MU update for Wednesday:
In the wake of the front, clearing skies will accompany temperatures that'll drop into the 30s by daybreak. A high pressure system settling over the Commonwealth on Tuesday will cause winds to become light later in the afternoon, but it will feel much chillier outside with highs only reaching the mid-40s to around 50. It's no secret that the "word is out" with regard to a late-season snow threat on Wednesday, with the weather slated to do a "total 180" in a matter of just 36 hours or less. A disturbance currently located over the Four Corners region will move rapidly northeastward into the central US on Tuesday and "stay the course" as it pushes into western PA by Wednesday. To the southeast of the disturbance, a storm system will develop across the Tennessee River Valley and move to a position off the Delmarva Peninsula by Wednesday. The air mass settling overhead Tuesday night should be "just cold enough" for a period of snow to occur across southern PA and far northern Maryland on Wednesday morning. Of course, it will only take a subtle shift north in the storm's track, or the chilly air mass to not push quite as far south, for snow to be mixed with or just end up being plain rain Wednesday morning. At this time, I actually do favor (with about 60/40 odds) a period of all snow Wednesday morning since the storm should track well to our south. I expect snow to overspread the Lower Susquehanna Valley from southwest-to-northeast around daybreak or in the pre-dawn hours Wednesday and continue throughout much of the morning. Despite the fact that it may snow for 4-6 hours, there are still plenty of reasons to be skeptical of anything more than a "nuisance" event.
First, low temperatures Tuesday night will only fall into the low-to-mid 30s and remain above freezing while it's snowing Wednesday morning. Secondly, ground, roadway, and sub-surface temperatures are way above freezing following the record-breaking warmth and will stay that way through the rest of the week. Finally, the snow will be occuring primarily during the daylight hours and have to battle the strong March sun, whose angle is now equivalent to that of early October. Under these circumstances, it will have to snow at the rate of 1-2" per hour for snow to stick on paved surfaces. It may even be challenging for the snow to stick on non-paved surfaces, especially in the valleys and lower elevations. A few hundred feet of elevation can make a 1-2 degree difference in temperature, and this is often vital when talking about snowfall accumulations in March. So, I suspect that most locations will pick up a general coating to one inch of snow, with 1-3" more likely along the South Mountain Range and higher elevations in northern Lancaster County. Higher elevations to the north/west of Harrisburg also have a greater chance of receiving 1-3" of snow. If the snow arrives a few hours earlier than anticipated, then there's a higher probability of greater snowfall amounts since more would fall before sunrise. Keep in mind that these accumulations are with respect to grassy surfaces. I expect paved surfaces to generally remain wet, with any slushy spots being confined to untreated, secondary roadways in the highest elevations. The primary weather-related travel issue will be reduced visibility in any heavier bands of snow during the Wednesday morning commute. Be sure to reduce speed if you encounter areas of low visibility, and exercise caution on the wet roadways. As temperatures slowly climb into the upper 30s and precipitation lightens around midday, snow will mix with and change to rain before ending late Wednesday afternoon.