The mild pattern's demise will be gradual but Monday marks the beginning of the end of the mild stretch of weather in the Delaware Valley. Most computer modeling is now hinting that the cold front that ends our warm weekend will slowly slide through the region on Monday.
The front will not be a sharp, sudden transition -- in fact, the cold transition will be in a couple of steps down. The first is Monday as we go from around 60 for a high in the morning hours (yes, morning) into the 40's in the evening as a cool front slides slowly across the state through the course of the day. You can see how the European computer model depicts that transition as it moves through. The first map below shows morning temperatures pushing 60 around Philadelphia and in the 60's in South Jersey and Delaware (lighter green).
By the evening rush, cooler northwest breezes will begin, some showers will be falling, and temperatures will be slipping back slightly towards the lower or middle 50's around the region, with colder temperatures to the north and west. Don't be surprised if it's quasi-balmy going out the door on Monday morning but a bit cooler heading home Monday evening.
This temperature drop won't be sudden and the onslaught of "brutal" cold is not in the offing. Temperatures on Tuesday will be more typical for January as a wave of energy may ride along the front and brings another round of showers in the morning hours. These showers could end as snow showers or sleet in the Poconos or Lehigh Valley but accumulations are not likely. 40's are likely on Tuesday and Wednesday for highs...perhaps a notch milder on Wednesday than Tuesday will be as it will be dry through the day.
We mentioned a second step to the cooldown -- step number 2 brings us to colder than average and that takes place on Wednesday night into Thursday. Thursday and Friday do look colder, perhaps a few degrees below average for highs. "Arctic" air this is not -- that looks to be confined to the Midwest and shifting into New England. Colder, this will be. No 60's in a week's time but highs in the 35-40 range look quite likely. That does include the possibility of a low below 25 next Thursday or Friday night.
It might be called a true comeback but winter is going to try to make an appearance from a temperature standpoint next week...and it does look likely the pendulum will swing much closer to the colder side of the fence over the coming weeks. I don't think a 2007-style icebox is in the offing but the pattern does suggest a more typical "winter" pattern ahead.