Update, 7 PM: Another burst of heavier precipitation (snow and sleet) pushing northeast through Chester County towards Montgomery County and the Philadelphia suburbs. Temperatures across the northern and western burbs holding tight around freezing, with mid and upper 30's along and east of I-95. We should see some moderation in temperature as the night progresses but this heavier burst of precipitation won't help boost temperatures one bit...probably will help aid in keeping things relatively close to freezing out there as we see more snow and sleet continue. We should gradually see that transition from snow to rain lift north through the night as warmer air down to our south finally makes some progress...might take until late evening (11 PM) for that shift to take place.
Update, 6:30 PM: Winter weather advisories out now for Bucks, Montgomery, and Chester Counties until 1 AM for snow and sleet. Could see an inch plus on the higher elevations over Upper Bucks and Montgomery, with coating accumulations down to the lower halves of those three counties.
Update, 6 PM: As the line of sleet, snow, and rain moves towards the region from the west, the heavier precipitation that's showing up on radar is generally in the form of snow (large flakes), with sleet falling on the front end of the line along with rain. Coating accumulations have occurred so far across the far western and northern burbs, with an inch or so having fallen in Reading in the past hour. This line is working east and northeast, with it earmarked to work through Philadelphia over the next hour or two.
Radar is showing the front edge of precipitation approaching the western suburbs at 4:30 PM. To our west, it was a thump of snow that was able to coat roads and make travel less than ideal in Harrisburg. This line of snow is moving east-northeast and is working into slightly milder air as it approaches our region. The result locally may be a mixed bag along I-95 once precipitation starts. However, north and west of Route 202 could see some snow on the front end before a quick transition over to sleet and then rain as milder air pushes in aloft.
Coating-type accumulations are still a reasonable call but the best bet for this will be north and west of Route 202 in Chester, Montgomery, and Bucks Counties...I doubt areas to the south really get in on any snow outside of a mix at this point as temperatures down there are sufficiently mild aloft and at the surface, with all rain likely south of Wilmington to Long Beach Island.
Farther north over the Poconos, precipitation looks to remain frozen longer up there, with Winter Weather Advisories upgraded to Winter Storm Warnings due to the prospect of snow and ice for the lion's share of precipitation this evening.
We'll keep an eye on this over the next couple of hours as this moves east. Your comments and updates on this are welcome!
Updated 5:20 PM: Precipitation generally is in the form of sleet as it moves through, although snow is common over higher elevations and farther north and west (Reading, Allentown). It will be a slick commute home in areas where sleet is falling as sleet will quickly stick to lesser traveled surfaces.