What is a ‘guerrilla rainstorm’?

The term first gained traction in Japan in 2008 as a way to describe sudden, powerful rainstorms that occur throughout the country. These downpours come on with little to no warning, and can be dangerous and even deadly, especially if there is hail or flash flooding.

Current weather prediction technology can only detect these storms at the last minute. That’s why researchers in Tokyo are developing an alert system that predicts a guerrilla rainstorm up to 30 minutes before it occurs, giving residents time to prepare. The system can measure the size and structure of an approaching cloud, as well as estimate the size of the raindrops, based on a 3-D map of the cloud generated by the weather radar. This information, combined with estimates on the levels of water vapor in the air, provide a more accurate prediction of when the heavy rainfall will occur and where.

Currently in testing phase, the technology is set to be launched citywide in Tokyo in time for the 2020 Olympics.