Simplify the following text for a Grade 11 native English-speaking person.
Original: The system developed as a subtropical depression on September 21 in the northeastern Gulf of Mexico from the interaction of a tropical wave , the tail end of a cold front , and an upper-level low .
Simplified: The storm formed as a subtropical depression on September 21 in the northeastern Gulf of Mexico from the contact of a tropical wave , the end of a cold front , and a low pressure system .