Hurricane Sam continued to strengthen on Sunday as it moved northwest across the Atlantic Ocean, forecasters said.

The Category 4 hurricane was 850 miles east-southeast of the Northern Leeward Islands as of 11 pm Eastern time Sunday, with maximum sustained winds of 145 mph, according to the National Hurricane Center.

A subtropical ridge northeast of the hurricane should steer it northwestward for the next three or four days. Sam should then veer north-northwest and begin to increase its speed but it is not expected to threaten land, the center said.

Forecasters described the storm as “small but ferocious” and said it was expected to remain a major hurricane for several days.

“It would not take much further expansion of the convection and cooling of the cloud tops over the inner-most core of Sam for it to become a rare Category 5 hurricane on the Saffir-Simpson hurricane wind scale,” the center said. “Although plausible, given the ideal environmental conditions over the next couple of days, this strengthening is not explicitly forecast to occur.”

The Saffir-Simpson scale classifies major hurricanes as Category 3 or higher, with maximum sustained winds above 110 mph. Category 4 storms have wind speeds of 130 to 156 mph.

The swells generated by the hurricane were forecast to reach the Lesser Antilles early this week and have the potential to cause life-threatening surf and rip current conditions, the center said. There were no coastal watches or warnings in effect.

Sam, which formed Thursday in the central Atlantic, is the fourth named storm to develop in less than a week and the 18th overall in a busy 2021 Atlantic hurricane season.

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Subtropical Storm Teresa formed north of Bermuda on Friday, becoming the 19th named storm of the hurricane season. Teresa fizzled out over the weekend and “no longer meets the definition of a tropical cyclone,” the hurricane center said Saturday.

After Sam and Teresa, the next named storms would be Victor and Wanda.

If forecasters go through the list, they will turn to an additional set of names approved by the World Meteorological Organization this year. That list begins with Adria, followed by Braylen and Caridad.

“With more than two months to go in the hurricane season, it is certainly possible that the 2021 Atlantic list of names will be exhausted,” Feltgen said.

Last year, there were 30 named storms, including six major hurricanes, forcing meteorologists to exhaust the alphabet for the second time and move to using Greek letters. It was the most named storms on record, surpassing the 28 from 2005, and the second-highest number of hurricanes.

This year, the arrival of peak hurricane season, August through November has led to a run of named storms that formed in quick succession, bringing stormy weather, flooding and damaging winds to parts of the United States and the Caribbean.

Tropical Storm Odette formed September 17, followed days later by Peter and Rose. All three storms have since dissipated.

Tropical Storm Mindy hit the Florida Panhandle on September 8, just hours after it formed in the Gulf of Mexico, and as a powerful Hurricane Larry was simultaneously churning in the Atlantic.

Ida battered Louisiana as a Category 4 hurricane August 29 before its remnants brought deadly flooding to the New York area.

The links between hurricanes and climate change are becoming more apparent. A warming planet can expect stronger hurricanes over time, and a higher incidence of the most powerful storms though the overall number of storms could drop, because factors like stronger wind shear could keep weaker storms from forming.

Hurricanes are also becoming wetter because of more water vapor in the warmer atmosphere; scientists have suggested storms like Hurricane Harvey in 2017 produced far more rain than they would have without the human effects on climate. Also, rising sea levels are contributing to higher storm surge the most destructive element of tropical cyclones.

Ana became the first named storm of the 2021 season May 23, making this the seventh year in a row that a named storm developed in the Atlantic before the official start of the season June 1.

In May, scientists with the National Oceanic and Atmospheric Administration forecast that there would be 13 to 20 named storms this year, six to 10 of which would be hurricanes, including three to five major hurricanes of Category 3 or higher in the Atlantic.

NOAA updated its forecast in early August, predicting 15 to 21 named storms, including seven to 10 hurricanes, by the end of the season on November 30.