Rain has hit Churchill Downs at the start of an expected a three-day stretch of wet weather leading up to the Kentucky Derby.

The outlook for the Kentucky Oaks on Friday (Saturday AEST) isn't encouraging, with a 60 per cent chance of rain forecast.

But there are two silver linings.

The first is that the serious preparations for the Oaks and Derby contenders are completed, and the weather wasn't a factor on Thursday morning, with most of the Derby horses out for routine maintenance gallops.

The second is that the rain could end well before the Derby on Saturday. While showers are forecast in the morning, the storm could move out before the scheduled post time of 6:46 pm (8.46am AEST).

The dirt track at Churchill Downs has a reputation for rapidly draining and drying. While there undoubtedly will be some residual moisture on the surface, it might not be a muddy Derby if the sky clears in time.