The onset of a summer monsoon involves processes on a range of spatial and temporal scales, from the large-scale temperature gradients that drive the seasonal transition to the small scale convective processes and interactions that determine monsoon rainfall locally. Predicting monsoon onset at the local scale remains a challenge but is crucial for agriculture and planning in countries where millions rely on the timely arrival of monsoon rains for their livelihoods. The South China Sea summer monsoon onset is a known indicator of the broadscale seasonal transition that represents the first stage of the onset of the Asian summer monsoon as a whole. Subsequent development of rainfall across East Asia is influenced by sub-seasonal variability and synoptic events. Here, we highlight recent studies which have shed light on the processes and scale interactions involved in the onset of monsoon rainfall, on the representation of such processes in seasonal forecast models, and on the spin-up of biases in the Asian monsoon region during the pre-monsoon. We find that year to year variations in South China Sea summer monsoon onset are predictable with useful skill at up to 3 months lead time using the GloSea5 seasonal forecasting system. The source of this predictability is the Pacific sea surface temperatures associated with El Niño and La Niña. Preliminary results also indicate that there may be useful skill for predicting mei-yu rainfall in June in GloSea5. Both occur despite systematic biases in the actual rainfall amount and horizontal zonal wind gradient that are found both in the seasonal forecasting system and in climate configurations. Such biases appear to develop during the broadscale seasonal transition over the South China Sea and the Western Pacific and persist through the summer. We investigate the spin-up of these biases using nudging experiments.