Tropical Storm Eugene was strengthening off western Mexico, about 700 miles west-northwest of Zihuatanejo, the National Hurricane Center said Monday. Eugene was expected to intensify during the next couple of days and could become a hurricane by Tuesday, the Miami-based center said in its 5 a.m. Eastern time advisory. No watches or warnings were in effect. Eugene, with maximum sustained winds of 60 mph, was traveling west-northwest at about 12 mph, the center said, and was expected to maintain that track for the next two days.