it occurred to me that because it isn't the Moto Social people's first rodeo, and since they do a lot of events in places where it actually rains, that they may have a policy for weather related eventualities. so I AI'd it:
"Policy: Rain or shine.
"The Moto Social San Francisco events typically operate on a "rain or shine" basis. The community gatherings continue regardless of weather conditions, and they encourage riders to attend even if it is raining."
and there ya go ... guess that's the answer for lightweights like me who worry about rain. I was rooting for "rain cancels." (I wuz roundly mocked for being a little-boy man for bringing the weather issue up this am on the SFMC email list)