It will be raining when I get up. It will begin raining late tonight. I will get up at 9:00 tomorrow morning. The action of raining will begin before I get up, and it will be in progress at a particular time in the future. Probably, the rain will continue.
It was raining when I woke up. It started raining late last night. I awoke at 9:00. It was still raining. The rain began before and was in progress at a particular time in the past. It continued after I awoke.
This will happen at a particular time in the future. It will rain tomorrow. It is going to rain tomorrow. Maria will eat fish next Friday. Maria is going to eat fish next Friday.