SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... The strong arctic cold front will be approaching from the west on Saturday night and will cross through the region early on Sunday morning. With a substantial cold air advection push behind the front, this will usher in a much colder airmass. As it does so, an area of low pressure will be developing over the Deep South along the tail-end of the arctic front. This area of low pressure will ride along the frontal boundary and deepen as it moves off the coast near the Virginia/North Carolina border by Sunday afternoon. From here, forecast guidance has come into better agreement this afternoon and has shown an overall northwestern shift to the track of the surface low by most global and ensemble guidance. This will place the surface low just off the New Jersey coast on Sunday afternoon. Still expect that the 12Z Canadian/GEM guidance is underdoing the cold air advection regime on the northwest side of the low, so suspect that the low will remain just off our coastline instead of tracking directly over southern New Jersey. Heading into Sunday night as the low exits, it appears that the deepening surface low will pass right over or just off to the northwest of the 40N/70W benchmark. Also, available guidance suggests an overall uptick in forecast QPF values by nearly 0.1-0.2", especially north and west of the I-95 corridor. Factoring in rather high snow-to-liquid ratios to the north and west, there again has been an increase in forecaster confidence in plowable snowfall for areas northwest of the I-95 corridor. Thus, our forecast this afternoon generally incorporates a blend of the situation discussed above. Still, this does not mean that areas south and east, including the Philadelphia metro will see snow through the entire duration of the event. Some short-range guidance does depict a subtle warm layer at 850mb so there may be at least a brief rain/snow mix for areas directly along the I-95 corridor at the onset. For this reason, have kept a rain/snow mix for these areas through Sunday morning and early afternoon. However, as the cold air begins to pour into the region as the low departs, we should see an eventual changeover from rain to rain/snow mix to all snow even down to the coast by Sunday night. In coordination with neighboring forecast offices and WPC, have opted to issue Winter Storm Watches for Sunday morning into Sunday evening for all of our Pennsylvania Counties except for Delaware, Philadelphia and Lower Bucks. Also, have issued Winter Storm Watches for Hunterdon, Warren, Sussex, and Morris Counties in New Jersey. These are the areas which are likely to observe warning level snowfall with lesser amounts expected elsewhere. In terms of amounts, generally expecting a widespread 5-8" snowfall in the Watch area with localized amounts up to 10" possible (especially across the higher terrain). Stepping southeast into and along the immediate I-95 corridor, expecting a 3-5" snowfall event due to potential mixing at the onset. Of course, these totals can be higher or lower depending on the duration of mixed precipitation. Closer to the coast where prolonged periods of mixing and at times plain rain is to occur before the changeover, a 1-3" event is expected. As always, the forecast can vary over the next 36-48 hours as more hi-res guidance becomes available. So stay tuned for the latest updates to the forecast through the holiday weekend!
