Chilly and wet for this Sunday's Bay to Breakers

I've been looking at the weather forecast for Fran San Frisco this Sunday, May 15, Bay to Breakers (2011) day.
Yahoo weather is predicting a high of 55, with a 50% chance of rain. My experience with this (this will be my 5th B2B) suggests that a 50% chance of rain means it will be sunny for one block, raining in the next.

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