No, they are not equally idiomatic. People never say "雨下了" and “风刮了”.

下雨了= it is raining or it is rainy. 刮风了=it is windy,or it is blowing. "下雨了” “刮风了” ,used as whole sentences. "雨下了”“风刮了”，not whole senteces, we need add some complements. eg. 雨下了一夜yi1 ye4 =it was raining for one night 一夜=one night。

"下雨了/刮风了" 是在形容一种天气情况，一种变化（describing weather conditions, a change） "雨下了/风刮了" 是不完整的中文表达，一般应在其后加上限定性的副词短语，表达一种既成的事实。例如，“雨下了一整夜”和“风刮了一整天”(you should add restrictive adverb phrase to complete this sentence, like how long did it rain) 理解这两种表达方式的一个关键在于对“了”的理解，前者为语气词，后者为助词。

下雨了 ，风刮了 are in common usage in daily talk. Just like it is raining and it is wind blowing in English. But 雨下了， 风刮了 can appear in two scene in my opinion. One is in the Chinese poem, lyrics and prose. I mean some literary work. And the other is a little bit complex. You have already known it will rain or blow wind, and when the rain and wind start outside, you can say it to others who do not believe it will rain or blow wind. Just like "Look, the rain falls down", and "Look, the wind blows" in English.

Thanks for all your answers! ;-) I see you have different judgements but that s precisely what is interesting ^^