We're currently in the middle of a Pineapple Express weather pattern in Seattle. This is a recurring winter pattern in which the jet stream conveys tropical moisture to us and keeps it aimed at us like a fire hose. This one is unusually massive: we are expected to get about 6" of rain in a couple of days in the lowlands and as much as 20" of rain in the mountains. There is already significant flooding and huge natural avalanches in the Cascades.
This weather makes bike commuting especially fun. Personally, I like the challenge. I assume even more than usual that drivers won't see me, so I take all street crossings and such with extra caution. I have ample reflective gear and obviously lights front and rear and I wear light-colored clothing (I'm amazed how many idiots I see on bikes after dark in black clothing and no lights or reflectors).
I don't bother with waterproof clothing. My commute is only 25-35 minutes depending on direction and wind, and I always ride pretty hard so I generate plenty of heat. Rain gear would make me as wet inside from sweat as I get outside from rain. I do wear clothing that is warm even when wet and I have my system down pat after several years. I've learned what pieces of clothing to wear for any conditions.
Today I happened to leave work just as a big wave of rain was arriving. Riding in heavy rain adds some challenges. I have to take my riding glasses off because the rain drops refract the city and vehicle lights and make the world way too psychedelic. And I have to remember that my brakes aren't going to work for several seconds after I apply them - they have to dry themselves off first. So I have to think way ahead and anticipate when I'm going to have to slow down or stop.
The worst part of bike commuting in Seattle winters is how filthy my bike gets. I don't clean it as often as I should - perhaps once every two weeks. We just had a huge snow event last week and the roads on my route were heavily sanded. Wet tires pick up all that sand and grime and deliver it right on to the rest of the bike. The grit also wears the brake pads faster and produces brake dust, which when mixed with water produces a dark gray paste that gets everywhere. Changing a flat tire on a bike coated in all that material is an especially dirty job (and not very fun when standing in a downpour next to a highway).
One of these days I plan to obtain a mini camcorder
and make a speeded-up movie of my commute. For now, you'll have to settle for photos of the aftermath.