We obtain uniform asymptotics for deformed Airy kernel determinants, which arise in models of finite temperature free fermions and which characterize the narrow wedge solution of the Kardar–Parisi–Zhang equation. The asymptotics for the determinants yield uniform initial data for an associated family of solutions to the Korteweg–de Vries equation, and uniform lower tail asymptotics for the narrow wedge solution of the Kardar–Parisi–Zhang equation.