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.

Another winter storm will approach the area late Sunday and Monday. This storm while not huge, can still make for tough commute to work on Monday morning.

One low will head into Ohio and a secondary low will develop near New Jersey. With an artic airmass already in place, everyone will start as snow. However, the mid-level lows including the 850mb low are gradually trending north with this storm. Including the 850mb, which almost every track into Pennsylvania. Models soundings show a warm nose between 800mb and 900mb early Monday morning. This could lead to mix and change over with sleet for parts of Northeast New Jersey, New York City metro area and Long Island. The shortwave through for this system continues to trend more amplified. This will support the primary low in Western PA before transferring to coast. The more sleet mixes in the lower end of totals we are likely to see.

If the 850mb and surface lows track farther north like the GFS and UKMET suggest today. More places quicker changeover from snow to sleet/freezing rain then plain rain. If the cold air holds out longer, and there is little mixing with sleet, higher snowfall totals may be realized. After new guidance comes in tonight, I may update this forecast, if they are any significant changes.

The National Weather Services in Upton, NY and Mt. Holly, NJ have issued blizzard warnings for Extreme Southeast NY, Northeast New Jersey, and Southern Connecticut. They are forecasting 20 to 30 inches for almost their entire forecast area. Winds are expected to gust between 55mph to 65mph. With the strongest winds being on Eastern Long Island.

A shortwave trough associated with an Alberta Clipper, is moving eastward over Mississippi River Valley now. The is trough will amplify, meaning take an extreme negative tilt, and phase with northern stream shortwave energy coming down from Central Canada tomorrow. When the northern stream shortwave energy starts phasing with the southern stream, the trough will being to close off at 500mb. All models agree on this general scenario. But slight differences in handling of shortwave energy and interaction, are causing models to show different precipitation fields.

Mid-level centers closing off just south and east of the Tri-State area. These features will help with maturation of the cold-conveyor belt or the comma-head. The closed 500mb low also captures the low before goes out to sea then slow meanders off Long Island. As this low occludes, dry air will start intruding in the storm’s circulation. This is most likely closer to the low pressure center. Further away center, near the northern fringes, features a stronger deformation zone. It hard to pin exactly where we would see the banding. So the purple area in the forecast below shows most snowfall totals in purple area.

Strong vertical velocities (lifting in the atmosphere) along deformation zone, will cause snowfall rates of 2 to 4 inches per hour. Also this a deep arctic cold airmass coming in tomorrow, with strong high over Quebec.  So snowfall ratios are likely higher then typical average of 10 to 1. Right now, I expect ratios to be near 15-20 to 1, during the height of storm, Monday night and Tuesday. Where this band or another setups, we’ll be the areas that receive locally higher snowfall totals of 30 inches. Stay tuned for more detailed analysis of model runs tonight and tomorrow, before worst of the storm arrives. My current snowfall forecast is below. This forecast will updated, if necessary.