Complex sentence: By lap 28 the rain was torrential and the safety car was deployed, several cars were out due to spin or crash, however, the rain didn't improve and become worse and the race was red-flagged on lap 33.
Split into simpler sentences: By lap 28 the rain was torrential and the safety car was deployed, several cars were out due to spins or crash. The rain became worse and the race was red-flagged on lap 33.