Erin barreled through the Atlantic as a “formidable” Category 4 hurricane near Puerto Rico, after earlier reaching the top of the five-step, Saffir-Simpson scale.

Erin’s top winds reached 140 miles per hour (225 kilometers/hour) as the storm churned 145 miles north northeast of San Juan, the US National Hurricane Center said in a statement at 11 pm New York time.

“The outer bands of Erin will continue to produce areas of heavy rainfall through Sunday across the northern Leeward Islands, the Virgin Islands, and Puerto Rico,” according to the hurricane center. “Locally considerable flash and urban flooding, along with landslides or mudslides, are possible.”

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Current forecasts show the center of Erin is expected to begin moving away from the Virgin Islands and Puerto Rico on Sunday, and pass to the east of the Turks and Caicos Islands and the southeastern Bahamas Sunday night and Monday.

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Warm ocean water, the lack of major wind shear and its escape from dry air allowed Erin to rapidly intensify since early Friday. At 8 am Friday, Erin was a tropical storm with 70 mph winds, and by 8 am Saturday it already reached Category 4 status with winds of 145 mph. The National Hurricane Center issued a special statement at 11:20 am to say Erin became a “catastrophic” Category 5 with 160 mph winds.

Rapid intensification means a storm’s winds increase by at least 35 mph in a 24-hour period, and can be dangerous because coastal residents and emergency crews may be caught by surprise. There have been numerous examples of storms rapidly intensifying, including Hurricane Milton, which struck Florida in October.