Complex sentence: Snowfalls are not common in the city because of the urban warming effect but the Pennine and Rossendale hills that surround the city to its east and north receive more snow that can close roads leading out of the city.
Split into simpler sentences: Snowfalls are not common in the city, due to the urban warming effect. However, the Pennine and Rossendale hills that surround the city to its east and north receive more snowfall that can close roads leading out of the city.