A cold front with actually cold air behind it passed through the area just before midnight last night. It has taken a few hours, but the cold air is now arriving in earnest and the temperature is going to slowly fall throughout the daylight hours. The temperature has dropped ten degrees since our midnight high of 57 and it will be down to the lower 40s by early evening. The warm start to the day will probably make this the ninth straight day where the temperature has been at least ten degrees warmer than normal.
There is no chance of a warm day tomorrow as the core of the cold air arrives. Saturday is shaping up to be the coldest day of the season to date, and the first colder than normal day of the month, as the temperature is going to drop to near freezing overnight and only warm to around 40 in the afternoon. To compound the cold, there is a strong pressure gradient between the departing storm and the arriving high, making for a very windy start to the weekend. Wind chills will be in the 20s from late tonight until Sunday morning.
Sunday is likely to only be slightly warmer than Saturday but with much calmer winds and plenty of sunshine it will be a very pleasant December day. Abnormally warm weather returns on Monday as a warm front moves through the city at the same time a huge ridge in the jet stream develops over the eastern half of the continent. Look for daily highs to climb from the lower 50s on Monday to perhaps the upper 60s on Thursday before dropping back to the upper 50s on Christmas Day.