Humberto, with maximum sustained winds of 60 mph -- up from 35 mph at the start of the day -- was about 120 miles west-southwest of the southernmost Cape Verde islands, prompting the government there to lift its tropical storm warning, the National Hurricane Center in Miami said in its 11 p.m. EDT advisory.
Humberto, with tropical-storm-force winds extending up to 80 miles from its center, could grow into a hurricane Tuesday, the forecasters said.
The storm's forward momentum had slowed from 19 mph earlier in the day to 10 mph. The storm was expected to slow even further during the night when it was to shift from its westerly course to a west-northwest track.
Humberto is expected to turn sharply to the northwest and north-northwest Tuesday and Wednesday.
The Cape Verde islands were expected to receive 2-4 inches of rain with up to 6 inches in isolated areas. Flash floods and mudslides were possible as a result of the drenching.