You have to remember that -에 and -에/에서 are time and place markers respectively. The reason why it is 오후에 and not 오후에서 is because it doesn't denote a place, BUT a time. -에서 is appropriate for place where a verb is happening, and it doesn't apply to a time when a verb is happening. It is only -에 as a time particle. Examples: 그는 이번 주에 일을 전혀 하지 않고 있어요. ( He hasn't done any work this week.) 오늘 아침에 일기 예보 들었어요? (Did you hear the weather report this morning?) Because you are describing that this morning it was beautiful, 아침 in that sentence is the subject of the sentence so you shouldn't use a time marker but a subject/topic particle. But you are right about "It snowed in the morning", you would have to say 오늘 아침애 눈이 왔어요, because if you look at the sentence, you aren't talking about the morning, but that it SNOWED this morning.