As a former frequent club ride leader that would drive me crazy. Like you mentioned, there will be a 20% chance of scattered later afternoon showers. People would bag and it would either not rain or, like the detailed forecast predicted, there would be scattered showers after the ride was over. After rides like that I would intentionally post ride reports to the club's email list mentioning how those who showed up did not get wet. Then for the next week or two I would run into people who would tell me "Your ride looked really nice. I was going to go, but they were predicting rain." People need to pay attention to the details of the forecast. I swear some people just see rain drops on the graphics and think "washout."