Before the rainy season even started, Jeju and the southern coast of Korea experienced over a week of rain.
The mountainous areas of Jeju received over 200mm of rain, and the southern coastal areas received around 100mm of rain. Why did this rain last so long?

■ This rainy season… The cause is a “stagnant front”

This is a recent satellite image of the Korean Peninsula taken by Tianlian 2A.

Source: Clairvoyant 2A satellite image (May 30-31)

For several days now, the rain clouds along the southern coast have been forming long bands like a rainy season front, which is called a “stagnation front.

Stagnation fronts are usually formed when two layers of air with different properties collide and fight it out, usually for a long time until one side exhausts its strength. The “rainy season front” we’re all familiar with is a type of stalled front, and in this case, it was formed by cold, dry air in the north and a mobile high pressure system in the south, and it centered on the south coast and stayed there for over a week.

However, if you look closely, you can see that the tip of the stagnation front stretches all the way to southern Japan as well as Korea.

Source: NHK Weather

Due to this effect, a lot of rain fell in southern Japan during the same period, and the Japan Meteorological Agency viewed it as “rainy season rain.” Although the rain fell on the same front, Korea Meteorological Agency analyzed it as a “stagnation front” and Japan Meteorological Agency analyzed it as a “rainy season front.

Japan Meteorological Agency declares early “start of rainy season”

The Japan Meteorological Agency announced that the rainy season began five days ago, on the 29th of last month, in northern Kyushu (where Fukuoka is located), Kinki (where Osaka is located), and Shikoku. This is when the rain started due to the effects of the stagnation front.

The start of the rainy season as announced by the Japan Meteorological Agency,

Source: Japan Meteorological Agency (Rainy season start times by region)

With the exception of Okinawa, southern Japan, including Kyushu, recorded a rainy season about a week earlier than normal. In particular, it is the first time in 10 years since 2013 that the rainy season started in May in the Kinki region. Compared to this time of year, when the rainy season usually starts around June 5 in southern Japan, we can see that this year’s rainy season is early.

Korea Meteorological Agency, “Not the beginning of the rainy season”

Despite the rain clouds on the same front, the Japan Meteorological Agency has decided that it is not the beginning of the rainy season. This is because Japan and Korea have different criteria for determining the rainy season.

The main criterion for the Japan Meteorological Agency to determine the rainy season is the “North Pacific High”. This is the air mass that is colored red in the image below over the Pacific Ocean,

Source: Japan Meteorological Agency Rainy메이저놀이터 Season White Paper (Placement of air masses during the rainy season)

The Japan Meteorological Agency defines the “rainy season front” as a stagnant front that forms at the boundary between the hot “North Pacific high pressure” extending from the southern sea and the cooler high pressure to the north, the “Okhotsk Sea air mass” or the “continental air mass.” However, this stagnant front is formed by the aforementioned mobile high pressure.

Unlike us, Japan puts a lot of emphasis on the timing of the rains. Regardless of the air mass, if a stagnant front forms around May and June and the rain continues under its influence, it is considered the beginning of the rainy season.

This year’s early rainy season in Japan is not unrelated to Typhoon No. 2, Mawar, which is moving north.

Source: Japan Meteorological Agency (Typhoon No. 2 Mawar’s expected path as of 16:00 on June 2)

While the typhoon was moving up from the sea south of Japan, it seems that the stagnant front that formed in Japan played a role in supporting it from below so that it would not retreat to the south. Therefore, it is analyzed that Japan was affected by the front for a long time, leading to the early start of the rainy season.

When does the rainy season start in Korea?

In a normal year, our rainy season starts in late June. It starts on June 19 in Jeju, June 23 in the southern region, and June 25 in the central region, and lasts about a month.

Although the forecast is still subject to change, there is no clear rain forecast until the 13th of this month. The Korea Meteorological Administration has not forecast the start and end of the rainy season since 2009, so it is too early to determine the exact timing, but by the middle to end of next week, when reliable forecasts are produced, we should be able to determine the approximate start of the rainy season in Jeju.