The National Weather Service issued Winter Storm Warnings and Winter Weather Advisories which were valid from Saturday through Sunday, in advance of a major winter storm which is forecast to impact the area. Beginning later Saturday, snow is expected to spread throughout the region from southwest to northeast. The snow will likely be light initially, as it battles with drier air. But as a disturbance moves northward and eventually develops a stronger surface low near the Mid-Atlantic coast, heavy precipitation will move towards the area on Saturday afternoon and evening.
Temperatures, both aloft and at the surface, remain a major challenge for forecasters and will have a significant impact on how the event shakes down. Warmer temperatures aloft are expected to begin to move into Southern and Central New Jersey by later Saturday afternoon, and progress northward through the evening. At the coast, this will spell the end for frozen precipitation, as the warm layer’s depth expands and surface winds turn southeast off the warmer ocean waters. But inland, the battle will just be beginning, as the warmer air moves in aloft but is forced over the top of a shallow cold layer which remains stubborn near the surface. The combination could lead to snow changing to sleet and freezing rain across parts of the area suburbs, resulting in not only a moderate snowstorm, but significant icing thereafter.
10:00pm Update: Much of the area has changed over to a sleet and freezing rain mix, due to warm air advection occurring aloft, but cold air remaining drained at the surface. When temperatures are above freezing well above the ground, precipitation melts and cannot recrystallize into snowflakes because the rain drops have left their snow growth regions. But if enough cold air is drained at the surface, the rain drops will either refreeze before they hit the ground (sleet/ice pellets) or when they hit the ground (freezing rain). When the cold air at the surface is a bit more shallow, it tends to be freezing rain instead of sleet.
Anyway, for areas east of NYC, sleet and freezing rain should not last more than an hour or two, and precipitation should change over to plain rain if it has not already within that time.
However, areas west of NYC will have to be on the lookout for at least 0.1″ of ice — if not lots more, which could cause lots of problems. Surface wind direction and isobars show this very well.
The red line outlined is where we expect 0.1″ of ice to fall, or greater — and the amounts increase as you head further north and west of that line. The reason we believe this will be the boundary for ice is because of the exact source of the surface winds. With the strong primary low pressure system in West Virginia and Ohio giving way to a secondary surface low pressure system on the Delmarva, counterclockwise flow dictates that the isobars will generally flow from east to west, or east-south-east to west-north-west. Generally speaking, winds follow these isobars, and when they are in balance with the flow of the isobars, we call those winds geostrophic. Notice how especially in the Atlantic Ocean, the surface wind barbs nearly exactly match the flow of the isobars. Thus, these winds are aided by what we expect from a low pressure system that is very close-by — e.g., warm winds that are south of due east right off the Atlantic Ocean, warming surface temperatures.
However, as one heads further west, the winds start to deviate from that flow. The surface wind barbs begin to turn a bit more northeasterly, and the isobars begin to increase in slope and align in a bit more of a south-to-north direction. What this means is that the winds are now essentially flowing perpendicular to these isobars, so these winds are ageostrophic. The implications of this are that these winds are not attached to the warmer Atlantic Ocean, as the source region of these winds is cold, dry land. The winds are going against the flow that is “supposed” to yield warmer temperatures at the surface. This often happens when a cold high pressure system is located to the north.
Thus, regions west of the boundary between geostrophic and ageostrophic winds (outlined in red) can expect several hours of ice, with even some isolated power outages. Most amounts should be between 0.1 and 0.3″ of ice, but an isolated spot picking up 0.4″ of ice is not out of the question. We will keep you posted.
