Morning update. My confidence level is moderate here. I’m looking more frontogenesis and vertical velocities here than qpf. Particularly on the NAM, RGEM, and GFS. Bust potential here is mainly with be mesoscale bands, especially with the inverted trough, producing more snow in some areas and cause more subsidence on other areas  The CCB could be weaker and further east reducing totals further northeast over CT or the east of end of Long Island. Or if the inverted trough and CCB are both weaker and further east. Then many of these totals will be too high.

A polar shortwave at 500mb and clipper low at the surface will dive south towards the Northern Mid-Atlantic coast this weekend. Then close off at 500mb and rapidly intensifisy as it heads offshore. The snowfall forecast below is my guess how much snow will fall in general. How this forecast is not easy, as there several areas of lift on models on focusing of banding. Some of these band can produce heavier snowfall in some areas. While time same creating subsidence for other areas. So snowfall may seem to be as uniform as portrayed here. As storm explodes offshore, the pressure gradient with tighten and high winds become an issue. There is potential for damaging wind gusts over 60mph. Saturday night and Sunday morning. Stay tuned for more details and updates on this storm, as necessary tonight and tomorrow.

This is update to for snowfall forecast for tonight and tomorrow winter storm. You can read a more technical discussion below. Stay tuned for another update this afternoon:

A winter storm over Central US is forecast to move into Mid-Atlantic and Northeast US tonight.  The primary low will tracking into Western PA. While a secondary low develops over Southern  New Jersey. The -NAO ridge over Greenland and the trough over Southeast Canada, will be lifting prior to this storm arrival. A strong 500mb shortwave trough will support the 700mb and 850mb low to track north of the area. This will cause warm, moist air to advect into mid-levels, with rising heights ahead of the storm. This setup typically doesn’t support a major snowstorm for the New York City metro area. However, surface high pressure over Central Canada will be nosing into area ahead of this storm. This will support cold-air damming over the Tri-State region for awhile. The primary low will be forced to transfer to secondary low developing over Southern NJ. This low will move offshore, keeping surface winds mostly out east or northeast direction. The result in potential for significant snow and icing event from NYC north.

Light snow should begin to overspread the area between 7pm and 10pm Sunday evening. It will become moderate to heavy at times for much of the area, during the overnight hours. Thermal packing at 850mb, will cause strong frontogenesis at this level. However in 600-800mb layer, where snow growth is occurring, frontogenesis is not as strong with a flatter thermal gradient. I expect snowfall rates 1″ – 2″ per hour across Northeast New Jersey, Lower Westchester, NYC and Long Island tonight, before a mix and changeover to sleet and freezing rain early Monday morning between 4am and 7am. If this changeover is any slower to occur, snowfall totals could be higher that what is shown in the forecast. Enough warm air with a low-level easterly flow, could change this area to plain rain during mid-late morning hours on Monday over NYC and Long Island. Otherwise a up to a .25″ of ice could fall on top of snow and accumulate on roads and trees. Making for very hazardous travel for tomorrow mornings rush hour. As the secondary low moves offshore tomorrow afternoon, winds will back to northeasterly direction and colder air will advect into region. This could mean that any rain or sleet falling changes back to snow tomorrow before ending later tomorrow afternoon. Any additional snowfall accumulations look to be minor, at this time. Due the 500mb trough not going negative or closing off in time. It’s possible that precipitation just tapers off quickly to drizzle tomorrow afternoon.

Over Northwest NJ, Northeast PA, the Extreme Lower Hudson Valley, and Interior CT, expect more snow to more dominate for most of this storm. However, model sounding show a warm nose between 800mb and 900mb that could cause some sleet to mix at times. Whether this warm nose can be overcome, will be dependent on amount lifting and intensity of precipitation. If there is ore sleet the more likely lower end of snowfall totals will be. Little or no sleet, the higher snowfall totals will be. Further south over Central and Southern NJ and Southeast PA, warm air in mid-levels should move in pretty quickly tomorrow morning, with the expect track of storm. So expect some snow before quickly change to sleet then rain.