red sky in the morning, shepherd's warning
A red sky at sunrise is a sign that bad weather will follow. The full phrase is "Red sky at night, shepherd's delight; red sky in the morning, shepherd's warning." I think we're going to get a bad storm today. Look at that sunrise—red sky in the morning, shepherd's warning.
Scarborough warning
No warning. The phrase might refer to a surprise attack on the castle of Scarborough in the 16th century. Primarily heard in UK. Well, that thunderstorm came through with a Scarborough warning—it wasn't supposed to rain today at all.
warning shot
1. Literally, the firing of a weapon to signal an impending attack and/or to urge the opponent into surrender. We're almost at the shore—send up a warning shot before we land.
2. By extension, something used to signal future events and/or to urge people into a particular course of action. This petition is just a warning shot, saying that we'll be arrested if we keep playing our music so loud. I say, turn it up!
trigger warning
A statement at the beginning of an article or video advising that its content might be upsetting, especially for trauma survivors. At least that graphic article came with a trigger warning.