We seem to be bottoming out at 22 for a low at Philly International every morning, but I'm not going to get into that(the urban heat island effect/airplanes taking off and mixing the air around the weather station) today because we have a Winter Storm Watch in place in advance of a potent system that will provide snow and sleet across the region late Wednesday night into Thursday. While pinpointing exactly how much snow/sleet/slop will fall is still far in advance(there will most likely be a map by Tuesday as we meteorologists likes to hold off until ~48 hours before an event to minimize mass panic and hype....unlike other social media sources).
Anyway, today's high was just 29 at the airport despite plenty of sunshine, but that fresh snow pack reflected a lot of the sun's rays back into the sky and also a northwest wind between 10-15mph didn't help much either(providing a wind chill in the upper teens all day). High pressure is still nestled on top of us so a) it will be another chilly and clear night with lows in the teens in Center City(but single digits in the suburbs) and b) provide another chilly yet sunny day on Tuesday with highs in the upper 20s.
Wednesday's high temperature won't really dictate what happens overnight EXCEPT if there's late cloud cover(meaning that if it reaches sub-freezing by the late afternoon and then clouds roll in it will be extremely hard for the atmosphere to cool down; conversely if we get to freezing during the day with no clouds before the sun sets all of that "heat" will escape back into the atmosphere). As I mentioned before an area of low pressure will ride up the coast and reach the region by Midnight providing snow at the start; I mentioned how the SURFACE temperatures may be sub-freezing at the start, but more importantly is the 850-millibar(5000 feet) temperature as if it's above freezing whatever is falling aloft(that is frozen) will melt. Now if temperatures at the surface is still below freezing that droplet can refreeze into sleet OR if it's above freezing just be pretty much rain(that's the laymen's understanding for it; it's more complex than that, but since you are all not meteorologists I won't get into it).
Warmer air will nudge in and provide a changeover from whatever is falling that is FROZEN(snow, sleet) to RAIN. Now where exactly that rain/snow line sets up is up in the air right now, but it could be right along the I-95 corridor through Philly(which could mean the difference between 3" and 13"). One this low cranks up offshore it will push to the northeast and then pull in colder air behind it so everyone will see a changeover to snow on the back end for some accumulation as well. As stated before, it's too early to pinpoint a great snowfall map at the moment.
Yes, there are more chances for light snow come Friday night and then early Sunday morning, but we'll take ONE SYSTEM AT A TIME! So stay tuned and remember that you can check out the rest of the seven day at www.scottderekwx.com/forecast.html.