Sub-tropical storm Debby has strengthened slightly to a tropical storm over the far North Atlantic, according to the US National Hurricane Center (NHC). The storm, which is about 1,890km (1,175 miles) west-northwest of the Azores with maximum sustained winds of 75 km/h (45 miles per hour), is expected to weaken slowly from late on Wednesday, the NHC said. The storm system is forecast to dissipate over the far northern Atlantic by Thursday night, Reuters reported, citing an NHC statement on Wednesday.