Summary: It's game on for a significant to major snowstorm for most of the area, even down to 276/195 and maybe even Philly/adjacent SNJ. The 18Z and 0Z models continue to show the storm being further south meaning less warm air drawn in due to the track and a faster changeover to heavy snow as the storm is modeled to have high precip rates. I think it's reasonable to expect 5-6" or more along/N of 78 with up to 10-12" as one heads towards 80/84 in PA/NJ and north of 287 in SENY and that it's reasonable to expect 2-3" along 276/195 meaning nearly all of CNJ, including 95 from Trenton to NYC would likely be between 2-6" (from south to north), with the potential for that to be more like 4-8" from 195 to 78. And even Philly/nearby SNJ could be in the 1-3" range.
Also, the worst of this will hit CNJ with heavy snow from about 5 am through noon or so, meaning the morning rush hour will likely be a mess, even if treated roads keep accumulations down, as visibility is absolutely going to be an issue - and areas that see the heaviest 2"/hr bands will see accumulations even on treated roads (and local roads will likely see accumulations assuming everyone gets the 1"/hr rates being shown on all of the models. The chances of this being only a sloppy inch or so for areas south of 78 (the NWS forecast) are now quite low, but the amounts I mentioned above could certainly be what we're seeing in the less snowy models, i.e., I think 2-4" between 195 and 78 is likely now the "floor" for this event (if 7.5:1 ratios and the less snowy models verify, as per the details below). This is probably the hardest I've disagreed with the NWS ever (or at least since Nov-18, which was somewhat similar with mostly rain predicted until 6 hours before the event, but everyone in CNJ getting 4-8" and a nightmare on the roads as the snow hit around noon, like a wall.
I'd guess that the NWS-Philly will upgrade snowfall amounts at 4 am and go with advisories for 2-5" for most of CNJ (Hunterdon/Somerset/Middlesex/Mercer/Monmouth) and for Bucks, Montco, Berks and maybe W. Chester and warnings for 6" or more for all of the counties north of 78 in PA/NJ (Poconos/Lehigh Valley and Sussex/Morris/Warren) and that the NWS-NYC will also go with advisories for 2-5" for Union/Essex/Hudson, NYC/LI and probably warnings for 6" or more for Passaic/Bergen and SE NY north of the Bronx.
Don't want to forget that minor to moderate coastal flooding is likely for DE/NJ/NY coasts on Tuesday and 40-45 mph winds are likely for coastal areas and 30-35 mph inland as the storm cranks on Tuesday.
Details: Every 0Z model (GFS, Euro, CMC, UK, NAM, RGEM, HRRR) is showing at least 6" of 10:1 ratio snow along and N of 78 in PA/NJ/NYC and at least 3" along 276/195. Also, several of the models have that 6" line as far south as 276/195 in PA/NJ (or even past Philly on the Euro) and those models (Euro/GFS/HRRR/CMC) all have 6" along 195/276 increasing to 10" or so as one moves up to 78. Even if one cuts those snowfall totals by 1/4 due to ratios/melting (i.e., a 7.5:1 ratio), that's still 4.5" along 195/276 and up to 7.5" along 78 for the snowier models and at least 2" along 195/276 and at least 4.5" along 78 for the somewhat less snowy models.
Yes, it's possible the forecast could trend back to the north some, but no experts I've read at AmericanWx or Twitter or elsewhere expects a shift as large as what we saw today to be realistic, as the error bars on forecasts continuously shrink as one gets near the event and we're only ~24 hours before the event starts (as rain), meaning the current NWS forecast is almost impossible to defend at this point, i.e., <1" of snow south of 78. It's also worth noting that their discussion says, "From there, latest model guidance has the low just a bit farther north from previous runs." which is completely wrong as the low has been shifting south since the 12Z runs at noon and they know that.
The rest of the 18Z models and all of the 0Z models moved the low further south for very good synoptic reasons (convective interference going on in the SE US). That combined with the storm being forecast to strengthen quickly along the baroclinic zone as it travels just off the coast, leading to development of a well modeled cold conveyor belt, which will dynamically cool the column due to the strong lift of moist air into the cold air in the DGZ (dendritic growth zone around 15K feet up), will change the rain over to heavy snowfall for most of us from about 5 am to noon on Tuesday (1-2" per/hr rates likely), which will quickly start to accumulate at those rates, despite 33-35F surface temps. And with good growth and once the accumulation starts, most experts I've seen are talking about snow accumulation ratios in the 7-9:1 region (even if Kuchera says lower).
See the AmericanWx link for details and John Homenuk's Twitter page for some of the best insights around. He's similarly bullish on this storm, predicting 2-4" for NYC metro and more N/W and discusses how the models are showing a lot more and that that might be what we see even south of 78.
https://www.americanwx.com/bb/topic/60026-significant-winter-storm-rain-snow-mix-on-feb-13/page/38/
https://twitter.com/jhomenuk
