A very weak area of low pressure will move from the Mississippi and Tennessee river valleys to the west of the area tomorrow afternoon through tomorrow night. This will spread moderate to heavy precipitation into the norther Mid-Atlantic region starting just around lunch through early afternoon from west to east, initially in the form of snow for almost everyone.
Yesterday’s overnight model runs first started picking up on the amount of cold air that would be in place prior to this storm. They continue to see the influence of this cold with each run and are nearing a consensus. In general, the upper atmosphere will be cold enough to support snow for everyone; however, with the low pressure center tracking to our west, warm air that initially rides up and over the cold dome producing the snow will eventually make inroads in the lower portions of the atmosphere, first in southern and eastern areas and I-95, then further northwest, to a point.
The problem in this case, and arguing against a more substantial I-95 snowstorm comes at 925mb, or about 2500 ft up in the air, down to the surface.
You can see on the above map from the latest 12z run of the NAM that 925mb temperatures go above 0c (32F) in southeastern PA and all of DE and southern and parts of central NJ around 7pm EST (0 Zulu time). Snow will fall though the majority of the atmosphere until it reaches approximately 2500 ft, and then begin to melt on it’s way to the surface, (surface temperatures also very closely mimic the map above for this storm). So, it’s safe to say, based on this model, that PHL and surrounding areas in southeast PA will start to transition to rain, after a very brief period of ice, around 7-8pm in the evening Saturday. Those farther south, it will be earlier after a very brief thump of snow, before changing to plain rain, while those farther north will hang onto snow longer and in some cases for nearly or all of the duration of the heavy precipitation. The 0c/32F line at 925mb and at the surface gets to about the blue ridge/ just north of I-78 during the large majority of the precipitation, and not much farther than that. Looking across the board at the models, there is near consensus on these variables on the NAM, RGEM, and European. The GFS I believe is just too warm, given the extreme cold preceding this storm and deep snowpack particularly in the northern 2/3 of Pennsylvania.
So what exactly does this mean? Precipitation will start sometime around or just before lunchtime for those farthest west, and early afternoon for those further east in New Jersey. It will start as snow for everyone, but quickly change to rain for those well southeast of I-95, within an hour or two (probably by 4-5 pm). In and around the city of Philadelphia including southeast PA and central NJ, expect the changeover to occur sometime around 7-8pm. Farther north and northwest, including places like Lancaster, Doylestown, Trenton, up towards NYC, expect a changeover around 10-11pm. The Allentown-Bethlehem-Easton area will changeover in the early morning hours, in the realm of 1-2am, if the changeover occurs at all. I suspect that in these areas, it may be a case of snow to drizzle/freezing drizzle, and really not that much of substantial rain/ice, but that’s kind of a tough call. Even if it does rain in these areas, it won’t be for more than an hour or two. IN the Pocono mountains, expect all snow, from start to finish.
Given all of the above, I think 2-4″ is a good call in and around PHL and southeast PA and its immediate eastern and southeastern suburbs. This will include the NYC area, and the central NJ coast just below the city as well.
Southeast of that, down toward the pine barrens and inland sections of NJ, expect 1-2″, and along the coast of southern NJ, little to no accumulation.
Farther north and west, in the Lehigh Valley, the Lancaster and Harrisburg area, and the Poconos, expect 4-8″, with the least in the south (places like Quakertown/Lancaster), and most in the north (along the blue ridge and obviously in the mountains to the north of that themselves).
Following the storm, expect relatively mild (by the past several weeks standards) temperatures on Sunday, in the mid 30’s to low 40’s from north to south. Then, it’s back into the deep freeze for the foreseeable future, with additional chances at snow around the 25th/26th and March 1st/2nd.
More as we get closer as needed, if there are any substantial changes.