Student or Learner
"It's likely to rain tomorrow."OK?
"It's likely to be raining tomorrow."OK?
Actually, I'd say "Rain is likely for tomorrow."
But of your two, the first sounds more natural, unless you are referring to another activity that relates to the rain: You're going to film your demo video tomorrow? Really? Because it's likely to be raining tomorrow, and how will that look?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.