Abstract The nowcasting of localized precipitation (LP) is often limited by insufficient observation of the vertical cloud evolution preceding rainfall. Here we systematically tracked rapid cloud development within 2 hr before LP onset, utilizing cloud data collected in 2024 from the millimeter‐wavelength cloud radar network across China, together with other meteorological data. Results reveal a coherent precursory signature from 40 down to 20 min prior to precipitation onset, characterized by a lowering cloud base, intensifying radar reflectivity, and enhanced turbulence intrusion into the cloud layer. This synergy creates a positive feedback mechanism that accelerates the collision‐coalescence process. Crucially, a simultaneous surge in cloud‐base descent rate (∼240 m/min) and reflectivity (10–20 dBZ) within the final 20 min provides a robust early warning signal for LP onset. These findings underscore that integrating cloud precursory information is essential for improving the accuracy of precipitation nowcasting.