Chicago’s Christmas cold snap is history as temperatures are forecast to climb back into the lower 30s this afternoon. The brief intrusion of arctic air brought the city its coldest Christmas in three years, with Friday’s highs reaching the lower 20s after early morning single-digit lows. The city missed out on a white Christmas, but lake-effect snow squalls brought more than a foot of snow to many locales in Lower Michigan and northwest Indian near Lake Michigan. Even milder weather is on tap for Tuesday when the mercury is likely to reach 40 degrees. A new and potentially significant winter storm is forecast to develop in the Plains by mid-week, but in keeping with this season’s lack of snow here, Chicago is likely to remain in the storm’s warm sector with the bulk of the precipitation falling as rain.