My Light & Motion Urban 500 has been fine in the rain. Rode it home in the rain right after buying it. And while L&M recommends mounting it only with the sealed USB port facing downward toward the road, I've used it several times mounted sideways on my bikes' head tube or fork. The USB port seal is very snug and effective -- in fact I've chipped fingernails several times prying the seal open for recharging.

And no problems in the rain with my Serfas SL-255, despite the twist apart opening for swapping AA batteries. It has a very effective seal that may be replaceable. Based on my experience with that light I wouldn't hesitate to get a USB rechargeable Serfas for all weather rides.

And there are dozens of bright LED flashlights on Amazon, ebay and elsewhere that are claimed to be weatherproof. The tricky bit is finding a mount that won't pop loose on rough roads, potholes or crossing railroad tracks, but some folks report good results with several mounts. The types that supposedly hold the flashlight barrel with just tension from a U-shaped block of rubber don't seem reliable, based on my observations in local group rides. I've seen those things pop loose several times on rough stuff. Get a strap that wraps around the flashlight barrel and handlebar for security.