It will snow a lot tomorrow.

It stands for "weather", and when it exists, be is followed by an adjective to indicate the weather. Another example: sunny/windy/cloudy/rainy.

Try to compare:

There will be a lot of snow tomorrow.

There is there be here, so it is followed by a noun. This sentence is not about the weather, but about what will happen tomorrow.

I wish you progress in your studies and go up a storey still higher! (*^__^*)

Ask again if you don't understand! Please remember to adopt it, thank you!