Keeping in mind, most Derby Days occurred in the month of May, usually within the first week. The coldest temperature recorded was 36 degrees, on May 4th in the years 1940 and again in 1957. The warmest temperature was on May 2nd, 1959.
You have a roughly 48% chance seeing at least a little bit of rain at some point in the day. 69 out of 145 Derby Days have had precipitation.
The wettest Derby Day was very recent. 3.15″ of rain was measured on May 5th, 2018.
For the first time, the Kentucky Derby will be held in September. The forecast in Louisville for Churchill Downs is expected to by dry, mostly sunny, with a morning low temperature of 57 degrees, and and afternoon high around 82.