With continuous heavy rains, why did the pre-flood season in South China start so high and end so fast this year?

In April, South China enters the pre-flood season, which is the first rainy period of the year. Generally speaking, rainfall has a gradual process. In May and June, it is the heaviest rainfall in South China. For example, in Guangzhou:

However, this year's pre-flood season in South China can be said to have started strong and ended wildly. Throughout April, heavy rainfall occurred one after another. Take the heavy rainfall since the 19th as an example. The daily rainfall at 17 national meteorological observation stations in Guangdong and Guangxi exceeded the extreme value in April; the second flood occurred in the Beijiang River in the Pearl River Basin.

For example, in Shaoguan , the cumulative rainfall since April has reached 754.4 mm. Even if it is placed in the whole year, it can rank among the top three monthly rainfalls, which is very rare.

△On April 21, due to heavy rainfall, Datang Village, Dazhan Town, Yingde City, Qingyuan City, Guangdong Province became a "swamp". (Photo source: Visual China)

So the question is, why did the pre-flood season in South China start so high and end so fast this year?

The reason for this is the name that we often hear in summer: subtropical high pressure.

The subtropical high pressure in the northwest Pacific is the main weather system affecting my country in summer. Its location determines the progress of my country's rain belt.

The subtropical high pressure is like a pot lid. Inside it, there are sinking air currents, which are mainly sunny and hot. The abundant warm and humid air currents from the ocean will move northward along its western and northern edges, bringing abundant precipitation to my country. Therefore, as it gradually expands from south to north, the main rain belt in my country during the flood season will also gradually shift from south to north.

As shown in the figure below, in April every year, the main body of the subtropical high pressure is still on the ocean surface, so naturally not much water vapor can be transported to the land.

This year, the position of the subtropical high is relatively northerly, and abundant water vapor is transported to South my country in advance. Combined with the active plateau trough and southern branch system, the center of rainfall has been near South China, so the momentum of May and June has been brought forward.

△On April 22, the weather forecast on the Xinwen Lianbo explained the cause of the heavy rainfall in South China

There will still be continuous rainfall in South China in the future, with heavy rainfall mainly from the night of April 24 to the day of April 26, and from April 28 to 30. Moreover, the heavy rainfall area highly overlaps with the heavy rainfall area from April 19 to 22. The cumulative rainfall in central and northern Guangdong and northeastern Guangxi since April will exceed the monthly rainfall extreme value in April.

【hint】

Under the influence of heavy rainfall, combined with the previous rainfall, it is expected that from the 24th to the 26th, there will be a high risk of rainstorm disasters in central and northern Guangdong, eastern Guangxi and other places, affecting an area of ​​about 35,000 square kilometers and a population of about 55.66 million. Urban waterlogging is prone to occur, and slippery roads and waterlogging are prone to traffic jams or accidents; central and northern Guangdong, eastern Guangxi, western Fujian, southern Jiangxi, southeastern Hunan and other places are prone to waterlogging disasters; central and northern Guangdong, eastern Guangxi, eastern Jiangxi and western Fujian are at high risk of flash floods and geological disasters, among which the risk of local flash floods and geological disasters in central and northern Guangdong is high, and the water levels in the Beijiang and Dongjiang basins of Guangdong are rising, which may exceed the warning water level. We must be highly vigilant against possible flash floods, mud-rock flows, landslides and other disasters, avoid going to mountainous areas and areas prone to geological disasters, and pay attention to prevent the adverse effects of strong convective weather.