WHEN +S + VED/P2 ====> WAS +V_ING ?????

Honestly most people would say It was snowing hard when I left the house. The same as raining hard. OR snowing A LOT. OR We would use snowing as a present participle "It was SNOWING heavily when I left the house." That said the word you are looking for here is "the snow was falling heavily when I left the house."

The snow was falling heavily.....