China's top observatory forecast Saturday that rain and snow will sweep a vast part of the country over the next three days to challenge a new period of heavy holiday traffic. From Saturday to Monday, the Qinghai-Tibet Plateau in southwestern China, and most regions to the east of the plateau will be pelted by a new round of precipitation, the National Meteorological Center (NMC) said in a statement. On Saturday, snow will fall in the south of Tibet Autonomous Region and the southeast of Qinghai Province, with some parts dumped with heavy snow or snowstorms, the center said. Rain and snow will then move eastward from Sunday to Monday. The weather will likely disrupt the upcoming return-trip travel peak in China's chunyun, a period of travel rush around the country's Lunar New Year. A large number of migrant workers and students will be on their return journeys in the next few days. This year's chunyun runs from Jan. 16 to Feb. 24. Chinese passengers are expected to make 3.62 billion trips during the period, with road trips hitting 3.2 billion, according to the transport authority. The NMC warned the affected regions to take precautions against negative impacts of the rain and snow on transportation. The center also issued a yellow alert for smog in Beijing, Tianjin and some areas in the provinces of Hebei, Shandong, Henan, Shanxi and Shaanxi, partly triggered by firecrackers and fireworks set off Friday to celebrate the Lantern Festival. China's meteorological alerts are labeled blue, yellow, orange and red as the severity ascends.