Visitors walk along a coastal path near Mount Songak in Jeju Island's Seogwipo, Monday. Yonhap

Heavy rain is expected to soak parts of Jeju Island, as the southern resort island braces for the annual monsoon season starting Wednesday, according to the national weather agency, Tuesday.

The Korea Meteorological Administration (KMA) announced during a press briefing that the rainy season for this year will begin with rain arriving on Jeju Island on Wednesday night.

Korea's rainy season is a period of heavy rain typically from June to mid-July when the North Pacific high-pressure front and the Northern Okhotsk pressure front collide over the Korean Peninsula to form a huge rain band stretching across a large section of East Asia.

This year's rainy season will begin June 19, which is a week earlier than last year's but on the same day as the annual average for Jeju over the past 30 years.

The island's southern mountainous areas are expected to receive 5 millimeters to 200 millimeters between Wednesday and Thursday.

Some areas of the island are forecast to experience torrential rain, reaching up to 30 millimeters per hour, along with strong winds, thunder and lightning from Wednesday night through Thursday morning. The rain will continue until Friday morning on the island.

Also, other southern areas of the country including South Jeolla Province and the Gyeongsang provinces will also receive 5 millimeters to 20 millimeters of rain starting Thursday night.

Notably, the sea surface temperatures around the Korean Peninsula this year are about 1 to 3 degrees Celsius higher than usual, and the torrential rain is expected to increase the size of giant rain clouds, according to the weather agency.

Other parts of Korea that don't experience much rain will see scorching heat lasting for more than a week, the KMA said. The weather agency issued a heat wave warning, Tuesday, for southern Gyeonggi Province, eastern South Jeolla Province and North and South Gyeongsang provinces.

Seoul's daytime maximum temperature is expected to rise to 35 degrees Celsius on Wednesday, and the first heat wave warning is likely to take effect.

In the eastern coastal region of Gangwon Province, tropical nights are likely to occur as the rising temperatures during the day remain overnight. Yangyang and Gangneung have already seen tropical nights as the lowest temperature on Monday night reached over 24 degrees Celsius.

It is still unclear when the monsoon season will begin for other major areas in the country, including Seoul.

Historically, the southern part of the country typically begins its rainy season around June 23, while the central regions start around June 25.