According to weather.com's hour by hour forecast, there is only a 10% chance of rain until 10:00, and at 10:00 it jumps to 30% chance, and at 10:30 it's all the way up to a 50% chance.
Regardless, that gives us about 2 and a half hours until the rain is supposed to arrive, and even then it's a coin flip (although in Tennessee, a 50% chance of rain really means a 95% chance of rain). Either way, the chance for rain never exceeds 50% tonight at Fenway. Should be able to get in the mandatory 5 innings. Jump out to an early lead.