"There is heavy rain in my city."
Translation:هُناك مَطَر ثَقيل في مَدينَتي.
hmm as an Arab myself I don't think there is something wrong in your sentence.
however, there is something wrong with the word thaqiil here. Thaqiil is used in Arabic to mean "heavy" in weight... for rains we say Ghazeer غزير
Duolingo is using quite a word-to-word translation