Since mid-May, the northern boundary of the subtropical high has extended to around or north of 27 degrees latitude, roughly corresponding to its typical position in June rather than May, he said.

The moisture transport associated with the system has already reached Hunan and Hubei provinces. At the same time, active cold air masses moving southward from midlatitude regions have collided with warm and humid air, triggering intense precipitation, he added.

Forecasters also said the South China Sea summer monsoon is approaching its onset stage and is expected to set in soon.

Normally, within two weeks of the monsoon's onset, stronger southwesterly airflows carry abundant moisture from the tropical Indian Ocean and the South China Sea into East Asia, significantly increasing convective rainstorms south of the Yangtze River.

Sheng Jie, chief forecaster at the National Meteorological Center, said southwesterly warm and humid airflows have strengthened since mid-May, supplying eastern and central China with abundant moisture needed for extreme rainfall.

Meanwhile, long-term warming trends and the development of El Nino have created background conditions more conducive to extreme rainfall.

Chen Lijuan, chief forecaster at the National Climate Center, said sea surface temperatures in the central and eastern equatorial Pacific have continued to rise, with warming conditions associated with El Nino emerging in May.

"The tropical atmosphere has responded rapidly to the ocean warming, which favors strengthening of the western Pacific subtropical high and creates favorable moisture conditions for increased rainfall in southern China," Chen said.