"Honey, I'm going cycling. Yes, I know a typhoon is approaching. I left my life insurance policy on the kutsubako."
Unless you're the Wicked Witch of the West, it's not a big deal. You get fully soaked immediately, which is part of the fun. I suppose in an open area you'd have to worry about wind blowing you around, but that wasn't an issue where I was.
I once saw a sheet metal street sign go slicing down the street in Ibaraki City. Glad you were safe. On a related note, I've decided to start wearing a helmet when I ride long distance.
Why would you not wear a helmet at all times you're on a public street? Have you never seen how people drive today? Almost used mine today... barreling down the Rt 1 descent from the pass on the return from Otsu, I was doing the speed limit (~50kph) and someone pulled out in front of me and stopped the car blocking the entire lane, because they'd pulled out, apparently, without looking in either direction and once they got halfway across the road realized that there was traffic the other way so had to stop. I squeezed by behind them, between them and the car waiting after them, yelling expletives at full volume. I didn't have my front-facing camera (it crapped out yesterday). Had I caught this on video, I would have gone straight to the police.
That sounds pretty much exactly like how I had my "sprawling-over-the-hood-of-a-car-like-a-1970s-cop-show" accident in June. Now that I'm venturing further into the mountains, I think it's a good idea to wear a helmet, but maybe I need it more in the city, because the smartphone zombies walking around randomly are scarier than most of the trucks out in the mountains.
A friend riding at walking speed hit his head on the curb when a rut caught his wheel in the city. A helmet would have saved him some grief. People who don't wear a helmet strike me as nuts, but then, I think the same of people who don't wear a seatbelt in a car. I've been using car seatbelts for 40 years and have never benefited from it, and I hope it stays that way, but it's cheap and easy and the protection it provides, should you need it, is vastly more than the hassle. Same with bicycle helmets, except I've shattered a bicycle helmet.