Simplify the following text for a Grade 15 native English-speaking person.
Original: Instantly after Erin become a tropical depression , the National Hurricane Center gave out a tropical storm watch from Freeport , Texas to the United States\/Mexico border ; at the same time , the government of Mexico issued a tropical storm watch southward to Rio San Fernando .
Simplified: Immediately upon becoming a tropical cyclone , the National Hurricane Center issued a tropical storm watch from Freeport , Texas to the United States\/Mexico border ; at the same time , the government of Mexico issued a tropical storm watch southward to Rio San Fernando .