(.)

Aug 30 (Reuters) -

Millions of residents braced themselves as

Hurricane Idalia

slammed into Florida on Wednesday morning and headed into Georgia as a Category 1 hurricane.

The storm had hit the Big Bend region of Florida just before 8 a.m. EDT (1200 GMT) as a Category 3 hurricane with winds of 125 mph (201 kph), but weakened to Category 1 with wind speeds of 90 mph as it moved inland, the National Hurricane Center (NHC) in Miami said.

Mandatory evacuations were in place in 16 of Florida's 67 counties as of late Wednesday morning. Most of Georgia and South Carolina were under storm warnings or advisories.

Here are the most important things to know about Idalia's projected impact.

WHAT IS IDALIA'S PATH?

As of 11 a.m. EDT, Idalia was packing maximum sustained winds of 90 mph as it whirled into southern Georgia near Valdosta, the NHC said in its

latest advisory

.

Idalia's center crossed Florida's coastline over the Big Bend region, where the state's northern panhandle meets the Gulf side of the Florida peninsula, and was headed northeast toward Savannah, Georgia, and up through Charleston, South Carolina, before it was expected to spin out over the Atlantic Ocean.

The NHC forecast Idalia would become a tropical storm while moving near the coasts of northeastern South Carolina and North Carolina on Wednesday night and Thursday.

DEADLY RAINS, SURGING SEAS

Two motorists died

in separate rain-related crashes on Wednesday morning, according to the Florida Highway Patrol.

The deadliest threat Idalia poses is a surging wall of seawater that could flood low-lying areas along Florida's coast, according to authorities.

Surge warnings were posted for hundreds of miles of shoreline, from Sarasota to the western end of Apalachicola Bay. In some areas, the surge could rise as high as 16 feet (5 m), according to the NHC.

"Storm surges" occur when high winds and atmospheric pressure from an oncoming hurricane force ocean water onto land. The resulting floodwaters may take a couple of days to subside.

A "king tide" - the highest type of high tide, caused by the extra gravitational pull that occurs when the sun and moon align with Earth - on Wednesday will likely exacerbate the surges from Idalia.

EVACUATION ZONES

Camden County in Georgia, where the chairman signed a state of emergency, issued an evacuation order, specifically for the residents of Cumberland Island and Little Cumberland Island. No other evacuation orders have been listed in Georgia as of 11 a.m. EST.

The Florida emergency management agency listed 28 counties with evacuation orders, 16 as mandatory for certain residents, especially those living in coastal and flood-prone areas or in mobile homes, recreational vehicles or structurally unsound housing.

POWER OUTAGES

Some over 275,000 homes and businesses were without power in Florida as of midday, Poweroutage.us reported. More than 64,000 in Georgia were also without power.

Some 30,000 to 40,000 electricity workers had been placed on standby to help restore power quickly after the hurricane passes.

FEMA

Deanne Criswell, the U.S. Federal Emergency Management Agency's administrator, said on Wednesday morning that more than 1,000 personnel from FEMA's rapid assessment teams were ready to hit the ground to assess storm damage once Idalia passes.

(Reporting by Julia Harte and Rachel Nostrant; Editing by Andy Sullivan, Cynthia Osterman and Marguerita Choy)