The most powerful storm to hit the United States in over a decade is fast approaching the southern coast.
Tens of thousands of people have been evacuated in Texas, which lies directly in the storm's path.
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Seven hundred members of the National Guard, a reserve military force, are on standby, as well as several military helicopters in preparation for search and rescues and further evacuations.
Forecasters say Hurricane Harvey will bring "life-threatening" amounts of rain, and has the potential to raise the water level by up to 3.5 metres.
"In all these years, it's rare that I've seen a hurricane threat that concerns me as much as this one does," hurricane expert Rick Knabb told The Washington Post.
It's predicted to be the worst storm to hit the United States since 2005 when Hurricane Katrina barrelled into New Orleans, killing at least 1200 people during the impact and subsequent flooding.
It will potentially be the first major disaster of Donald Trump's presidency, and all eyes are on the President to see how he reacts.
So far in response to the emergency situation, Mr Trump has tweeted a candid video telling people to "plan ahead". The video shows Mr Trump walking around speaking to officials to a stirring music track.
"The U.S.A is the most resilient nation on earth, because we plan ahead," a scrolling message on his Twitter video says. "Preparedness is an investment in our future! President Donald J. Trump."
Several counties along the Gulf coast have ordered mandatory evacuations in low-lying areas.
Hurricane Harvey is forecast to hit the Texas and Louisiana coast at 7pm on Friday (local time).