“First Warn” is improper (or at least nonstandard) English.
“Warn” is a verb. The correct noun is “warning.” So the natural English would be “first warning” or, more commonly in weather and safety branding, “First Alert.”
“First Warn” is just clipped marketing language that chops off the “-ing” from “warning.” It’s the same kind of shorthand you see in product branding—short, snappy, and easy to trademark—rather than proper English. Stations keep copying it because the graphic is simple enough for anyone to understand at a glance, and local TV marketing departments tend to follow whatever everyone else is already doing.
Marketing to those with lower IQs. TV does it all the time.
