That means three out of every four years it's rained on this date, on average. The next wettest date on the chart is 82 on November 15 (also dry this year though, amazingly).
Looking back, the last time it rained on November 19 was 2008 and going back 16 years, it's rained on four of them, so that statistic seems to hold true fairly well.
Guess that means don't plan any outdoor activities on November 19 in 2012, 2013, or 2014, but 2015? Picnic at Alki!
(P.S. Yes, I know it actually doesn't work that way, but I'm calling first dibs on the picnic shelter at Alki anyway :) )