SEATTLE - It's hard to look up at a glance because most weather records just track daily rainfall, not when it rained (we'd have to pour over every hour's data back to 1945).
We managed to look back through 1998, and since then, there were only one time it was that warm and rained -- a 7-minute rain shower at 80 degrees on June 13, 1999. And even that is shaky because it seemed like a sunny day and other short rain showers recorded earlier in the day by automated equipment were later corrected to remove the rainfall. So it's possible this one slipped through the cracks and it never really did rain and might have been an equipment glitch. (Or, maybe they were washing a plane too close to the rain gauge and some of the spray got onto the rain receptor?)
Otherwise, the next closest was 0.06" of rain on Aug. 4, 1999, but the high was 79 and the rain fell near midnight.
It is pretty hard to have it be 80 and rain, since what makes it that warm here makes it dry too. It'd have to be like a few weeks ago when we had the thunderstorms form over the mountains and then drift west into Seattle. Had those storms arrived earlier, it would have rained over 80, but the temp had dropped to 74 before they arrived.