The National Meteorological Centre (NMC), China's national observatory, yesterday said that heavy rain would sweep regions south of the Yangtze River.
Some areas will see torrential rain accompanied by thunder or hail, the state-run Xinhua news agency quoted the NMC as saying.
Local authorities were asked to prepare for emergencies, halt outdoor work and prevent floods and landslides.
Blue alert is the lowest in a four-tier warning system in China. The country has a four-tier colour-coded warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.