SHANGHAI may be entering its earliest autumn in 25 years, but because the temperature is expected to bounce back after several cooler days, the city's weather authority is holding off its declaration.

The city is expected to be cloudy today with the high rising two degrees to 23 degrees Celsius, said the Shanghai Meteorological Bureau. The sky should turn sunny and cloudy from tomorrow through the rest of the week, and the high is forecast to gradually reach 26 degrees over the weekend, said the observatory.

According to the standard meteorological changing of season, autumn is announced by the bureau when the 24-hour average temperature drops below 22 degrees for five consecutive days after li qiu, or beginning of autumn on the Chinese lunar calendar, usually around August 7. The first day of the five will then be considered the beginning of the season.

The city's average temperature slid below 22 degrees on Monday and Tuesday but there are three days to go. The mercury may rise above 22 from tomorrow to Friday, said Zhu Jiehua, a chief service officer of the bureau.

"The temperatures of the next three days should be climbing and there should be a lot of uncertainty for the autumn announcing," Zhu said yesterday. "The average of the next three days should around 22 degrees, according to our prediction. We'll see how it goes."

If the average of the next two days stays below 22 degrees Celsius, Monday will be announced as the beginning of autumn this year, which would be the city's earliest fall since 1986.

Over the last 30 years, Shanghai's autumn has started on an average date of September 27, but during the last 10 years it usually started in October.

"Even if autumn is announced this week, it's not early in history," Zhu said. "September is a normal timing for season changing."

Zhu explained that the strong feeling of autumn is likely caused by a cold snap that has affected the city.

"The high last Friday was still above 34 degrees and suddenly dropped to around 24 degrees over the weekend, which might cause the feeling of summer to autumn in one day," Zhu said.

In any case, Zhu said the mercury should still experience some ups and downs after the fall season becomes official.