We've seen two trends establish themselves overnight with the guidance -- one, a slightly faster arrival and exit to the precipitation with the developing coastal low. Second, a slower arrival of the Canadian chill with the cold front from the west. The two pieces looked to interact just well enough to bring snow to most everyone...even if it meant a start as rain. The faster arriving precipitation is "outrunning" the cold...and the developing coastal simply isn't going to crank up fast enough or close enough to produce significant dynamic precipitation that will help drag cold air down from aloft. The result will be a light precipitation event, which this was going to be from the get go, but one that is going to start this evening as opposed to later tonight. Expect the first fingers of the system moving into the region after 8 or 9 PM, with the steadier precipitation hitting overnight.
Those few hours make the difference on temperature and the result will be an event that starts as rain for nearly everyone before we see the changeover to snow take place for many of us along I-95 and points northwest.
The above graphic is last night's Euro computer model -- showing the 4 AM time frame. The "blues" on the map suggest one inch snow rates over the prior three hours. You can see how the Airport is on the edge of the snows in this timeframe while the northern and western burbs are getting some snowfall accumulation from this system. The NAM is slightly warmer than the Euro, with the GFS slightly colder in terms of overall forecast. Slight differences may yield a different result though...as is usual with these marginal "thread the needle" storms a little bit of difference in temperature, a little slower timing in precipitation moving into the region and we could see that "snow for most" type outcome that was being advertised yesterday at this time.
Taking the early morning long-winded nature of this post and breaking it into simple terms, the bottom line is this: Light rain this evening, mixing and changing to snow from northwest to southeast overnight. That changeover will happen towards Midnight in the Pennsylvania suburbs, after 1 or 2 AM in the hilly parts of the city although the Airport may hold out with a mix of rain and snow for a little while longer, with rain ending as snow south of Route 70 in South Jersey in the predawn hours of Saturday morning. The timing element might change a little bit on either side and if it's a bit earlier than anticipated...we could see a bit more snow down to the coastline and even the Shore above Cape May could squeeze out an inch of snow.
We'll have more information late this afternoon on the upcoming system.