First take a few days of constant heavy snow, and spread liberally around the city. Then thaw for two days until the entire area is an inch thick lake of grey slushy mess. Be sure to leave not a single square centimeter uncovered. Then freeze down to -4C, creating a delightful glace effect. Finally spread the whole confection with nice, wet, fresh snow and hey presto you have an entire city whose only purpose is to destroy human life. Tomorrow, how to add 40 mph winds for real effect.