Altimeter issues in the rain?

A number of times I've seen erratic behavior from the barometer/altimeter during activities (see attached image miles 5 to 7.5) and have been trying to figure out the cause. Usually the altimeter works fine, but occasionally I see a ton of noise on it. I am wondering if maybe it has to do with rain getting into the barometer sensor or something. Has anybody else observed similar issues with the altimeter while running in the rain? More often than not, it works fine, so I don't think I have a hardware problem or dirty sensor.