Texas Gov. Greg Abbott issues disaster declaration as Tropical Storm Alberto approaches Mexico

Kids enjoy cool spraying water during a hot weather week on Monday, June 17, in Brooklyn, New York City, United States. Selcuk Acar | Anadolu | Getty Images

The first named storm of the hurricane season strengthened slightly late Wednesday as it moved toward Mexico's Gulf Coast, threatening rainfall of up to 20 inches there, as well as flooding and heavy rain in Texas, officials said.

Tropical Storm Alberto formed over the western Gulf of Mexico on Wednesday morning, and its center is expected to reach Mexico's Gulf coast early Thursday, according to the National Hurricane Center.

The storm is large, with tropical-storm-force winds extending out 460 miles.

The storm's maximum sustained winds strengthened to 50 mph from 40 mph, the National Hurricane Center said in a 10 p.m. local time update, Once it hits Mexico, it is expected weaken rapidly as it moves inland.

The center of Alberto was about 120 miles east-southeast of Tampico, Mexico, and around 290 miles south-southeast of Brownsville, Texas, at 1 a.m. Central Standard Time, the hurricane center said Thursday. It was traveling west at about 9 mph.

Mexico's civil protection coordination agency said late Wednesday on social media that rain was beginning to fall in Tampico, and warned people to stay inside, stay safe, and not drive during the storm.

A tropical storm warning is in effect for the northeastern coast of Mexico, as well as the Texas coast from the mouth of the Rio Grande up to San Luis Pass, which is near Houston. High winds and as much as 10 to 15 inches of rain are expected in Corpus Christi.

Gov. Greg Abbott issued a disaster declaration for 51 Texas counties "to ensure Texans and at-risk regions have the resources and personnel needed to respond to this storm," he said in a statement.

The National Weather Service said moderate coastal flooding was observed across the Texas Gulf Coast as the storm approached Mexico.

Dustin Leeds and Kristine Martin, of Houston, were on vacation in Freeport, Texas, and woke up Wednesday to floodwaters that were thigh-high.

Martin woke up at around 5:30 a.m. after having heard heavy rain: "I opened the blinds and was like, 'Huh, the ocean looks closer,'" she joked.

Residents seemed to be taking it in stride. Homes are raised in the area because of flood risk, and many people had moved their cars to safety in advance.

"Everybody's still in their homes. There's two other families in houses right around us that were just — they were hanging out, living their best life," Leeds said.

A storm surge of 2 to 4 feet is possible from Sargent, Texas, to the Sabine Pass.

There could also be a few tornadoes in deep South Texas overnight Wednesday into Thursday, the weather service said.

On Thursday, the storm's center is forecast to move west into Mexico then weaken, most likely dissipating by Thursday night, the National Hurricane Center said.

Some parts of Mexico, including Coahuila, Nuevo León and Tamaulipas, could get maximum totals of 20 inches of rain, the hurricane center said.

The Texas A&M Forest Service has mobilized four teams comprising 100 personnel and 24 vehicles, while the Texas National Guard has three platoons of more than 40 personnel in total, alongside 20 vehicles, including Chinook helicopters.

The weather service advised people in areas affected by the storm to have five to seven days' supplies of food, water and other necessities on hand.

More than 82 million people were under some form of heat advisory Wednesday. The heat wave is set to last through at least Friday.