BEIJING, July 18 (Reuters) - Searing summer heat waves are expected to return this week across large parts of China, lasting through late August, the state weather forecaster said, despite brief interludes of seasonal rain.

Temperatures from 39 degrees Celsius to 42 degrees Celsius (102.2°F to 107.6°F) are expected in the southern region after July 20, including the provinces of Jiangxi, Zhejiang and Fujian, the China Meterological Administration said on Sunday.

Despite some weekend respite for provinces such as Hebei, Hunan, Hubei, Guizhou and Jiangxi, maximum temperatures elsewhere are expected to hover over 37 degrees Celsius (98.6°F).

The sweltering heat will last from July 16 to Aug. 24 nationwide for an "extended period" of 40 days, up from the usual 30, the forecaster said on its website.

The so-called "sanfu", or "three periods of laying low" during China's summer refers to three annual 10-day stretches between July and August when temperatures and humidity peak.

But this year, the second phase is expected to run for 20 days from July 26 to Aug. 14, the forecaster said.

China has broiled in average temperatures of about 35 degrees Celsius (95°F) for the past two weeks, bringing buckled roads and more hospital visits because of heat stroke, sparking discussion on social media.

Heat waves have gripped several nations as raging forest fires sweep parts of Europe. Temperatures on the U.S. west and southeast coasts have regularly hit 40 degrees Celsius to 45 degrees Celsius (104°F to 113°F) for several days.

On Monday, the Hong Kong observatory again warned of very hot weather in the global financial hub.

Chinese forecasters said the number of cities with high temperatures has gradually increased over the past 30 years. Many experts blame persistent high temperatures on global climate change. (Reporting by Beijing newsroom; Writing by Bernard Orr; Editing by Clarence Fernandez)