Monsoon onset over the South China Sea occurs in April--May, marking the
start of the wet season over East Asia. Nevertheless, skillful
prediction of onset timing remains an open challenge. Recently,
theoretical studies using idealized models have revealed feedbacks at
work during the seasonal transitions of the Hadley cells, and have shown
that these are relevant to monsoon onset over Asia. Here I hypothesize
that monsoon onset occurs earlier in years when the atmosphere over the
South China Sea is already in a state where these feedbacks are more
easily triggered. I find that local anomalies in lower-level moist
static energy in the preceding Jan--March are well correlated with South
China Sea Monsoon onset timing. This relationship remains consistent on
decadal timescales, while correlations with other teleconnections vary,
and is used to develop a simple forecast model for onset timing that
shows skill competitive with that of more complex models.