Forewarn
/fɔːrˈwɔːrn/
Definition 1 of 1
Pro Tip 1/3Nuanced Warning
Forewarn often implies a thoughtful, considerate warning about possible future problems. 
I must forewarn you that the trail gets very treacherous beyond this point.
Pro Tip 2/3Not Immediate Danger
'Forewarn' usually refers to future dangers, not ones that are immediate or already occurring. 
The weather report forewarned of a powerful storm arriving next week.
Pro Tip 3/3Serious Tone
Using 'forewarn' can carry a serious tone and suggest that the warning should be taken seriously. 
I must forewarn you, the company's financial situation is more serious than you think.