READING, Pa. -

As the weather continues to warm up, officials in Reading are urging swimmers to be extra cautious, especially in natural water.

"Conditions are changing and they're very unsafe," said Deputy Chief Michael Lessar, Reading Fire Department, standing alongside the Schuylkill River, where he and crews rescued a teenage boy Monday evening.

Lessar said the 14-year-old boy, Jose Zalapa, was entering the river near Canal and South streets with a group of friends shortly after 5 p.m. when he was swept by the current.

"[He] lost hold of the others, started to drift out, was uncertain of his swimming abilities," said Lessar.

"Honestly, I thought it was over," Zalapa told 69 News.

Fortunately, Zalapa was able to cling to a rock, and his friends phoned for help. A diver with the Reading Fire Department's scuba team was able to rescue him safely, officials said.

Still, that isn't always the outcome, according to Lessar.

"Unfortunately, throughout the years, we've seen many people that we've had to pull from the river, that just didn't survive and they were uncertain of their swimming abilities," he said.

But regardless of one's swimming experience, Lessar said anyone can be vulnerable to unpredictable currents.

"We get different currents. Somebody that may have swam here last week, now is going to experience a different current… different temperature of the water," said Lessar.

Capt. William Stoudt Sr. with the scuba team added that currents will become more intense following heavy rain.

"The current can run anywhere from 5 miles an hour to… [with] a good, heavy rain, 50 miles an hour… and it can wash you down. It'll wash a vehicle downstream," said Stoudt.

Stoudt advises people to avoid entering the water following a heavy rain, and to never swim alone, no matter the condition.

"Don't go in alone. Always have somebody there," Stoudt said.

Swimmers are also encouraged to wear life vests whenever possible and to be mindful of water temperature.

"Any water under 70 degrees is considered cold water, you could start getting hypothermia," said Stoudt. "Use common sense. That's the whole thing."