the mountains that border the coastal California cities.

“Rain gets pushed against the mountain slopes, which is where the most significant flooding is likely to be,” Overton explained.

Experts say the forecast rain is “unheard of” for this time of year in southern California, and comes after the state was battered by freak weather events last spring that caused widespread flooding. Hilary could even break Nevada’s rainfall record.

Officials as far north as Los Angeles are bracing for Hilary, rushing to get the homeless of the streets, setting up shelters and preparing for evacuations, according to reports.

“I don’t think any of us — I know me particularly — never thought I’d be standing here talking about a hurricane or a tropical storm,” said Janice Hahn, chair of the Los Angeles County Board of Supervisors.

Major League Baseball games have been rescheduled and the National Park Service pre-emptively closed Joshua Tree National Park and Mojave National Preserve.

The Federal Emergency Management Agency has positioned personnel and supplies to respond in Southern California or other areas, President Biden said Friday.

“I urge everyone, everyone in the path of this storm, to take precautions and listen to the guidance of state and local officials,” Biden told reporters at Camp David.

Hilary formed 470 miles off the coast of Manzanillo, Mexico, on Wednesday and became a Category 3 hurricane by Thursday night. It strengthened to a Category 4 storm Friday but lost some of its power overnight prior to the downgrade.

With Post wires