Summer rain is a rare weather event in Davis.
It occurred on September 20 and 21 2013. (barely summer, it should be noted — Autumn began on September 22 in this year, but Summer anyway). That day, September 21, around 6.15 PM there was a complete cloudburst including hail & thunder. The hail was observed for about 15 minutes.
It occurred on August 5, 2014 too and in the night of 5-6 August 2014. Because this rain was very mild and because of the exceptional drought California is in, unfortunately, it will not bring any substantial relief.