Miami, Florida (CNN) -- Forecast to gain "major" Category 3 status, outer rainbands association with a strengthening Hurricane Earl were affecting the islands of Antigua and Barbuda on Sunday, with conditions forecast to deteriorate, the National Hurricane Center said.

Earl became a Category 1 hurricane on Sunday. As of 8 p.m. ET, its center was about 100 miles (160 kilometers) east of Barbuda and about 185 miles (300 kilometers) east of St. Martin. Its maximum sustained winds had increased slightly to 85 mph (140 kph) and higher gusts, and the storm was heading west-northwest at near 14 mph (22 kph).

The center of Earl is forecast to pass near or over the northern Leeward Islands Sunday night and Monday, the center said.

A hurricane warning was in effect for Antigua, Barbuda, Montserrat, St. Kitts, Nevis, Anguilla, St. Martin, St. Barthelemy, Saba and St. Eustatius. As of 5 p.m., the government of Antigua and Barbuda also had added the British Virgin Islands to the warning, forecasters said. A hurricane watch and a tropical storm warning was in effect for the U.S. Virgin Islands and Puerto Rico, including the islands of Culebra and Vieques.

Tropical storm-force conditions were forecast in the warning area Sunday afternoon and evening, with hurricane conditions expected early Monday, forecasters said. Tropical storm conditions were expected to spread across the Virgin Islands and Puerto Rico on Monday, with hurricane conditions possible on Monday night.

"Strengthening is expected during the next 48 hours, and Earl is forecast to become a major hurricane by Tuesday," forecasters said. Earl was forecast to turn west-northwest and slow down over the next day. Tracking maps show it moving northward away from the Caribbean and possibly affecting the U.S. East Coast by Friday.

Easterners should pay attention to Earl, as it could affect areas anywhere from the Carolinas to Cape Cod, Massachusetts, over Labor Day weekend, said CNN meteorologist Jacqui Jeras. It is too soon to tell whether the storm could make landfall, she said. However, outer bands of the storm are likely to hit North Carolina beginning Thursday, kicking up large swells and possibly generating rip currents through Labor Day weekend, Jeras said.

Earl is expected to dump between 3 and 5 inches of rain over the northern Leeward Islands, with isolated amounts of 8 inches possible in some areas. Puerto Rico is expected to receive between 4 and 6 inches of rain, with isolated amounts of 12 inches possible over higher elevations. In addition, Earl is expected to produce a 2-to-4-foot storm surge within the hurricane warning area, and 1-to-3-foot storm surge in coastal areas, accompanied by high surf.

Meanwhile, Hurricane Danielle remained away from land, and was gradually weakening as it moved northeastward, the hurricane center said. The center of the storm was about 605 miles (975 kilometers) south of Cape Race, Newfoundland. Once a Category 4 storm, Danielle had weakened Sunday to a Category 1 hurricane with winds of near 80 mph (130 kph). It was expected to continue to weaken, and was moving north-northeast at near 29 mph (46 kph).

Large waves and dangerous surf conditions should diminish near Bermuda on Sunday, the hurricane center said, and "swells from Danielle are expected to gradually subside during the next day or two along portions of the United States East Coast."

A number of people were rescued along the East Coast because of those swells and riptide currents. Nearly 250 were rescued Saturday in the waters off Ocean City, Maryland, authorities said.

In addition, an area eastward from Earl could develop into a tropical depression and then potentially into Tropical Storm Fiona, Jeras said.