By analyzing high spatial and temporal resolution data of wind profile radar provided by Hong Kong Observatory, a thorough study of the heavy rainfall from 19 to 20 August 2005 in Shenzhen is conducted. It is proved that the hourly wind field derived from wind profile radar data could reveal the correspondence between the occurring of every heavy rain and the rapid strengthening and downward expanding of the southwest jet stream. The low-level jet stream index I illustrates a close relation between the extent of downward expanding of low-level jet stream and the meso-micro scale precipitation. This index is therefore significant for forecasting magnitude and intense of heavy precipitation.