http://www.weather.com/activities/events/mlb/index.html

OK, so it is just the same forecast that you can get anywhere but I like that they have made it MLB specific. It gives the 10-day forecast then the average temps/rainfall for the rest of the season in each MLB city.

One thing that is different from most forecasts is that it shows the wind direction with the orientation of the field where the game is being played.

And for a little fun trivia, it gives the team weather performance. For the Yankees in 2005:

Winning Weather

The most favorable weather conditions for the New York Yankees
are Day games, temperatures between 74 and 83 and winds
cross-field left to right.


Games Played

Games with wind above 10 mph:88
Games above 80°F:47
Games below 60°F:28
Day games55
Night games107