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Live Blog: Steady snow stays well south of NYC

March 2, 2014/0 Comments/by Doug Simonian

1:45am Update: Steadier and heavier snow is beginning to develop near Philly and Northern Delaware. It is heading to South Jersey as we speak. Over the next couple of hours, any initial rain or mix in Burlington County and Ocean County and southward will quickly turn over to snow, and accmulate about 1″ per hour — perhaps even up to 2″ per hour in quick bursts. Due to the quickly accumulating snow and the fact that there is no solar insolation to warm the roads, roads will become slick pretty quickly.

Radar Imagery as of 1:33am EST. You may need to click to animate.

This heavy band of snow is falling as a result of a strong area of frontogenesis. In essence, frontogenesis is an area of a strengthening front, where the temperature gradient is increasing (cold areas are getting colder, and warm areas are getting warmer). This leads to enhanced lift in the atmosphere. Not surprisingly, the strongest frontogenesis is headed for Southern New Jersey.

Latest 850mb frontogenesis is pretty strong in northern Delaware, and is heading for Southern New Jersey.

There are some weaker areas of frontogensis trying to streak northward towards NYC. Thus, even in areas north of Central NJ, light to at times moderate snowfall will be developing throughout the overnight. We still do not anticipate more than an inch of snow north of NYC, but an inch or so could still fall in the NYC area and Long Island — max potential is 2″. This includes western suburbs as well. So although not much snow is expected, enough could still fall to create some minor issues on the roadways.

9:45pm Update: The south trend never stopped. As time has gone on, models have better picked up the true impacts of a strong Polar Vortex, which is to suppress precipitation to our south. For more on why this has occurred, check out our article from yesterday.

High resolution model trends, along with radar and surface observations are indicating that dry air and subsidence will be the rule for NYC and points north, where perhaps only a dusting of snow will fall. Perhaps an inch of snow or so for the south shore of Long Island, through Staten Island and Union County and points westward. Middlesex County looks to receive 1-3″, with the 3″ being on the southern end of the county. Snowfall totals exceeding 3″ will not be reached unless in southern Monmouth, and Mercer County, and points southward. Burlington and Ocean County could still receive 4-8″ of snow (closer to 4″ in northern sections, closer to 8″ in southern sections). Camden County and southward could still receive snowfall totals in excess of 8″, and perhaps even approach a foot, with heavy snowfall at times, so we will make sure to keep this blog updated for these residents.

Although the GFS shows totals a little higher for northern sections, we believe that its low resolution is not picking up on how sharp the cutoff in precipitation will be.

Here is the most recent HRRR Model’s precipitation output valid through 10:00am tomorrow, with steady snow still falling in Southern Jersey.

The most recent run of the HRRR (High Resolution Rapid Refresh model) shows heavy precipitation remaining to the south.

We believe ratios will be about 12:1 or so, so multiply these numbers by 12 to get an expected snowfall total. Heaviest snow in Central and Southern Jersey looks to from 3:00am through at least noon, with the heaviest centered between 7:00am and noon. Be sure to give yourself a lot of extra time for the morning commute if you are in these areas.

Why is the storm shifting south?

March 1, 2014/1 Comment/by Doug Simonian

A few days ago, most models had the area getting hit with 10-15″ of snow, which generated a lot of hype. Even up until yesterday, some of the models were still showing that same solution. However, models were beginning to trend towards a change in the orientation of the Polar Vortex, which started to sound the alarms for a south trend. This is why we wrote this article yesterday, detailing the potential trends and why this storm might not produce that much snow.

Today, every model has shifted southward, and barely gives New York City a few inches. And the south trend might not be done.

On the left, we have Wednesday’s European Model forecast at 500mb valid for Monday morning, which showed a lot of snow. On the right, we have today’s European Model forecast at 500mb valid for Monday morning, which showed little in the way of snow. Note the differences with the handling of the Polar Vortex. On the left, the vortex was much more elongated and oriented from southwest to northeast. Additionally, energy is diving down and interacting with the southern stream storm. On the right, the vortex is compressed together, and has a due north to south orientation. This squashes the height field in the Northeast, and leaves little room for precipitation to advance northward. This also allows northern stream energy to dive down further east — out ahead of the storm — so the pieces of energy do not interact. Instead of the northern stream energy enhancing the storm like it did on the left, it inhibits the storm on the right.

