A nice inversion is expected to set up across our region this evening and this should help trap moisture at the lower levels. With the wind expected to be non-existent, dense fog will likely develop. The higher terrain and the shore points will be the first to see this dense fog, followed by the rest of us. Visibilities will be around or below one-quarter of a mile at times, especially overnight. Some patchy drizzle is also expected.
Temperatures will have a difficult time falling back this evening under the clouds which will act like a blanket.
How fast does the fog burn off? It is difficult to say. But the longer it hangs around, the less likely Philadelphia International Airport can hit 60 degrees. I am thinking that after 9 AM or 10 AM, the fog should be almost gone in the city. It may hang on longer north and west of the city and along the coast. If it is gone sooner than that, maybe the city does better than 60 for a maximum temperature. Either way, we expect well above normal temperatures on Sunday even if they fall a degree or two short of 60. The Delmarva could get very close to 70 degrees if there is modest clearing and the fog ends quickly.
While a weak warm front moves through on Sunday, a cold front follows for Monday. The cold front will be slow to move southward and out of town and it may not do so until late Wednesday. Colder air may slowly trickle into our region as waves of energy ride along the front. This creates a tricky precipitation type forecast as borderline temperatures in the profile are indicated on the latest guidance for Tuesday and Wednesday. More details are contained in the post below. For now, Tuesday and Wednesday may feature a mixture of rain, snow, and sleet with the best chance for frozen precipitation types later Tuesday into the first part of Wednesday. Some models don't have much precipitation and faster clearing for Wednesday. So there is a lot of uncertainty right now.
Temperatures are colder behind the front, but already the models are continuing a trend that they have had since the fall and that is to warm up the temperatures as each day approaches and comes closer. So this colder air-mass arriving already doesn't look as cold as it did a few days ago.