Bermuda braced for a close encounter with tropical storm Gabrielle, on track to hit or pass closely by the popular vacation spot by early Wednesday, US forecasters said Tuesday.

Meanwhile, tropical storm Humberto, churning in the Atlantic west of the Cape Verde Islands, was expected to gain hurricane force within hours, the US National Hurricane Center said.

Packing maximum sustained winds near 65 miles per hour (100 kilometers per hour), Humberto was about 220 miles (355 kilometers) west of the southernmost Cape Verde Islands, the National Hurricane Center said.

"Some strengthening is forecast during the next day or two and Humberto should become a hurricane later today or tonight," the Miami-based forecasters said in an update at 1500 GMT.

The system was expected to turn toward the northwest later Tuesday, followed by a sharp northward turn, coupled with a pickup in forward speed, by late Wednesday.

However, no coastal watches or warnings were in effect.

Gabrielle, with maximum sustained winds near 40 miles per hour, was some 120 miles south of Bermuda, according to another 1500 GMT update from the National Hurricane Center.

"Gabrielle is expected to pass over or near Bermuda tonight and early Wednesday," the forecasters said.

"Some strengthening is forecast during the next 48 hours."

A tropical storm warning was in effect for Bermuda, which could see up to seven inches (18 centimeters) of rainfall and storm surges of two to three feet (0.6 to 0.9 meters) above normal levels.