A wintry mix of rain and snow moved into several parts of our area, causing slippery spots on roads and sidewalks. Snow fall totals Tuesday ranged from none in most of South Jersey, Delaware and Philadelphia, to nearly 5 inches inches in parts of Chester County.
What a difference even a mile made during the storm! The atmosphere gets colder at higher elevations, and that was the key. Even a degree or two lower temperature can mean the difference between hours of accumulating wet snow instead of hours of a cold rain.
This type of setup is more typical both early and late in the winter seasons. A place on a hill can get several inches more than in the valley only a mile away. It also makes for very tricky forecasting. It also could lead to some icing into Tuesday night even in areas that didn't have major snowfall.
South Jersey and Delaware have very low elevations, as does Philadelphia. As you travel to the north and west, the elevation above sea level changes quickly as you hit the “Fall Line”. (Here’s a description from USGS).
It's no coincidence that the rain/snow line stayed pretty constant during the day. The slightly higher terrain north of the Fall Line made the difference. In a “typical” snowstorm, with a wind from the east, the air is forced to rise as it moves toward higher terrain. This ends up producing more snow, and is a major reason parts of Chester, Bucks and Montgomery Counties are normally a lot snowier than the I-95 corridor. So, if you like snow, move north of the Fall Line to guarantee a whiter future.
Most roads in the area were treated by PennDOT. The messy mix still caused hazardous road conditions however. SEPTA also announced all of their Regional Rail lines may experience delays due to slippery rail conditions.
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A Winter Weather Advisory was issued by the National Weather Service for all counties north and west of Philadelphia until 7 p.m. Tuesday because of lower temperatures in those areas.
After sunset (4:38 p.m.) will be the period with the highest risk of slippery roads. Colder air will move in during the evening, changing rain to snow, not only in Philadelphia and surrounding areas, but south and east of the city as well.