Heavy rain & Low level jet
Look at the upper graph. They show the area that has a huge precipitation. The area of August is larger than one of July.
And the graph below represents the trend of precipitation. we can recognize that precipitation of July pretends to be decreased, and August increased.
*This graph shows comparison of increased precipitation with July and August. Increment of August is larger than July. Why did precipitation of August increase more?
Discussion
A seasonal rain front is formed in July. That causes large precipitation of July as we know. Currently the precipitation of August increased rapidly. It is called the autumn rainy spell. The frequency of severe rain storm that is difficult to forecast started to increase.
The severe rain storm of August is caused by low level jet. The low level jet is warm and humid, because it came through the warm ocean, so it goes upward, and forms cloud as you can see figure 4. and next, figure 5, If it rises more, it meets upper level jet. The upper level jet is cold, so the vapor condenses drastically.
Consequently, the high-level-cloud is formed and suddenly the rain pours down. This process happens in 3 or 4 hours at the most, so it's very hard to forecast. It is how the severe rain storm occurs in August.