I've seen a bunch of people bring umbrellas to shul on Shabbat and leave them open during in the coat room during service. I know you're not supposed to open an umbrella on shabbat but there's a different rule for carrying an already open umbrella. If you open the umbrella before shabbat it's permissible in an erev. The [torahmusings link][1] suggests that you may give the appearance of breaking shabbat rules. That varies from shul to shul. In areas where it rains a lot, more observant people know the umbrella nuances. 


  [1]: http://torahmusings.com/2010/11/umbrellas-on-shabbat