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Urban-induced thermal stress can threaten human health, especially during heat waves (HWs). The growth of cities further exacerbates this effect. Here, weather research and forecasting (WRF) with an urban canopy model (UCM) is used to assess the effects of megacities and their growth on the thermal regime of proximal cities during heat waves. Analysis of the heat fluxes shows that advection impacts cities downwind. Results indicate that as urban areas change size (50%-100% and 100-150% of their current size), the local 2 m temperature increases by 2.7 and 1.7 degrees C, and the 2 m specific humidity decreases by 2.1 and 1.4 g kg(-1), respectively. A small city downwind is impacted with a 0.3-0.4 degrees C increase in 2 m temperature. Green roof is a potential mitigation strategy for these regions (i.e., beyond the megacity). With 50% green roofs in an urban area, a 0.5 degrees C decrease in 2 m temperature and 0.6 g kg(-1) increase in specific humidity is simulated. Urbanization upwind of a megacity will contribute to regional climate change.