。
This time tomorrow it'll probably be raining.
Literal
Tomorrow [possessive-の] this-about-time [topic-は] rain [subject-が] fall [progressive-ている] thing probably.
The phrase ~ことでしょう is a slightly more emphatic or literary version of plain でしょう. Literally 'thing probably' — treating the whole preceding clause as a noun + probability. Common in narration, forecasts, and mildly poetic statements.