Only one Category 5 hurricane has ever made landfall on the island of Puerto Rico, an unincorporated territory of the United States. That was the 1928 Okeechobee hurricane that crossed the island with sustained winds of 160mph and caused more than 300 deaths on the island. Later, that storm would become the second-deadliest hurricane in the history of the continental United States, with 2,500 deaths in Florida.

Now, Hurricane Maria seems likely to become the second Category 5 to hit Puerto Rico. As of 5pm ET, the storm is intensifying, with 165mph sustained winds. Critically, the storm's central pressure is also falling, and it is down to 916 millibars as of Tuesday evening. At that central pressure, Maria would rank among the 10 most-intense landfalling hurricanes in the Atlantic basin in the last 150 years.

In response to the threat to the US Virgin Islands and Puerto Rico, warnings from the National Hurricane Center have become increasingly dire during the last 48 hours. In a Tuesday evening public advisory, forecasters warned of Maria's "potentially catastrophic" winds, rainfall, and storm surge. They urged that final preparations be rushed to completion.

Category 5

The 2017 Atlantic hurricane season has now seen the formation of four major hurricanes—Harvey, Irma, Jose, and Maria—all of which but Jose have threatened land as a Category 4 storm or stronger. Of those storms, Irma and Maria have reached rare "Category 5" status, with sustained winds of 160mph. Over more than the last hundred years, only six seasons have produced more than one Category 5 hurricane.

A handful of seasons have produced more Category 5 hurricanes, such as the hyper-active 2005 year that produced four in the Atlantic basin. But it is rare for hurricanes to remain this strong, this long. Combined, Irma and Maria have now retained Category 5 status for more than four days, a tally not exceeded since the 1932 hurricane season, according to Colorado State University hurricane scientist Phil Klotzbach. That year's storms included the legendary 1932 Cuba hurricane, which killed more than 3,000 people.

After moving across Puerto Rico, Maria is expect to turn to the north-northwest, which more likely than not will keep the worst of the system's winds and rains away from Turks and Caicos as well as nearby islands, which were battered by Hurricane Irma earlier this month. A further turn to the north, and later northeast, should keep Maria away from the continental United States.