Why does the rain and snow line in the weather forecast draw a big bend in southwest China?

Because of the topography of Sichuan basin. The west of Sichuan Basin is Qinghai-Tibet Plateau and Hengduan Mountains, the north is Qinling and Daba Mountains, the east is Wushan, and the south is Yunnan-Guizhou Plateau. So the Sichuan Basin is low in the middle and high around. The Qinling Mountains and Daba Mountain in the north block the cold air from invading Sichuan Basin, while the central and eastern regions of China are dominated by plains and are easily affected by cold air. So the temperature in Sichuan basin is higher than that in the same latitude area. So when it snows in central and eastern China, it usually rains in southwest China.

However, in special years, some cold air is particularly strong, and even a cold wave appears, so cold air will cross the mountains and invade the Sichuan Basin. By then, there will also be snowfall in the southwest.