、。
Don't go outside — it's raining hard.
Literal
Outside [to-に] going [must-not-てはいけません] terribly rain [subject-が] falling [progressive-いるよ].
表に出てはいけません — 表 means 'outside' (literally 'the front/surface'). いけません is the polite prohibition. ひどく雨が降っている explains the reason. The よ draws attention to the heavy rain.