The Warning Lexicon is a tool to help risk communicators write effective warning messages for Wireless Emergency Alerts (WEAs) using a common set of statements about hazard impacts and protective actions. The research has developed a Warning Response Model, a theoretical framework that identifies five key types of warning message content and their style of presentation that motivate people to take timely and appropriate protective action. Based on a multiphase study using qualitative research involving content analysis, subject matter expert interviews, and focus group interviews to create, verify, and validate the contents of the Warning Lexicon. The results have generated a comprehensive list of 48 hazards, their associated impacts, and 112 protective action statements spanning atmospheric, technological, biological, and human-induced events. The Warning Lexicon is a theory-based, expert-informed, and practitioner-reviewed resource for warning message composition.