Hurricane Irma will be a Category 4 or 5 storm as it approaches Florida — here's everything you need to know

(National Hurricane Center)

• Hurricane Irma slammed into several Caribbean islands as a Category 5 storm on Wednesday and Thursday, killing at least 23 people.

• The National Hurricane Center's projected path shows Irma arriving in South Florida over the weekend and traveling up the center of the state.

• Florida Gov. Rick Scott has activated the state's National Guard, and parts of South Florida and Georgia are under mandatory evacuation orders.

• President Donald Trump has declared a state of emergency for Florida in anticipation of the storm.

After steamrolling several Caribbean islands, Hurricane Irma made landfall on the coast of Cuba late Friday night.

The National Hurricane Center says Irma has weakened slightly to a Category 4 storm, as it moves over the Camaguey Archipelago of Cuba.

Irma was briefly upgraded to a Category 5 storm late Friday, but now has maximum sustained winds of 155 mph. The National Hurricane Center placed the storm about 245 miles from Miami, and moving in a westward direction at about 12 mph, the NHC said.

Several hurricane and storm-surge warnings and watches were extended late into the night on Friday, stretching northward along Florida's east and west coasts.

Hurricane warnings have been issued for Florida, the Bahamas, and parts of Cuba, where the storm is hitting Friday afternoon and evening.

Florida's hurricane warnings extend from Sebastian Inlet southward around the peninsula to Anna Maria Island, as well as the Florida Keys, Lake Okeechobee, and Florida Bay. Storm-surge warnings are in effect from Sebastian Inlet around the peninsula to Venice.

"Catastrophic and life-threatening wind and storm surge" are expected to hit the Florida Keys as early as Saturday night, NHC said.

Hurricane watches in Florida are in effect from north of Sebastian Inlet to the Flagler/Volusia County line on the east coast and from north of Anna Maria Island to the Suwannee River on the west coast.

Storm-surge watches are in effect from north of Sebastian Inlet to the Flagler/Volusia County line on the east coast and from north of Venice to the Anclote River on the west coast and for Tampa Bay.

Irma, one of the most powerful Atlantic hurricanes in recorded history, has killed at least 23 people, according to reports.

Irma's intensity

Irma is the strongest hurricane ever measured in the Atlantic Ocean (outside of the Caribbean Sea or the Gulf of Mexico, where Hurricane Allen still holds the record). The storm's winds are as powerful as those of a tornado, which can tear off roofs, level homes, toss cars, overturn trains, and uproot large trees.

Initial damage from the storm was observed on Wednesday in Barbuda, an island east of Puerto Rico. Gaston Browne, the prime minister of Antigua and Barbuda, said Barbuda was "totally demolished," with 90% of its buildings destroyed. Communication with the island was cut off because of the destruction. As Hurricane Jose approaches the island, Barbuda officials are trying to evacuate the entire population of 1,800.

Irma also destroyed an estimated 95% of buildings in parts of St. Martin and devastated popular tourist destinations in St. Barts. It slammed the Virgin Islands before passing just north of Puerto Rico. Winds were still strong enough to cut off power to half of Puerto Rico's residents, however, and reports suggest some may not regain electricity for months.

Irma is now a Category 4 storm, with sustained winds of 155 mph, but officials caution that it should not be referred to as "downgraded" — it's still a life-threatening, catastrophic storm with major population centers in its path, and it's likely to fluctuate between categories 4 and 5 before making landfall in Florida.

As seen with Hurricane Harvey, which made landfall late last month as a Category 4 hurricane but caused most of its damage with heavy rain, the number doesn't always accurately predict a storm's impact.

"I fear that people will say, 'Oh, it is only Category 4 now, we are safe,'" Marshall Shepherd, a meteorologist, wrote early Friday. "This would be devastating and potentially deadly. It is a Category 4 storm with a track that will bring it directly into South Florida with the most populated cities of the region on the dangerous right side of the eye."

The Category 4 label means Irma has sustained winds of at least 130 mph near its core, though it's on the verge of the Category 5 level of 157 mph — the latest "hurricane hunter" airplane measurements suggest the storm's sustained winds top 155 mph, and gusts of more than 215 mph were previously recorded.

Even now, measurements are showing winds above the storm of up to 190 mph, which are likely to affect tall buildings.

Additionally, Irma is passing over some of the warmest water it has encountered, which could help it maintain its intensity or even pick up steam. The National Weather Service says Irma could regain Category 5 strength at some point before landfall.

