Last week started off cold, then warmed to just below freezing, then back to cold. During Deep Winter, it's not unusual to have a warmish bump bracketed by walls of cold. Typically that second round of cold feels a little more chilly and a lot more icy.
During the bump, air on the ground stayed several degrees below freezing, but the air above that must have been warmer. The drizzle that fell froze instantly as it hit roofs, roads, and treetops, making for a slight icing event and an exciting time to drive on any untreated lane.