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Thunderstorms, due to their high frequency of occurrence over southern Africa and their dominant contribution to summer rainfall, are the primary focus of very short range forecasting and nowcasting efforts in South Africa. Most southern African countries are heavily reliant on satellite technology due to the limited number of surface and upper-air observations and the limited availability of numerical model output. In developing tools for the first 12 forecast hours, the South African Weather Service has to address both national and regional needs. The blending of techniques in an optimal manner is essential in achieving this. In this paper a description is given of how the Global Instability Index product derived from the European Meteosat Second Generation Satellite was adapted to fit to South African circumstances using a different numerical model as background information, creating the Regional Instability Indices (RII). The focus of the study is the development of a new convection indicator, called the Combined Instability Index (CII), which calculates the probability of convection based on satellite derived instability indices and moisture as well as orographic lift early in the morning when the sky is as cloud free as possible. The CII is an objective way for operational forecasters to calculate the probability for convection later in the day based on satellite and model data. Early morning CII values were evaluated statistically against the occurrence of lightning as well as against satellite derived precipitation later in the same day. It is shown that the CII not only performs well, but also outperforms the individual RII. The CII will be beneficial to operational forecasters to focus their attention on the area which is most favourable for the development of convection later in the day. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]