Tropical Storm Harvey is gaining strength and is expected to evolve into a "major hurricane", the National Hurricane Center has declared.

The entire coast of Texas has been placed on high alert, with the hurricane expected to make landfall by Friday evening (25 August).

Major hurricane is one designated at Category 3 (winds of at least 111 mph or higher). Harvey had been previously marked as a Category 1 storm.

If the predictions are accurate, the first major hurricane to hit the US since Wilma in 2005.

While winds are a major concern, the storm is expected to stall over Texa,s bringing extreme levels of rainfall which could lead to flooding.

Hurricane expert Dan Kottlowski said: "Since Harvey is forecast to stall, we expect 10-20 inches of rain over a large part of southern and eastern Texas and southwestern Louisiana from Friday into early next week."

On Wednesday, the Texas Governor Greg Abbott declared a state of disaster to allow emergency services to "quickly deploy resources for the emergency response."

"Texans believe in taking action and always being prepared in the event of an emergency," Abbott said. "That is why I am taking every precaution prior to ... Harvey making landfall."

Students and staff at the Texas A&M campus at Corpus Christi on the coast have been evacuated with the campus closing today (24 August).

Hurricane Wilma in 2005
The storm is expected to be the first 'major hurricane' since Wilma (pictured) in 2005Getty