Heavy rain will hit South China until Thursday after combining with strong winds and hailstones to batter North China on Saturday.
The China Meteorological Administration on Sunday also issued a grade-IV response for the upcoming rain. The grade-IV response for torrential rain, the lowest in China's emergency response system, means a 24-hour alert, daily damage reports and the allocation of money and relief materials within 48 hours.
He Lifu, chief forecaster at the center, said the rainfall in the south will be the heaviest since the flood season began last month.
"Areas with accumulated rainfall of more than 100 and 200 millimeters will be larger than in previous rounds," he said. "From June 7 to 10, precipitation in southeastern parts of Guizhou province, northwestern parts of Guangxi Zhuang autonomous region and northeastern parts of Hunan province will exceed 300 mm, and the maximum will reach 400 mm."
He said the main cause of the heavy rain in South China will be cold air from plateau areas meeting warm, humid air from southwestern China, with the rain spreading from west to east.
"In that case, rainfall will first be concentrated in southwestern China in provinces such as Sichuan, Yunnan and Guizhou," he said. "Afterward, it will gradually move to southeastern parts like the provinces of Anhui, Jiangxi and Zhejiang, as well as Shanghai."
He warned that soil will be loose as there has already been a lot of rainfall, and mountain torrents and mudslides are more likely to occur. Residents of Guizhou, Guangxi and Hunan should be especially cautious about the risk of flooding.
On Saturday, winds and rain hit northern China, with some regions battered by hailstones.
Lan Yu, a senior engineer at the center, said the weather was caused by a cold vortex that originated in northeastern China encountering warm, humid air in areas along Yellow River and Huaihe River.
"High temperature accelerates evaporation of the ground water, which provides favorable air conditions for severe convection," Lan said. "That's why this kind of weather always happens in summer.
"However, in dry areas like the Xinjiang Uygur autonomous region, it is rare because there's not enough water there."
The Anhui Meteorological Center issued a yellow rainstorm alert, the third-highest level, at 8:50 am on Saturday for the northeast of the province, including the cities of Huaibei, Suzhou and Bengbu, in the following six hours.
Heavy rain and hail started to hit Huaibei at about 11:40 am, with 37.5 mm of rain falling in an hour.
The center then raised the alert to orange, the second-highest level, at 2:30 pm, warning that rainstorms would hit Suzhou, Bengbu and Chuzhou in the following two hours.
Heavy rain and hail hit Tianchang in Chuzhou and parts of neighboring Jiangsu province in the afternoon.
Videos shot by residents showed hailstones as large as eggs hitting cars, roofs and windows.
Many residents said they had never seen such strong hail before.
In Fujian province, landslides were reported on Sunday in villages near the Wuyi Mountains due to heavy rain. No casualties were reported.