Simplify the following text for a Grade 11 native English-speaking person.
Original: It strengthened into Subtropical Storm Otto several hours later ; however , additional tropical cyclogenesis occurred during the following hours , and Otto transitioned into a tropical storm by October 7 .
Simplified: It then strengthened into Subtropical Storm Otto , and a few hours later the National Hurricane Center believed that it was a fully tropical storm , and it became Tropical Storm Otto .