AKIPRESS.COM - China warned residents across a large part of northern China on Friday to prepare for a wave of choking smog arriving over the weekend, the worst of which is expected to over Beijing, prompting the capital city to issue its second ever red alert.

The red alert — the most serious warning on a four-tier system adopted a little over two years ago — means authorities have forecast more than three consecutive days of severe smog, Al Jazeera reports.

The National Meteorological Center said the smog would stretch from Xian, home to the Terracotta Warriors, across part of central China, through Beijing and up into Shenyang and Harbin in China's frigid northeast.

The pollution is expected to begin appearing on Saturday evening and last until Tuesday, with visibility in the worst affected areas such as Beijing likely to fall to less than 0.6 miles, it said.

In Beijing and parts of Hebei province, which surrounds the capital, the pollution index will probably exceed 500, it said. At levels higher than 300, residents are encouraged to remain indoors, according to government guidelines.

Beijing officials issued the first red alert last week, which was the second time this month that notoriously polluted Beijing experienced a prolonged bout of smog, sending PM2.5 levels in the suburbs as high as 976 micrograms. Beijing was also shrouded in persistent smog for most of November, when power demand soared due to unusually cold weather.

A red alert is triggered when the government believes air quality will surpass a level of 200 on an air quality index that measures various pollutants for at least three days. The U.S. government deems a level of more than 200 "very unhealthy.”