Related Clues:

• APB, for one
• It stops traffic
• A serious one may be red
• Storm warning
• Flag or flare, perhaps
• Emulate Paul Revere
• Wave or wink
• "Fore!" for one
• Give a heads-up to
• Readiness condition
• Yellow flag
• Fully conscious
• Clear-witted
• Full of NoDoz
• Not dozing