9:19 PM

I’ve updated the snowfall forecast to increasing snowfall totals for the Philadelphia metro area.  I think the majority of the snow will fall Tuesday afternoon through early Wednesday morning as a second low pressure system develops off the New Jersey coast along an inverted trough  south of Long Island.  I think there is significant potential for heavy snow over Long Island through the New Jersey coast as this is where the best dynamics are located.  However, the strong PVA entering the forecast area should enhance snowfall amounts as far west as Philadelphia as well.  

Grey:  Flurries and passing snow showers.  No significant accumulations are expected.  

Light Blue:  Scattered snow showers and a period of light snow on Tuesday evening will produce a trace to 1 inch over most locations and an isolated 2 inches closer towards the coast.  

Blue:  Slightly higher snowfall totals expected in this location as lifting will be located closer to these locations.  Snowfall amounts of 2 to 4 inches is expected.

Purple:   The best upper level (500 MB PVA), mid level (850 MB frontogenesis, strong 700 MB omega), low level (strengthening thermal gradient at the vertical and horizontal levels) forcing will be located over these locations with 3 to 6 inches of snowfall expected.  Higher amounts will be possible, though not likely, under very heavy bands of snowfall.

Note:  These types of inverted troughs can produces surprising snowfall amounts in a localized event and will have to be NOW-Casted tomorrow afternoon through the evening.