The fact that the change in Polar Vortex orientation led to two separate changes to the weather pattern to inhibit the storm explains why the shifts to the south have been so dramatic.

We issued a snowfall map earlier, which can be viewed with this link.

However, there is a chance that these amounts will have to be pushed even further south. Wave #1 looks to come into the region on Sunday afternoon, and give light snow from Central NJ and northward through around 3:00am on Monday morning. This could add up to 2-4 or 3-6″ in some spots from New York City’s latitude and south. South of Trenton, however, may see rain or ice with this initial batch of precipitation. Then wave #2 arrives for Monday morning, but this should miss the area entirely, except for areas south of Trenton. The question becomes how far north does wave #2 go — but it is even conceivable that the south push is so strong, that the northern half of the 8-12″ zone may have to be chopped in half or even more. If wave #1 is rain or ice for these locations, and wave #2 misses, then not much snow will fall at all for the northern sections of the 8-12″ zone. Areas further south and/or east of Philadelphia could still see significant snow from wave #2.

We will wait until tomorrow morning to make any changes to our map, since tonight’s data could still change. However, the trend has continued to be south, and based on the orientation of the Polar Vortex, we foresee that continuing.

Explaining the uncertainties with the upcoming snowstorm

February 28, 2014/28 Comments/by Doug Simonian

Although yesterday saw pretty good agreement among most computer model guidance regarding a major snowstorm on Monday, we were still four days away from the storm, which is a pretty long time in the realm of computer modeling. In more plain terms, this means that despite this agreement, we were still susceptible to changes in modeling that would ultimately affect the outcome.

The current weather pattern is dealing with a strong storm system crashing into southwest California, as well as a strong Polar Vortex in Southern Canada, supplying cold air into much of the country. This storm system in southwest California is forecast to move eastward, and as it does, will be increasing the warmth and moisture out ahead of it as it interacts with the Gulf of Mexico. Additionally, the Polar Vortex is forecast to exert its muscles and intensity the cold around it and just to its south. This leads to a strong source of anomalously cold air in the north, and anomalously warm air to the south — creating a strong temperature gradient which should lead to widespread, heavy precipitation just north of this gradient. This part seems easy.

But it’s much more complicated than that. The Polar Vortex in a way is a double-edged sword. While it can supply the cold air to strengthen the gradient, it can also be so strong that it shears out the initial storm system. If the initial storm system gets sheared out, then its ability to generate warm air advection out ahead of it and spread northward is decreased, which means the thermal gradient weakens and gets pushed to the south.

Let’s take a look at some computer models. Today’s 18z NAM showed a major snowstorm of 10-15″, whereas today’s 18z GFS only shows a few inches. Below, we will see the 18z NAM valid for 10:00pm, Saturday night.

Today's 18z NAM at 500mb valid for 11:00pm Saturday night shows a favorable setup for major snow, due to the Polar Vortex becoming elongated, as opposed to compressed.

Today’s 18z NAM at 500mb valid for 10:00pm Saturday night shows a favorable setup for major snow, due to the Polar Vortex becoming elongated, as opposed to compressed.

Significant winter weather event possible next week

February 27, 2014/2 Comments/by John Homenuk

Forecast models have come into much better agreement regarding a storm system which is forecast to impact the area early next week. Several days ago we mentioned in a blog post that the next significant precipitation event wouldn’t come until early this coming week — and that idea continues. A powerhouse upper level trough will slam into the West Coast over the next day or two, providing much needed rains. But more interestingly for us, the associated energy will de-amplify somewhat, but eject northeastward towards the Tennessee and Mississippi Valleys. As a result, a surface low pressure system will develop from the Arkaltex towards the Mid Atlantic coast.

To our north, a piece of the Polar Vortex will be meandering in Southeast Canada, maintaining an impressive low level cold air source. The result of the ejecting shortwave and cold air to the north will be an impressive thermal gradient and stalled frontal boundary, which will extend from the Central US towards the Mid Atlantic. Along this thermal gradient, a plume of moisture is forecast to develop in response to enhanced lift from the aforementioned disturbance. Accordingly, areas to the north of this thermal gradient (including much of our forecast area) will be in line for a potentially significant winter weather event.

GFS model showing a large-scale winter weather event impacting the Eastern United States early next week.

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