It's all a matter of mood. Since it started raining a few minutes ago, Peter is referring to the feeling generated a few minutes ago, when it actually started to rain.

We can use the past tense in such a situation. There are two things that are very characteristic of English, two elements of grammar we stress, time and number, exactly when something happened in relation to the present and how many things are involved. English is unlike Chinese in that respect. 还有,一般来说英语重形合，强调结构的完整性和形态的严谨性，而汉语重意合，强调内容和表意的完整性。 The grammatical form of our words has to conform to every other element in the sentence, not only to the idea, as in Chinese. At the time Peter is making the statement, it has already started to rain. In Peter’s perspective the beginning of the rain, the change in the situation is already in the past, so he uses the past tense. Very often I receive a letter from a Chinese friend it starts with the words, “I am very glad to receive your last leter...” My Chinese friend uses the present tense. I would say “I was very glad to receive your letter...” because at the time I am writing those words, the receipt of the letter is in the past.

I think Peter is talking about the past-- when he actually realized that it was raining and was happy to have found it out.

I think it should be "That is a pleasant surprise." Where do you get that conversation from?

To be honest Peter stated something incorrectly. He should have answered That IS a pleasant surpise because Susan said it IS raining.