The purpose of this paper is to characterise and model the A1 and A2 screech modes in supersonic jets operating at off-design conditions. The usual screech-modelling scenario involves a feedback loop between a downstream-travelling Kelvin-Helmholtz instability wave and an upstream-travelling acoustic wave. We review state-of-the-art screech frequency prediction models and associated limitations. Following the work of Edgington-Mitchell et al. (2018), a new prediction approach is proposed where the feedback loop is closed by the upstream-travelling jet modes first discussed in