Some info from wikipedia on the big upcoming weather event.
"The East Asian rainy season, commonly called the plum rain (Chinese: 梅雨, méiyǔ; Japanese: 梅雨, tsuyu, baiu; Korean: 장마, jangma), is caused by precipitation along a persistent stationary front known as the Meiyu front for nearly two months during the late spring and early summer between eastern China, Taiwan, South Korea, and Japan. The wet season ends during the summer when the subtropical ridge becomes strong enough to push this front north of the region."

The weather front forms when the moist air over the Pacific meets the cooler continental air mass.

The most rain in a one-hour period as recorded in Japan was in Nagasaki in 1982 with 153 mm. The highest overall recorded rainfall during the rainy season in Japan 2003, Miyazaki Prefecture recorded rains of 8670 mm."

Will be interesting to monitor it's progress this year.

What we have now is a seasonal spring unsettlement in the weather, which is not officially the mei yu, but is it a prelude to the mei yu? The graph below sure looks something like it. The wind is almost 50-50 now south and north.

still pleasantly cool today, but any day now could be the last we get of that feeling again for several months.