Humberto, with maximum sustained winds of 40 mph, was about 1,150 miles west-southwest of the Azores, moving on a north-northeasterly path at 5 mph, the National Hurricane Center said in its 5 a.m. EDT advisory.
No watches or warnings were posted.
Forecasters said Humberto was expected to maintain its northward motion while picking up some forward speed through Wednesday night.
Humberto also was expected to slowly strengthen in the next 48 hours.
Tropical storm-force winds extended outward as far as 175 miles from the center.