It is going to rain because the sky is covered with black clouds.

[Alice Chu]

Please tell me if my understanding is correct.

1. The two sentences below indicate that the speaker predicts/thinks raining will happen tomorrow, but raining is uncertain.
1) It will rain tomorrow.
2) It is going to rain tomorrow.

2. The following sentence indicates that the speaker predicts/thinks raining will happen because of the evidence, black clouds, and raining is certain. In this case, “is going to rain” cannot be replaced with “will rain”.
1) It is going to rain because the sky is covered with black clouds.

3. The two sentences below are incorrect.
1) It is raining tomorrow.
This sentence is wrong because raining can’t be arranged.
2) It rains tomorrow.
This sentence is wrong because raining can not be scheduled.
Is “can not” equal to “cannot”?

 

[Tarheel]

We predict rain, not raining. And we say storm clouds, not black clouds.