If you were wondering…No, Trump shouldn’t be able to use the system to broadcast his tweets.
The law explicitly states, “The public alert and warning system shall not be used to transmit a message that does not relate to a natural disaster, act of terrorism, or other man-made disaster or threat to public safety.”
buzzfeednews.com

It could legally be used to report on Trump (by the letter of the law anyway), but not by Trump for his propaganda.