More than two million people were under a tropical storm warning along the Texas Gulf Coast in the early hours of Thursday as Tropical Storm Alberto, the first named storm of the Atlantic hurricane season, neared the coast of Mexico, bringing intense rain and storm surges.

The expansive storm system brought widespread coastal flooding in southern Texas on Wednesday, well before the storm was expected to make landfall. Officials in Mexico said they were cleaning drainage points to prevent flooding, and monitoring the levels of dams, rivers and streams.

At least three people were killed in storm-related events in the state of Nuevo León in Mexico, according to local media reports. One teenager was trapped by currents in a river and drowned, and two other minors were electrocuted crossing a pond that was in contact with a live cable, the El Universal newspaper reported, citing local emergency authorities.

In Texas, tides surged beneath elevated houses in some coastal cities, including Surfside Beach, about 40 miles south of Galveston, starting on Wednesday morning. The city closed its beach earlier this week and warned visitors to stay away as the flooding worsened.

The National Hurricane Center warned that Alberto was a large storm, with tropical-force winds extending about 415 miles north of its center in the Gulf of Mexico as it moved west toward northeastern Mexico. The storm had maximum sustained winds of 50 miles per hour on Wednesday night, but the main concern was rainfall of a foot or more that was predicted for parts of Texas and Mexico.

Forecasters predicted that Alberto could make landfall early Thursday near the Mexican city of Tampico, but its effects were expected to extend far beyond that. As of Wednesday night, a 700-mile stretch of coast from San Luis Pass near Galveston to Tecolutla in Veracruz, Mexico, was under a tropical storm warning, according to the National Hurricane Center.

Mayor Gregg Bisso of Surfside Beach said that while the flooding was slowly easing there on Wednesday evening, the city was bracing for things to intensify at any moment, as they did when Hurricane Nicholas slammed into the city in 2021, causing major damage.

“It’s a wait-and-see kind of deal,” Mr. Bisso said, adding that all of the city’s police officers and emergency service workers were on call.

Here are key things to know about the storm

• Alberto is expected to make landfall on the northeast coast of Mexico early Thursday, but the wind and rain extend far from the center of the storm.

• Heavy rain will continue across much of southern Texas and northeastern Mexico. Five to 10 inches of rain is likely, with the possibility of isolated amounts near 20 inches.

• Up to three inches of rain is forecast to fall per hour, potentially overwhelming streams and creeks. Storm surges of up to four feet are expected in coastal areas including Sargent, Texas, and Galveston Bay.

• Mudslides are also a concern in the hills of Mexico.

Preparations were underway in Mexico

On Wednesday, the authorities and residents alike were preparing for heavy rains and strong winds.

Government workers were setting up temporary shelters, and more than 1,500 electricians were deployed to Tamaulipas, Veracruz and Nuevo León — the three states expected to bear the brunt of Alberto’s force when it makes landfall — to respond to any power outages.

Still, for some states in Mexico, the storm’s arrival was a welcome respite amid a water crisis and scorching heat waves.

“We are waiting for these rains, which are going to be very beneficial,” Luis Gerardo González, the Tamaulipas state civil protection coordinator, said in a radio interview on Wednesday.

A disaster declaration was issued in Texas

Ahead of the storm, Gov. Greg Abbott of Texas activated a range of emergency management resources, including high-water boats and helicopters with hoist capabilities. On Wednesday afternoon, he announced a severe-weather disaster declaration for 51 counties.

On Wednesday, there were reports of property damage on North Padre Island after coastal bulkheads collapsed from the storm surge. And on South Padre Island, officials were distributing free sandbags to businesses and residents. In Corpus Christi, at least two people were rescued and taken to higher ground after their cars became submerged in floodwaters, officials said.

Hurricanes and tropical storms can produce tornadoes, often in rain bands far from their centers. As Alberto approached, the National Weather Service issued a tornado warning late Wednesday night for an area of Texas along the U.S.-Mexico border that included the cities of McAllen, Mission and Pharr. Tornado warnings that were issued earlier for counties near Corpus Christi were extended. A tornado watch was in effect until 7 a.m. local time for much of Texas south of San Antonio, an area that is home to about 2.5 million people.

In Houston, which only recently recovered from an unexpectedly deadly storm last month, officials were bracing for major flooding this week.

This hurricane season is expected to be busy

Forecasters have warned that the 2024 Atlantic hurricane season could be much more active than usual.

In late May, the National Oceanic and Atmospheric Administration predicted 17 to 25 named storms this year, an “above-normal” number and a prediction in line with more than a dozen forecasts earlier in the year from experts at universities, private companies and government agencies. Hurricane seasons produce 14 named storms, on average.