Final answer:
The flag used to indicate a Hurricane Warning (observed forecast winds of 60 knots or greater) is a red flag with a black square in the center.
Step-by-step explanation:
Hurricane warnings are indicated by a specific flag known as the hurricane warning flag. This flag is a red flag with a black square in the center. The flag is raised when observed forecast winds reach 60 knots (69 miles per hour) or greater. It signals that a hurricane is approaching and everyone in the area should take immediate action to protect themselves and their property.