Present Perfect vs Past Simple

1. Let's say the accident just happened and I want to ask my friend that is next to me:"Did you see that?"/"Have you seen that"? This could also apply to any unusual event: A reindeer in a shopping centre; a fox entering a butchers ; a lightning strike splitting a tree etc
Which one is correct?
2. Another situation: it's snowing outside and I want to ask my roommate whether she saw (has seen?) what the weather like today. What tense do I have to use? PP or PS?

I would say "Did you see the forecast for today?"

If I was talking about a film, I might ask "Have you seen that?"

Den

Present Perfect vs Past Simple

1.Let's say the accident just happened and I want to ask my friend that is next to me:"Did you see that?"/"Have you seen that"?
Which one is correct?
2. Another situation: it's snowing outside and I want to ask my roommate wether she saw (has seen?) what the weather like today. What tense do I have to use? PP or PS?

1. I think "Did you see that?" more appropriate. the action/accident has occured before the time of speaking .

2. use present progressive/continues. i.e: is it snowing outside? we use present continues for action in progress at the time of speaking.

i suggest you to visit anglo link channel on youtube for more explanation on tenses.

good luck.