The event was declared a national-level emergency and the Xinhua news agency said it was the worst tornado to hit China in half a century, according to the BBC.
The storm struck at around 2:30 p.m. local time Thursday, the Associated Press said.
Satellite imagery reveals vigorous thunderstorm activity developed in this region Thursday. Clouds bubbled up to extreme altitudes, signifying intense updrafts common in the most violent storms.
“The skies turned dark, and we thought the summer torrential rain was about to begin,” said a resident of Yancheng, according to a live blog published by Sina, a Chinese Web portal (via the New York Times). “We ran to our house. Suddenly, the roof was blown apart, bricks were falling and the house collapsed.”
The thunderstorm activity responsible for the twister developed along a feature known as the mei-yu (or baiu) front. Weather Underground meteorologist Bob Henson explains how this front tends to serve as a focal point for severe weather:
This semi-permanent feature extends from eastern China across Taiwan into the Pacific south of Japan, associated with the southwest monsoon that pushes northward each spring and summer. The AMS Glossary notes: “The mei-yu/baiu front is very significant in the weather and climate of southeast Asia as it serves as the focus for persistent heavy convective rainfall associated with mesoscale convective complexes (MCCs) or mesoscale convective systems (MCSs) that propagate eastward.” A number of studies have found that the Mei-yu rainfall tends to be particularly heavy in the summer following an El Niño event.
The front is clearly seen on the analysis of Thursday’s weather features shown below, from the GFS model.
Notice the position of Yancheng at the intersection of much-cooler-than-normal air to its north and much-warmer-than-normal air to its south— marking the front’s location. Yancheng sat squarely in the battleground of the colliding air masses, in an explosive environment for storms.
In addition, low pressure to the west of Yancheng was acting like a pump, feeding an incredible amount of moist, humid air into the area. The map below shows the low pressure area as well as precipitable water, which is a measure of atmospheric moisture, as analyzed by the GFS model. This extraordinary feed of moisture served as fuel for the deadly storm.
In part because of the mei-yu front and moisture convergence in this region, eastern China is a hot spot for tornado activity in the Eurasian continent.
Correction, 2:09 p.m.: This post originally included a tweet which claimed to be footage of the tornado. The footage was, in fact, of a tornado that struck China in 2011 – five years prior.