"This is a relatively new innovation," said White House spokesman Josh Earnest, "to expedite the provision of federal support in a situation like this."
The list of 28 counties includes every single one along the Atlantic Coast of Florida, as well as counties inland from the east coast of the state.
FEMA - the Federal Emergency Management Agency - has already been moving supplies to points near the coast, to allow for quick action after the storm has moved through.
Also, power crews from a number of states are already on the way to Florida, to help restore power and deal with any hurricane damage to the state's power infrastructure.
"Power is going to go out, and people are going to be challenged to operate," Earnest told reporters.
"We just want to try to do as much planning as we can in advance to get it back on as soon as possible," he added.
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