The NOAA-NASA satellite GOES-16 captured this geocolor image of Hurricane Irma passing the eastern end of Cuba at about 8:00 a.m. EDT on Sept. 8, 2017.

A forecast for Hurricane Irma has shifted the trajectory of the mega-storm slightly westward, which may mean direct hits on the Florida Keys and worst-case scenarios unfolding for major cities like Naples, Fort Myers and Tampa.

As of midday Saturday East Coast time, Irma had been downgraded slightly to a Category 3, but was expected to regain strength as it reached the outskirts of Florida. Modeling from the NBC Weather Unit now expects catastrophic wind and storm damage in the Keys early Sunday morning. Marco Island, Naples, Cape Coral and Fort Myers are likely to be struck later on Sunday.

A shift west would still mean that the east coast of Florida sees hurricane gusts and damage, NBC said, but it makes catastrophic effects less likely from Miami northwards. On Saturday, an updated forecast showed the South Florida redoubts of Miami, Fort Lauderdale and Palm Beach — all of which were predicted to be Irma's primary targets — nearly out of the storm's cone.

The outer band of Irma, which has killed at least 22 people in the Caribbean, was already lashing South Florida with tropical storm-force winds and left nearly 25,000 people without power, Florida Governor Rick Scott said.

Irma could inflict major damage on the fourth-largest U.S. state by population, which is braced for winds well in excess of 100 miles per hour and a huge storm surge that could trigger coastal flooding.

"This is a deadly storm and our state has never seen anything like it," Scott said at a Saturday morning news conference.

Irma, one of the most powerful Atlantic storms in a century, strengthened and made landfall on an island chain in Cuba as a category 5 storm, according to a late Friday night tweet from the U.S. National Hurricane Center (NHC).

NHC tweet

By Saturday morning, it was again downgraded to a category 3 storm moving along the Camaguey Archipelago, the NHC said.

The NHC also extended hurricane warnings northward along the Florida peninsula.

Earlier on Friday, the "extremely dangerous" hurricane was downgraded from a category 5 to a category 4, but it still packed winds as strong as 155 miles per hour at that time, the NHC said in an advisory.

Irma has already lashed the Caribbean with devastating winds and torrential rain, leaving behind at least 21 deaths and a swath of destruction.

Dozens of cities and counties in increasingly-northern regions of Florida are issuing mandatory evacuations that have affected millions of residents. Some highways have ground to a halt from congestion as evacuees flee the most dangerous hurricane areas.

Florida Governor Rick Scott is scheduled to deliver an update on the storm on Saturday morning from Sarasota.

A mandatory evacuation on Georgia's Atlantic coast was due to begin on Saturday, Governor Nathan Deal said.

Diana Olick tweet

Rick Scott Evac tweet 1

— Reuters, NBC News' Bill Karins, and CNBC's Javier E. David, Kevin Breuninger and Spencer Kimball contributed to this report.