英文文法關於天氣

這句話主要是說下午有可能會下雨，出門請記得攜帶雨具。

有的句型就是there is/ there are
因為要表示有可能、有機率，所以句子加了may
句子因此變成 there may be rain this afternoon.

如果你要用It 開頭的話，句子應該要寫成
It may rain/ It may be raining this afternoon.

希望這樣夠清楚。

There adverb;meaning in consequence, that being so, in that place, to that point meaning in consequence (take) an umbrella with you.
It=pronoun, neuter pronoun of the third person,no need to use here. Also It Might be should be used instead, for possibility.

It may be rain this afternoon.
今天下午, 那個東西可能是雨. (不合乎邏輯)