It is shown that by monitoring longwall leg pressures in real time, warning can be given for significant weighting events and the formation of roof instabilities, such as roof cavities, several hours in advance. Longwall Visual Analysis (LVA) is a software package that continuously monitors shield pressures and shearer position in longwall mines. LVA has been running on 22 Australian longwalls for up to five years, and as a result a very substantial database of shield pressure trends in a wide range of longwall situations has been collected. This database has been analysed to develop indicators that will give operators and geotechnical engineers advance warnings of developing conditions such as weighting events and difficult roof conditions. LVA displays live charts of data, such as shield leg pressures, loading rates, and yield frequencies. Data analysis techniques were applied to historical data records from multiple longwall sites in order to develop a “Cavity Risk Index” (CRI). The CRI indicates the relative risk of roof cavities developing, and it is based on pressure trends that indicate significant yielding and loading rates spanning a region of relatively low support. Case studies from two different longwalls are given, showing how the CRI can give real-time advance warning of the formation of roof cavities.