Falling prices may be a 'warning sign' of a recession. [=may indicate that a recession is coming]
There were storm 'warnings' [=warnings that a storm was approaching] issued for the area.
I'm giving you 'fair warning' that you had better not discuss politics at dinner tonight.
(..) How dare you say such things? - Are you mad? - I may be. But since I love some of you, I urge you again to take heed of my warning. The sons of Ragnar are sure to come! As for you, sweet sister, I counsel you, have a care for your own soul. (..)2017 TvShow: Vikings Title: The Great Army Season: 4 Episode 17