Ame ga furu sō desu – It’s likely going to rain.

“~Sō” is used when you are mentioning something that you are not sure that it is going to happen. Other examples :

Tenki yohou ni yoru to, kyō wa gogo kara hareru sō desu – According to the weather forecast, it will likely be a sunny day in the afternoon.

Terebi no nyu-su ni yoru to, kōtsūjiko ga maitoshi (fueru)fuete iru sō desu – According to TV news, traffic accident is increasing every year

Meaning : Tenki (Weather), Yohou (Forecast), Yoru To (According To), Gogo (Afternoon), Hareru (Be Sunny), Nyu-su (News)m Kōtsūjiko (Traffic Accident), Maitoshi (Every Year), Fueru (Increase)