A big MU update:
Regarding the Monday night-Tuesday system, there is still an immense amount of uncertainty regarding its track and intensity. Much depends on the interaction between the two disturbances highlighted below. The closer together they are, the stronger the system..
(2/4) The upper-level pattern across North America will also be a major player. A ridge axis over the Intermountain West (GFS model - 1st image) would allow the system to "dig" farther south & deepen, while a subtle trough (ECMWF model - 2nd image) would promote a weaker system..
(3/4) Regardless of the ultimate outcome, surface temps will be above freezing and very marginal for snow across northern MD, southeastern PA, and the Lower Susquehanna Valley. In order for rain to change over to snow and accumulate, precipitation will need to fall heavily..
4/4) The storm is still about 4 days away, so I won't be able to finesse the details until Monday. For now, I still favor an outcome with no accumulating snow south of the PA Turnpike. Don't pay attention to forecast snowfall maps until Sunday. They're bound to fail.