The storm sprang to life early on Monday south of Cape Verde, a country of small rocky isles off West Africa with a population of about 531,000. It won independence from Portugal in 1975.
Humberto lashed the southern Cape Verde islands with rain and wind gusts as it moved past them but by late afternoon all tropical storm warnings had been lifted.
Humberto's top winds had strengthened to 50 miles per hour, up from 40 mph on Monday morning.
The storm was moving west and was expected to swing to the north over open waters, strengthening into a hurricane with sustained winds of at least 74 mph on Tuesday.
It is not expected to pose any further threat to land, nor is there a chance it would threaten energy interests in the Gulf of Mexico, according to the hurricane center.
Humberto is the eighth tropical storm of the Atlantic-Caribbean hurricane season, which runs from June 1 to November 30.
The season is on track to be above average for tropical storm formation. But so far this year, shearing winds and patches of dry, dusty air have prevented any Atlantic storms from strengthening into hurricanes.
The first hurricane of the season usually forms by August 10. Since the dawn of the satellite era in the mid-1960s, the latest date for the first hurricane to arrive was set in 2002 when Hurricane Gustav made its debut on September 11.
If Humberto achieves hurricane status any time after 8 a.m. EDT (1200 GMT) on Wednesday, it would replace Gustav as the modern-day record holder, the forecasters in Miami said.
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