This study examines metaphorical expressions in American radio news magazines appearing in two linguistic forms: NP is a NP and It's like NP. It integrates Blending Theory (Fauconnier & Turner 2002) with a usage-based approach to grammatical constructions (Goldberg 1995, 2006;Tomasello 2003, and Croft 2001) and analyzes the forms within their dynamic discourse context in terms of noun phrase accessibility (Chafe 1980, 1994; Givón 1983; Ariel 1988) and grounding (Langacker 1999, Oakley & Coulson 2008). The findings indicate that the functions of the grammatical constructions used in the metaphorical expressions were directly related to the non-metaphorical uses of the constructions and that the analysis of the ongoing discourse was essential to understanding the form-meaning pairings inherent in the expressions.