My neighborhood recently had a pownpur as we didn't expect. It was so sudden and heavy, and I got scared about what was going to happen around my house. Luckily, the rain eneded after a while, but the rain reminds me of watar damages experienced past years. What we got this day seems close to "Gerrira Gou (ゲリラ豪雨, Guerrilla Heavy Rain) --- the term we've seen recently.
What does the buzz word mean?
The Japanese of Gurrilla Heavy Rain is a popular word heard oftentimes, because some people across the country come on to a sudden downpour, which is so sudden that it's hard for weather experts to forecast when and where. It's similar to Niwaka Ame (にわか雨), a general term for a sudden rain, but the Guerrilla rain has more emphasis of "all of a sudden." We don't see any crowd even shortly before the downpour.
Why did started to borrow the English word "guerrilla?" To be sure, there is hardly any guerrilla group in my country Japan, but we know they do have one in other unstable countries. The term seems to be used figuratively. We associate a guerrilla with a small group of terrorists or some illegal groups who do a wrongdoing unexpectedly. Therefore, I think, the term guerrilla was added to the term of rain.
Additionally, the term Guerrilla rain has been heard more often than before. It seems, because of the global warming, abnormal events of the past have now become normal occurrences. Like, a heavy rain in three decades might happen every year. So, we have to prepare for a disaster.