8:00pm Update: Coastal front continues to move slowly northward, but progress is being slowed even more by the cold air to the north and reinforcing northeast winds. Newark Airport fell to 19 degrees the past hour (for the record, well below any model guidance forecast). But just to the southeast, JFK Airport surged to 30 degrees — while rain was falling on much of Eastern Long Island and the NJ Shore (45 degrees at Cape May and 9 Degrees at High Point!!). So the concern now becomes the potential for a period of freezing rain after a few more inches of snow. Mid level warm with move northward by around 10pm…and begin making its way towards Northern NJ and NYC. But the surface remains very cold, and as the coastal front moves inland, areas that have warmth aloft and remain west of the coastal front could see a period of icing.
During this time, travel could become extremely hazardous. We suggest that you stay off the roads over the next several hours.
6:00pm Update: The coastal front is finally advancing northward, and heavier precipitation is also working its way from west-to-east through New Jersey. Snowfall rates will continue to get more intense, as steady moderate to heavy snowfall is right on NYC’s doorstep. Snowfall rates will be at around 1″ per hour — maybe even 2″ per hour for brief period of time — for the next 2-3 hours once the heavy band arrives.
3:30pm Update: Snow has gradually picked up in intensity across Central Jersey, and this area of heavier snow will trek northeastward throughout the rest of the afternoon and evening. This can partially be explained by a phenomenon we call a coastal front. New Jersey’s current temperatures better help to explain this.
Notice how in Monmouth County, temperatures are at or below freezing, but they quickly rise to several degrees above freezing once one enters Ocean County. This sharp temperature gradient is caused by a coastal front — a mini cold front that forms when warmer, Atlantic Ocean air collides with the colder, drier air source from the Arctic high pressure to the north. What this front does is cause an area of more sufficient lifting, which explains why precipitation has begun to blossom across Central New Jersey.
That warm air gets lifted slantwise somewhat, which will cause the heavier precipitation to move northeastward out ahead of the cold front. The implications of this are that as the coastal front gradually moves northward, snow will become heaviest. Also, areas just north of the coastal front will have the heaviest snow, given that they are so close to the lifting mechanism. Thus, in the prior two hours or so to a changeover to rain, expect snow to really increase in intensity — perhaps with 1 to 2″ per hour rates. This may help some areas to get about an inch or two of snow more than forecast.
Wherever the coastal front stalls/slows down for the longest in conjunction with the storm moving in later this evening will have the highest snowfall amounts. We will keep you posted on where this is — it’ll probably be in interior Northern New Jersey, given that by the time the coastal front approaches that area, the storm system will be closer to our area, further helping in lifting for precipitation.
12:40pm Update: Snow continues, light in most areas, early this afternoon. The best lift for heavy precipitation still remains back to our south and west but will be approaching the area within the next few hours. Temperatures seem mostly on track and we are still expecting a period of heavy snow across much of Northern NJ, NYC and CT this afternoon and evening. Snowfall map has been updated for the final time, but mostly just tweaks at this point. The rest of our products will be updated this afternoon (next few hours) including the Forecast Discussion and Forecast Brief. Below, the HRRR model shows snowfall totals of 6″+ across much of New Jersey by 12am tonight.
10:50am Update: Snow continues throughout most of the area, specifically northern portions of New Jersey, Southeast New York and New York City into Connecticut. A few reports of 2″ accumulation (yes, already!) in parts of Connecticut. This is likely owing to improved snowfall rates well in advance of any mid level warming. Speaking of which, there is already a dramatic temperature gradient on the Southeast Coast of NJ as evidenced by this NJ Wx Net map. This works well with out forecast of very little to no snow in that area. Meanwhile, the latest HRRR model continues to indicate the potential for 1-2″ per hour snowfall rates in bands later this afternoon and evening. Stay tuned as more develops, we will update this page more frequently.. – JH
9:50 am Update: Snow continues throughout the area, light to moderate at times in banding. The snow will continue over the next few hours but will continue to be somewhat spotty in nature. That said, some accumulations have already begun. Be careful of slick and untreated roads. Travel with caution. We continue to monitor forecasts and trends including radar and temperature.
Finally, if you haven’t yet, use the Link Hub at the top of this post for links to handy info. These will update through the day as well.