President Barack Obama has declared a federal emergency exists in Texas as Tropical Storm Alex builds toward hurricane strength and approaches south Texas and northern Mexico.

According to a White House statement issued Tuesday, Obama has ordered federal help to augment the state and local responses to the developing emergency.

The declaration authorizes the Federal Emergency Management Agency “to coordinate all disaster relief efforts” and “provide appropriate assistance.”

National Weather Service official Mark Fox said Alex is expected to make landfall as a Category 1 hurricane as early as Wednesday night, with a forecast for as much as 12 inches of rain. Alex had maximum sustained winds near 70 mph early Tuesday.

The storm has authorities distributing sandbags and getting emergency shelters ready.

Monday, Gov. Rick Perry activated up to 2,500 Texas National Guard members, eight UH-60 helicopters and three C-130 aircraft. Elite emergency personnel -- Texas Task Force 1 and Texas Task Force 2 -- are standing by in Weslaco and Kingsville.

The Alamo Regional Command Center has been activated, and 100 buses are on standby in case evacuations are in order.

The Texas Department of Public Safety has sent 25 additional troopers to the region plus more outside of it.

"We have 100 troopers and 10 supervisors from out of region from the Valley area who are also on standby at this time to deploy if needed" DPS Maj. William Diggs said.

Brownsville Mayor Pat Ahumada warned residents that Alex “is a very dangerous storm” and to be prepared. Crews in Brownsville cleared roadside ditches, put water pumps at flood-prone intersections and began boarding up windows to prepare for the storm.

Cameron County closed beach access at the north end of South Padre Island. The University of Texas at Brownsville-Southmost Texas College was closing Tuesday afternoon. Neighboring Hidalgo County declared a disaster in advance of expected flooding.

Red Cross volunteers from Austin and other areas are also headed to South Texas. Fifteen disaster relief vehicles were loaded Monday and headed to Harlingen.

"In driving the truck that we usually take in, we have to watch how high the level of the water is. We can only take it to a certain point, and if the water is higher than that, we can not go in. We have to back out and wait until somebody else goes in," Red Cross of Central Texas volunteer Marcia Marks said.

Red Cross officials say they have more volunteers ready to deploy if necessary.

Perry said Alex poses a threat of imminent disaster and has issued a disaster proclamation for the following 19 Texas counties: Aransas, Bee, Brooks, Cameron, Duval, Hidalgo, Jim Hogg, Jim Wells, Kenedy, Kleberg, Live Oak, McMullen, Nueces, Refugio, San Patricio, Starr, Webb, Willacy and Zapata counties.

Forecasters say the storm's likely path will take it away from the site of the huge Gulf of Mexico oil spill off Louisiana's coast, but added that it might push oil farther inland and disrupt cleanup efforts.

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