At 11 a.m. EDT, the hurricane center said "preparations should be rushed to completion in the warning areas" as "conditions will continue to deteriorate" throughout the day.
The approaching storm marks Trump's first natural disaster and one of the first major domestic problems that does not stem from political strife. FEMA, working with state officials, is tasked with preparing for and responding to natural disasters that are considered a significant threat to life and property.
Two former FEMA administrators believe Long, who was confirmed to the post only in June, is up to the task.
James Witt, a former FEMA head under President Bill Clinton, said Long brings experience from leading Alabama's emergency agency and will do a "great job." Aside from supplementing state resources with federal resources, Long will have a role in making sure that Trump stays up to date on developments and communicates effectively with local officials, Witt told CNBC.
"The president needs to give them his full support and [say] that he's going to be on top of it," said Witt, who is now CEO of consulting and lobbying firm Witt Global Partners.
Long, who actively tweeted updates on the storm and FEMA on Friday morning, told CNN that the agency has positioned management teams, commodities and search and rescue crews in Texas.
On Thursday, White House press secretary Sarah Huckabee Sanders said Trump "has been briefed and will continue to be updated" on the hurricane. She played down concerns about the Department of Homeland Security — which oversees FEMA — lacking a permanent secretary, expressing confidence in acting Secretary Elaine Duke.
On Thursday, Texas Gov. Greg Abbott said he with Trump and the heads of DHS and FEMA, who are "helping Texas respond."
Abbott has declared a state of disaster in at least 30 counties, and widespread evacuations have been ordered along Texas' Gulf Coast.
Trump tweeted Friday morning that he has spoken with Abbott and Louisiana Gov. John Bel Edwards about Harvey and noted that he's "here to assist as needed."
On Friday, Sanders said in a statement that "the storm will likely be very destructive for several days" and that Trump "encourages people in the path of this dangerous storm to heed the advice and orders of their local and state officials."
"The President's highest priority is the safety of the public and of first responders. Those who ignore evacuation orders could be putting both themselves and first responders in danger," she said.
Trump plans to visit Texas sometime next week, Sanders said.
David Paulison, who took over FEMA following the agency's heavily criticized response to Hurricane Katrina in 2005, told CNBC he is "very comfortable" with the officials overseeing the disaster response.
"With the politics aside, I'm comfortable from what I've seen that they're going be able to handle this as well as possible," said Paulison, who now works for emergency management and homeland security consulting firm Global Emergency Solutions.
Still, the federal government has no formal control over one crucial area of limiting potential damage and loss of life: issuing evacuations. Long acknowledged as much in the CNN interview on Friday.
"I do not have the authority," Long said, saying it falls on local county officials.
Witt said listening to evacuation orders is "the most important thing right now."
Said Witt: "You can replace material things but losing a life is very, very tough."