Forecasters and emergency agencies prepped for a long weekend early Friday as Tropical Storm Erika wobbled toward Puerto Rico and then the southern Atlantic coast, where it could be a hurricane when it arrives early next week.
A tropical storm warning was in effect for Puerto Rico and the U.S. Virgin Islands after Erika caused mudslides blamed for at least four deaths on the Caribbean island of Dominica.
More than 12 inches of rain poured down on Dominica in less than 12 hours, the National Weather Service said — and it could do close to the same overnight and Friday on Puerto Rico, where forecasters warned that major flooding is possible.
After that, it's anybody's guess.
The center of Erika started "wobbling" Thursday, according to the weather service, making it difficult to project its course. The storm was about 135 miles south east of San Juan, Puerto Rico, at 11 p.m. Thursday night with maximum sustained winds of 45 mph, the National Hurricane Center said.
"At this point, to quote Shakespeare, 'to be or not to be — that is the question,'" said Danielle Banks, a meteorologist for The Weather Channel. "There's still a lot of uncertainty with the track and the intensity."
Late Thursday afternoon, forecasters said Erika looked increasingly likely to reach the U.S. mainland Monday night or Tuesday morning, possibly as a Category 1 hurricane.
"We're still a long ways away from this thing affecting" the mainland, said Ari Sarsalari, another Weather Channel meteorologist. But Myrtle Beach, South Carolina, he said, "might have to start thinking about making preparations."
It already was — like emergency management agencies from the southern tip to the Panhandle of Florida on the Gulf side of the state and up to North Carolina on the Atlantic side.
"It's going to be all hands on deck this weekend," Carl Barnes, a meteorologist at the National Weather Service's regional office in North Charleston, South Carolina, told NBC station WCBD of Charleston.
Kim Stenson, director of the South Carolina Emergency Management Division, said: "We're watching Tropical Storm Erika very closely. If it looks like this storm is going to affect South Carolina, we want everyone to be ready."
The Salvation Army, meanwhile, began trucking tens of thousands of gallons of bottled water to a staging area Thursday morning at West Palm Beach, Florida.
"Water is one of the most critical needs when responding to a natural disaster," Maj. James Hall, the Salvation Army's regional commander, told NBC station WFLA of Tampa. But volunteers are also on standby to set up mobile food stations across the state, he said.
TODAY weather anchor Al Roker said coastal areas were likely to get 5 to 7 inches of rain by next week — with some getting as much as a foot."
"It's been 10 years since Florida had a significant hurricane, and this one could be the one," Roker said Thursday evening.
Florida's southeast coast could be especially vulnerable to a storm surge and enhanced risk of floods greater flood risk, said Mark Perry, executive director of the Florida Oceanographic Coastal Center in Stuart.
"This weekend — actually Friday to next Thursday — we'll have these very, very strong tides due to the full moon," Perry told NBC station WPTV of West Palm Beach.
This article originally appeared on NBCNews.com.