There has been much emphasis on the increased predictability of the extratropical tropospheric circulation after stratospheric sudden warmings and the potential value of this to weather forecasting. But it remains the case that the sudden warmings themselves, which are significantly (but not exclusively) driven by variabiliity in the troposphere, have limited predictability. This paper uses ensemble forecasts to identify tropospheric circulation features that, if poorly predicted in the period prior to a sudden warming, lead to a poor prediction of the warming itself and hence provides a potentially useful focus for future improvements to forecast models.