Frontal convection (FC) and diurnal convection (DC) are the two most frequently observed weather systems affecting the pre-summer (May and June, MJ) rainfall formation over Taiwan and Southeast China. Focusing on the time period of 1982–2012 MJ months, this study found that the occurrence frequency of FC has declined, but the occurrence frequency of DC has significantly increased over Taiwan and Southeast China. Diagnoses of the atmospheric thermodynamic conditions over the East Asian–Western North Pacific (EAWNP) region indicate that the area favourable for the FC formation (i.e.