I have a Rapha softshell and it is a phenomenal piece of kit. Not the most practical off-bike though, since it's cut for on-bike use.
Anyway, it can handle maybe 45 minutes of light to moderate rain before it starts to soak through (this is riding at a decent speed). This is true of all softshells and Rapha has arguably the best in the business (I've tried a bunch, I tend to agree). For anything more than that, you're really going to need a true rain shell if you want to stay dry/warm.
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