Tonight will be a good test of the weather models. Both the GFS and the NAM are showing the precipitation associated with an artic front to dissipate before reaching Philadelphia. The GFS cranks out a snow flurry around daybreak with a QPF of 0.01 inches water.
On Sunday, another weather forecasting dilemma presents itself as a stalled front in our area will allow disturbances to develop and move along the front. It’s impossible to know where the front will end up, but recent similar scenarios have the front stalling further south than model predictions. As a result, light snow is seeming more possible than light rain or sleet in PHL. A stronger low pressure system may develop along this front Monday, bringing snow. Amounts not predictable at this time. Stay tuned.
The low-pressure system will be taking a more northerly track, right across Pennsylvania. More warm air is expected to be brought into the storm changing the initial snow to rain by daybreak Monday and much of the morning will be rain in and around Philadelphia. Windy and very cold conditions will follow Monday afternoon and evening.
The models are trending a bit north with the Sunday night storm, suggesting a change to sleet during some of the heavier precipitation early Monday morning, limiting snow totals in PHL.
Most models suggest a wrap-around changeback to snow before everything ends late Monday. Impossible to predict final totals with this one, since degree of intensification and fine thermal structure will be indeterminable in advance. That said, expect a storm impact and don’t expect a fizzler.
We are flying back from an island vacation early Sunday evening, so I’m counting on the models having the storm not really starting much before 7pm.
For Sunday evening– Low pressure will develop in the middle of the country and move eastward as it intensifies over PA and NJ Sunday evening through Monday morning. Snow will overspread our area Sunday evening and become heavy after midnight and during the predawn hours of Monday.
Warm air is expected to move in at about 5000 feet in and around PHL, so a changeover to sleet and possibly freezing rain will likely occur towards daybreak Monday morning. A change back to snow will likely occur before ending early Monday afternoon.
Uncertainty about the northward track of the low and the amount of warm air mixing in will make accumulation prediction very difficult around PHL, but areas to the far north and west may have snow amounts around 12+ inches.
Unlike the ‘blizzard’ that wasn’t, this storm will more likely be impactful with accumulations of 6 or more inches possible (but not definite) around PHL by Monday morning.
I suspect the NWS is being overly cautious about over-predicting this one. I don’t think this one will fizzle out. Stay tuned.