AMNA NAWAZ: Hurricane Debby barreled into Northern Florida this morning with Category 1 winds.

The state has reported at least four deaths so far.

Now a tropical storm, it's slowly moving inland, expected to hit Georgia and the Carolinas later this week.

Its gusty winds are packing a punch, with hundreds of thousands of power outages reported.

But authorities are most concerned about Debby's rainfall, which is expected to be historic.

William Brangham has our report.

WILLIAM BRANGHAM: Debby lashed Florida's Big Bend today with fierce winds, powerful storm surges and torrential downpours.

The rain triggered catastrophic flooding, submerging whole neighborhoods and stranding drivers.

GOV.

RON DESANTIS (R-FL), Presidential Candidate: Good morning.

WILLIAM BRANGHAM: Governor Ron DeSantis was out early today with a warning.

GOV.

RON DESANTIS: The most important thing to do is to just protect yourself and protect your family.

Don't go out into this storm.

Don't drive on the roads, particularly when they're flooded.

WILLIAM BRANGHAM: Shortly after, authorities pulled a semitruck out of the water in Tampa after it fell over a guardrail, killing the driver.

Yesterday, in fierce winds, the U.S. Coast Guard had to pull two people out of the Gulf of Mexico after their sailboat was damaged.

On land, wind whipped Florida's Western Peninsula, as Debby slowly crept along offshore.

MAN: It was actually like pushing me.

Like, literally, I was -- I never had -- like, was outside in a storm that was actually pushing me.

WILLIAM BRANGHAM: Tampa's usually packed Bayshore Boulevard was a ghost town as waves crashed onto the sidewalk.

County sheriffs patrolled the streets of Fort Myers Beach, also inundated by rain and storm surge.

MAN: It's really treacherous out.

WILLIAM BRANGHAM: There were similar scenes across Florida's Southwestern coast.

JEFF BERARDELLI, Chief Meteorologist, WFLA-TV: In terms of rainfall, this is becoming a worst-case scenario because Debby has made its way parallel to the Florida coast.

WILLIAM BRANGHAM: Jeff Berardelli is chief meteorologist for WFLA-TV in Tampa.

JEFF BERARDELLI: And that means rain falling over and over and over again over the same areas.

And that means we could see two-and-a-half feet of rain in some places.

And there's no place in the country that can handle that much rain in that period of time.

There is going to be some dangerous and maybe catastrophic flooding in the Southeast.

WILLIAM BRANGHAM: As Debby continues to churn north, residents of Georgia and the Carolinas are preparing for those potentially historic rainfall totals.

JAMES STALLINGS, Director, Georgia Emergency Management and Homeland Security Agency: That will be a generational storm that we have not seen.

They're calling it a 500-to-1,000 year storm.

And so, with that, we're extremely concerned about flooding.

WILLIAM BRANGHAM: Berardelli says repeated disasters like this, events that are made worse by climate change, are stretching the nation's ability to respond.

JEFF BERARDELLI: What concerns me is this is yet again another billion-dollar disaster in the making.

And we can only handle so many of these before people become wary, before we exhaust our resources.

It's really hard to help the amount of people being affected by these big, extreme weather events.

And it's really starting to wear on us.

WILLIAM BRANGHAM: For the "PBS News Hour," I'm William Brangham.