How Is the Start of the Rainy Season Predicted? How JMA Decides

The start of Japan's rainy season is something many people hear about every year.
But many still wonder what forecasters are actually looking at, and why the announced date sometimes changes later.

The short answer is that JMA does not decide based on a single rainy day.
It looks at the recent flow of weather, the outlook for the next week or so, and the usual timing for each region.

What JMA looks at

JMA checks whether cloudy or rainy weather is likely to continue, where the seasonal front is located, and whether the timing matches the normal period for that area.
This is why the start of the rainy season is not treated like an instant switch.

Why the wording says "likely started"

The announcement includes a forecast element.
That is why JMA uses wording equivalent to "the rainy season appears to have started" rather than a hard declaration.

Why the date can be revised later

JMA first publishes a preliminary value, then later reviews the season using the actual weather progression from spring to summer.
Because of that, the final date can move after the season ends.