Temperatures in northern China are set to plunge as much as 20 degrees Celsius (68 Fahrenheit) after summer-like conditions in the final days of autumn, state forecasters said on Friday, extending a year-long trend of unusual swings in the weather.

A stream of cold air entering China on Saturday from the northwest will join with another that had already arrived on Thursday to cause a sharp drop in temperatures, the National Meteorological Administration (CMA) said.

Temperatures in the small towns, deserts and grasslands of the northern Inner Mongolia region could fall as much as 10C on Friday and a further 10C from Saturday. Blizzards could hit Xinjiang in China's arid northwest.

From next week, most of the northeast is expected to see daily maximum temperatures dive to the single-digits or even below freezing as cold air masses move east and south, in an abrupt reversal of the recent "big warming", according to CMA.

While cold and freezing temperatures in these regions are not atypical for the time of year, the sudden change is unusual, and resulted from the uncharacteristically warm weather recorded in late October to early November.

Parts of northern China posted record high temperatures of more than 30C earlier this week, while also suffering widespread smog.

Extreme weather has become more pronounced in China this year, destroying urban infrastructure as well as farmland, leading to hefty economic losses.

In the summer, typhoons dumped historic rainfall in parts of inland China less used to tropical storms. Typhoon Doksuri caused the worst flooding since 1963 in the vast Hai river basin that encompasses Beijing, Tianjin and Hebei province.

China is due to issue 1 trillion yuan ($137 billion) of sovereign bonds to help rebuild areas hit by this year's floods and improve urban infrastructure to cope with future disasters.

Earlier in spring, northern China basked in unseasonal heat with temperatures reaching summer-levels.

That spell of 30C weather was preceded by a very cold January, during which China's northernmost city of Mohe saw its minimum temperature dip to a record minus 53C. ($1 = 7.3127 Chinese yuan renminbi) (Reporting by Ethan Wang and Ryan Woo; Editing by Simon Cameron-Moore)