Can you warn your teammates that they are being surprised?

Question

Can I warn my party members of an enemy even if they're surprised. If I can does that mean that they are not surprised?

Answer 1

If combat has already started, it's too late

The rules for surprise say (emphasis added):

Any character or monster that doesn't notice a threat is surprised at the start of the encounter.

The important part here is the timing: surprise is determined "at the start of the encounter" – generally this is whenever initiative is rolled. So if you're already in initiative, then it's too late to prevent a creature from being surprised.

Note that surprise doesn't mean that the surprised creature is unaware of the enemy for the entire 6 seconds of the first round of combat. It means that the creature was unaware of the enemy at the moment the encounter started, and was therefore unprepared for combat, either mentally or physically. Once the first attack lands and the screaming starts, everyone is acutely aware that an enemy is attacking them, but if they are surprised, that means it takes them 1 full round to prepare for combat.

Think of a time when a sudden emergency happened when you were not expecting it. Did you jump immediately into action, or did you need a few seconds to get your bearings and assess the situation before you were able to react in any useful way? This is what the surprise mechanic is meant to represent.

You should probably still shout that warning

That being said, if your group is into role playing based on what their individual characters know and avoiding meta-gaming, shouting out information about what is attacking the party will be very helpful to the surprised characters once they are able to act. Beyond that, if you have allies nearby who may not have heard the start of combat, being loud can get you reinforcements. So it's probably still worth making some noise, even if it won't negate the surprised condition on your allies.