Less than a week after Hurricane Harvey tore through Texas and Louisiana, causing at least 60 deaths and billions of dollars in damages, Hurricane Irma appears ready to make U.S. history.

Irma on Tuesday grew into a Category 5 storm — with wind speeds exceeding 156 miles per hour — as it moved toward the Caribbean and southern Florida. If the storm maintains course and this ferocity, it will rewrite the history books.

“The U.S. has never been hit, since we started collecting records in 1851, by two Category 4 or stronger hurricanes in the same season.”

“The U.S. has never been hit, since we started collecting records in 1851, by two Category 4 or stronger hurricanes in the same season,” said Jeff Masters, a meteorologist and co-founder of Weather Underground. “If Irma follows the current National Hurricane Center projection, that will happen.”

Some uncertainty still surrounds Irma’s fate, so here’s what Puerto Rico, south Florida and the rest of U.S. should expect from the storm — plus an explainer on why this hurricane season is churning out so many extreme storms.

‘Florida’s best hope’ could be Cuba’s disaster. Computer models are strongly predicting that the eye of Hurricane Irma could make landfall in Puerto Rico, the British and U.S. Virgin Islands, as it moves toward Hispaniola and Cuba. Hispaniola and Cuba will largely dictate the fate of what happens to the Florida Keys, Miami and the rest of south Florida, Masters said.

“Cuba is a big island, with a lot of mountains. It could definitely destroy the inner core of Irma, if the storm were to spend more than 12 hours over Cuba,” he said. “That’s kind of Florida’s best hope right now.”

The storm would need to spend time over open water to rebuild its core, which it wouldn’t have before making U.S. landfall. But if the storm squeaks through these islands, then it’s likely to be a Category 4 or Category 5 hurricane when it hits, Masters said.

A Category 5 hurricane has not hit the mainland U.S. since Hurricane Andrew in 1992, and such an event has only happened to the nation three times since record keeping began in 1851. Because of the rarity, Masters and other meteorologists are not calling for a Category 5 landfall in the U.S. at the moment.

Meteorologists should become fairly certain about Irma’s severity and threat to the U.S. by Wednesday, Masters said, as certain variables in the weather forecast become clearer.

Here’s why so many hurricanes are heading toward the U.S. Masters said “the Atlantic is primed for making major hurricanes”right now due to three major ingredients.

“Typically, over the last 10 to 12 years, we tend to have a ridge of low pressure along the east coast, which has helped steer the storms away.”

First, and most pertinent, is weak wind shear. Hurricanes lose potency when winds near the ocean surface blow at one speed and direction while winds in the upper atmosphere blow another. This difference — or shear — causes a hurricane to physically tilt, like a top, which dampens the force of the winds emanating from it.

“Right now, the shears are pretty low, and as you can see the storm looks pretty symmetrical,” Philip Klotzbach, an atmospheric scientist at Colorado State University, told the NewsHour.

The other two ingredients are warm Atlantic ocean temperatures that extend deep underwater and high levels of moisture in the air. Irma is brewing where sea temperatures hover around 85 degrees Fahrenheit, which are ideal for fueling intensification. Meanwhile, the air has 55 percent relative humidity, which is a little on the dry side, Masters said, but still enough to make a strong storm.

Irma is following on the heels of Harvey, because of a change in air pressure along the East Coast, Klotzbach said.