The rain arrived at Churchill Downs as predicted. About 4:10 p.m., nearly two hours before post time to the Kentucky Derby (gr. I) it began raining hard at Churchill Downs. The track turned sloppy in a matter of minutes.The crowd headed to cover as the thunderstorm hit the Louisville, Ky., area. Weather forecasters had predicted for days it would hit between 4-7 p.m., and they were right. It poured and will now be the first Derby since 1994 (Go for Gin) to be run on a sloppy track.Rain had fallen overnight but by mid-morning, the sun was trying to come out and three races into the card, the track was changed from muddy to fast.However, more rain was forecast and arrived as predicted. The weather map showed a powerful storm headed toward Louisville and it hit prior to the Derby, which has a 6:04 p.m. post time.Churchill Downs sealed the track and cancelled all training Saturday morning. Sealing the track consists of compacting the surface so that water rolls off more easily rather than being absorbed.Churchill Downs spokesman John Asher said most of the horses for today's Kentucky Derby were walking under the cover of their shedrows since they could not be taken to the track for light exercise. Asher said today was the first Derby Day since 1994 when there was no training allowed due to the track being sealed. Later that day, Go for Gin splashed his way to victory in the Derby.The last Derby horse to arrive on the grounds was Lion Heart, who trainer Patrick Biancone kept stabled at Keeneland. He arrived on the Churchill Downs backstretch at about 9:30 Saturday morning after an easy 90-minute ride from Lexington.Overall, racing fans and handicappers should plan on wet conditions at the historic track. The forecast is for a 50% chance of showers and thunderstorms throughout the day, and severe weather possible later in the day.Those planning to attend the Derby should not try to protect themselves from the rain by taking in umbrellas: they are prohibited and will be kept at the entrance.Eighteen horses are heading for the starting gate at Churchill Downs -- and they better like mud. The field was reduced from 20 to 18 with the scratches on Friday of Wimbledon and St Averil.A sloppy track in Saturday's Kentucky Derby could favor Imperialism, Pollard's Vision, and the undefeated Smarty Jones. "We're ready to dance,'' said John Servis, who trains Smarty Jones. Todd Pletcher believes a wet track helps Pollard's Vision, but isn't a factor for Limehouse, his other starter. "An off-track is going to help some horses and hurt some others,'' Pletcher said. ``You've just got to go out there and do it.'' Nick Zito might be the trainer who is happiest about the prospect of stormy weather. His horses, Strike the Gold in 1991 and Go for Gin, were the last ones to win on rainy Derby days. He'll saddle early 4-1 favorite The Cliff's Edge and Birdstone. Then there's Borrego, whose sire El Prado is known for producing colts who like off-tracks.With additional reporting from the Associated Press.