Severe flooding has forced the evacuation of millions in China and displaced entire communities in Bangladesh as relentless torrential downpours accompany the summer monsoon.

Up to a third of Bangladesh has been affected by the floodwaters, which are forecast to remain steady or rise in the coming days. The flooding is noteworthy both for its extreme intensity and long duration.

The worst flooding was happening along the Brahmaputra River in Bangladesh, where water levels were predicted to climb even further. The water has reportedly remained above the danger level for more than 30 days, the longest such streak in more than 20 years.

The Padma River will also see worsening flooding. The rivers, along with the Meghna River, make up the Ganges Delta, which empties into the Bay of Bengal. It’s the largest river delta in the world, with over 41,000 square miles of surface area.

Fertile soil and the prospect of highly fertile land makes the Delta an attractive place to live, but it comes at the price of the summer rains, which can be fiercely intense and erratic.

China’s burden

In China, heavy rains have plagued the Yangtze, Yellow, and Huai rivers, affecting up to 50 million people. State media reports that 41,000 homes were destroyed and another 368,000 houses damaged, resulting in 158 deaths. Taiwan News called the officially reported death toll “suspiciously low.”

Continued monsoonal rainfall over the upper Yangtze River Basin has placed growing pressure on the Three Gorges Dam in Hubei Province, as floodwaters pour into the reservoir climb. According to the Asia Times, officials have reported some minor deformation in the dam, but expressed confidence in its structural integrity. The volume of floodwaters pouring into the dam’s reservoir is less than during the floods of 2012.

The Three Gorges Dam is the world’s largest dam and biggest power plant, spanning more than 7,500 feet in length. It is not only used to generate hydropower, but also to regulate the Yangtze’s flow downstream.

Flow out of the dam was temporarily reduced to less than two-thirds of what was entering the dam’s reservoir to buy time for cities downstream to prepare.

A backup of water flowing into the reservoir can affect the city of Chongqing, which has already seen destructive flooding as the Yangtze swells in heavy rain. Once the water is released from the dam, it will rush downstream and have an impact on cities such as Yichang, Yueyang, and Wuhan.

Wuhan found itself thrust into the international spotlight as the epicenter of the world’s coronavirus crisis in early 2020.

China is particularly susceptible to flooding due to its exceptional population density and the amount of infrastructure built within flood plains.

Monsoonal rains can be a blessing or a menace

Southeast Asia and much of the Indian Subcontinent has monsoonal rains every year as temperatures increase and moisture builds. They are most common from June through September, often at their worst in July.

Scientifically speaking, a monsoon is simply a seasonal wind shift, but that change in the breeze often trucks in a moist air mass that can result in widespread downpours. One such storm struck Xi’an on Friday, with serious flooding as a result.

Over southern China, the monsoon is made worse by the Meiyu Front, a stagnant stationary front that becomes entrenched as warm air from the south collides with cooler, dry air settling in from northern Asia and Siberia. As moisture pools near the front, waves of low pressure develop, sparking drenching deluges that can target the same areas repeatedly and cause havoc.

Japan recently suffered catastrophic flooding associated with monsoonal moisture along the Mauyi Front.

In India and near the Himalayas, the monsoon is enhanced by orography. Air over the Tibetan Plateau heats up, rising and pulling in moist air from the Bay of Bengal to fill the void. That self-repeating process generates heavy rains with exceptional rainfall rates.

Cox’s Bazar, a fishing city in southeastern Bangladesh near the border with Myanmar, picked up a staggering 37 inches of rain in June. It has received about a foot and a half of rainfall in July.

Sylhet, located in a mountain valley of northeast Bangladesh, received 26 inches during June and 27 inches in July.

Monsoonal rainfall rates are highly variable, the rains a game of feast or famine for many. The water is needed for agriculture, but it can do more harm than good when it falls in such concentrated, furious bursts.

Rangpur, a major city in extreme northern Bangladesh near the Teesta River, measured 9.8 inches of rain on July 19, but then didn’t report a single raindrop for the next five days.

More than half of all river gauges in India are reporting waters at or above flood stage, with severe flooding occurring on the Dhaleswari River at Elasinghat in the Tangail District of central Bangladesh. Water levels were a meter higher than “danger level."

Flooding forecast to continue

The Bangladesh Flood Forecasting and Warning Center does not anticipate that floodwaters will subside below threshold until at least a week or so into August.

“Ganges-Padma River may steadily rise,” wrote the center, which is particularly concerned about Dhaka, the capital of Bangladesh. It’s one of the most densely populated cities in the world.

“Rivers around Dhaka city may continue to rise. Lakhya River may continue to rise at Narayanganj. As a result, the flood situation in the low lying areas of the district is likely to continue for the next 7 days,” the center said.

Farther east, the Chinese Meteorological Association expects heavy rains to continue as well.

“Rain will plague Sichuan Basin,” the agency wrote. “Rainy weather will also affect North China, Northeast China, central-western South of the Yangtze River Region, and South China.”

Studies have shown increasing precipitation from the South Asian monsoon due to climate warming, which is expected to continue into the future.