Irma also threatens the Caribbean and US with storm surge, a crest of water formed ahead of a storm by powerful winds. The National Hurricane Center suggests that parts of the Bahamas, for example, may see a storm surge with a height of 20 feet above a typical high tide, though forecasts differ greatly among regions. The NHC said Friday morning that storm-surge concerns for southwest Florida were growing.

Irma heads to Florida

The NHC's latest forecast for Irma projects the centerline of the storm going right into South Florida before heading up the state and into Georgia. Irma is big and powerful enough to affect most of Florida — the storm is almost as large as Texas, with a huge eye that expanded overnight. If Irma heads up the middle of Florida, it's wide enough that both coasts could experience hurricane-force winds.

(National Hurricane Center)

Though it's too soon to say for certain whether the center of the storm will hit Miami, most predictive models show that southwest Florida is in Irma's path, and the warm water could help the storm maintain its strength the whole way. At the very least, tropical-storm-force winds are likely in Miami by Saturday and hurricane-force winds by Sunday.

And even if Irma goes west of Miami, the most dangerous part of a hurricane is to the right of its eye, so the city could still see catastrophic damage.

As a result, US and state officials are urging people to ready their emergency plans and supplies. Florida and Georgia have issued evacuation orders for many parts of both states, and Florida has closed all schools, colleges, and universities.

About 5.6 million people have been told to evacuate, leading to traffic jams and fuel shortages across the states.

"We are running out of time ... You need to go now," Gov. Rick Scott of Florida said on CNN on Friday morning, urging anyone in an evacuation zone to flee immediately. "This is a catastrophic storm that our state has never seen."

Scott said all Florida residents should be prepared to leave if necessary.

To prepare for Irma, Scott ordered all 7,000 members of Florida's National Guard to report for duty on Friday morning. President Donald Trump declared states of emergency in Florida, Puerto Rico, and the US Virgin Islands on Tuesday evening to free up federal resources for a major response to the storm.

Experts say to expect widespread power outages throughout Florida, with some areas likely to remain without electricity for weeks.

The growth of a monster storm

Irma formed off the coast of western Africa last week and almost immediately started crossing the Caribbean Sea. Phil Klotzbach, a meteorologist at Colorado State University specializing in Atlantic hurricanes, told Business Insider last week that a combination of conditions — including a warm tropical Atlantic, a weak wind shear, and a change from drier to wetter weather — made it easy for Irma to pick up strength.

Irma officially became a named storm on August 30 and was classified as a hurricane the next day. Since then, it has gained and kept strength from the moisture of unusually warm waters in the Atlantic Ocean and the Caribbean Sea. Its sustained wind speeds of 185 mph for 37 hours set a record for the longest a cyclone has maintained that intensity.

(The GOES-16 satellite's view on Tuesday of Hurricane Irma as a Category 5 storm.CIRA/RAMMB; NOAA)

Experts suggest that Irma could still get stronger before making landfall again.

James Belanger, a senior meteorological scientist with The Weather Company — the group behind the Weather Channel and Weather Underground — told Business Insider that it was "possible the storm could strengthen further" as it passes over warm waters near the Bahamas.

"One of the things we need to keep in mind is that some of the guidance is that the storm is going to maintain [strength]," he said.

(CIRA/RAMMB; GOES-16/NOAA)

A busy hurricane season

Irma, the season's fourth hurricane, has already put the Atlantic far ahead of the average accumulated cyclone energy — a measure of the energy of tropical cyclone systems — for this time of year. Klotzbach said that half of a season's cyclone energy usually occurs in September. But as of Wednesday, there had already been enough to meet the definition of an average season.

Both Colorado State University and The Weather Company predicted an unusually active hurricane season this year. Irma is the fourth hurricane of 2017, though the average date of the fourth hurricane in a year is September 21. Three hurricanes — Irma, Jose, and Katia — are swirling in the Atlantic, though the peak of the season is not until September 10.

Jose, a Category 4 hurricane east of Irma, is making its way toward some of the same Caribbean islands that Irma has already devastated. And Katia, a Category 2 hurricane off the coast of Mexico, is expected to make landfall in Mexico by Saturday afternoon.

A hurricane's category is determined by its wind force — here's what the scale means:

(Ana Pelisson/Business Insider)

This is a developing story. Find all of Business Insider's latest Hurricane Irma coverage here.

NOW WATCH: Forecast shows 'extremely dangerous' Hurricane Irma heading for Florida and Cuba — here are the latest updates

More From Business Insider

• Hurricane Irma looks monstrous in a new video recorded by the International Space Station
  

• How to track the path of Hurricane Irma — which could soon hit the US
  

• Irma is now a major Category 3 hurricane as it moves across the Atlantic