After a damp and unseasonably cold spell, most parts of China are set to see a dramatic warm-up over the weekend, with temperatures surging by as much as 17 C in some cities. Forecasters warned of widening temperature gaps between day and night that could catch people off guard.

On Wednesday and Thursday, a chilly mix of rain and cold air is expected to keep the middle and lower reaches of the Yangtze River and Northeast China significantly colder. Highs in most areas will stay below 20 C, with some places only reaching around 10 C, said Weather China, a website affiliated with the China Meteorological Administration.

In Hefei, Anhui province, the temperature struggled to reach just 11 C on Wednesday, more than 12 C below the seasonal norm, the website said.

However, from Friday, as rainfall decreases and a combination of solar heating and warm air advection takes hold, temperatures will climb quickly and turn higher than normal, the website added.

On Friday, in Harbin, Heilongjiang province, the high is expected to reach 21 C, marking the city's first 20 C day of the year.

From Saturday to Monday, many cities across China will see highs return to 25 C or above, with some areas touching 30 C.

Hefei, for example, is expected to hit 28 C on Sunday, a remarkable 17 C jump from its midweek low. Temperatures in Changsha, Hunan province, could reach 30 C the same day.

In northern China, frequent weak cold fronts will bring seesaw temperatures, particularly in Northeast China. Most of northern China will see a broad rise on Thursday. Parts of Heilongjiang province are expected to reach 20 C on Friday, and Liaoning province will see highs above 25 C in some areas.

Day-night temperature differences of 15 C or even more than 20 C are likely in parts of northwestern, northern and northeastern China.

After this round of warming, cold air will remain frequent but weak across the country. Temperature swings will narrow in most areas, while the summer zone in the South will gradually expand, the website said, adding that summer is approaching.