Meanwhile, the annual Bike to Work Day festivities scheduled for Friday, sponsored by the Baltimore Metropolitan Council, were canceled because of safety concerns, the organization said.
The State Highway Administration cautioned drivers to avoid areas where streams and rivers may crest and flow onto roadways.
So far this year, more than 21 inches of rain has fallen at Baltimore-Washington International Thurgood Marshall Airport. With Friday's forecast, 2014 is on pace to be among Baltimore's top 10 wettest years on record.
About three-quarters of an inch of rain has fallen this month at the airport, and Friday's rain could push that total close to or above normal May rainfall of about 4 inches.
The weather can always be a factor at Preakness, though the race itself has been mostly dry since at least 2009. Partly cloudy skies and highs in the lower 70s are forecast for Saturday.
In 2003, several inches of rain similarly threatened to have a say in the Preakness, with a Kentucky Derby winner's trainer dismissing the weather as a factor. Derby champ and Preakness favorite Funny Cide came to Pimlico a day early because of possible weather-related travel disruptions, but trainer Barclay Tagg said the horse could handle anything.
That he did, winning by a commanding 93/4-length margin.
But when heavy rain preceded the Belmont Stakes three weeks later, Funny Cide's Triple Crown bid died in the mud there.
Baltimore Sun reporters Childs Walker and Kevin Rector contributed to this article.