As of this morning, the weather.com forecast for Kansas City puts the chance of rain at about 80 percent, with the Kansas City Star already issuing instructions on what to do about tickets should the game get rained out. But an hourly breakdown of the puts the highest likelihood of a storm at around 4 PM—by the 7 PM CT start time those odds dip to 40 percent. So while the field may be soggy, chances are the game will start tonight with minor or no delays.
World Series games are often subject to inclement weather. The 2008 Rays-Phillies series was a particularly rainy set—Game 3 was delayed an hour and a half, and Game 5 became the first officially suspended World Series game when it stopped for rain in the top of the sixth inning, to be completed two days later. Philadelphia’s weather set things back again the very next year during the 2009 Phillies-Yankees series, with a Game 3 start delayed over an hour due to rain.
The Mets, too, have encountered some World Series rain, and fared well against it. The Game 7 washout in the 1986 series versus the Red Sox didn’t stall their comeback; they went on to win the World Series the next night.
But even if it stops raining in time for the first pitch, the batters will have to get their power stances on: the humidity is expected to stay high, at around 97 percent, which means the ball may not go as far as it would on a less humid night. The effects of humidity on a baseball's movement are dependent on multiple factors including temperature, but humidor experiments show an increase in humidity can make balls "less bouncy" so they travel less. Citi Field and Kauffman Stadium have comparable dimensions, with center outfield distances of 408 and 410 feet, respectively, so both teams may have to hit the ball a bit harder than normal to get it over the fence.