Video: Explaining the late-week snow potential

Much has been said over the past several days regarding the potential for a winter weather event and snow accumulation late this week. And in reality, the potential is quite high. Forecast models, however, have been waffling with the potential for a bigger system and one that remains disorganized and is more likely to produce a moderate event. As forecasters, this keeps our confidence in the event rather low. The specific details and intricate interactions between atmospheric disturbances will ultimately determine how the storm forms, where it tracks, and how strong it is. But at this range, forecast guidance will struggle mightily to pin down the details.

To help quiet the noise a bit, we’ve provided a video discussion which breaks down the setup. In this video you’ll find our concerns, thoughts and ideas as we continue to analyze and interpret the data. Despite the uncertainty, we should begin to gain confidence in the eventual outcome of the system over the next 12-24 hours. You can watch the free video by clicking below. If you are on mobile, copy the video link into your browser.

 

Confidence in a significant snowstorm is increasing for Thursday night and Friday

As colder air begins to filter in over the next few days, the weather pattern will become more favorable to support a snowstorm for later this week. While details still remain a bit uncertain, the trends in all of the latest model guidance support the threat for a potentially high-impact snow event.

Although December was a pretty active month as far as winter storms were concerned, there were no major, widespread snowstorms for the entire area. The main reason for this was the lack of any North Atlantic blocking, which helped to keep the flow in the pattern too fast for any major, organized amplification. However, this time around, some North Atlantic blocking has developed. Although it is not tremendously strong, it is located in a perfect position to slow down the pattern and support a major storm. Let’s go over all the pieces in the latest model guidance.

Today's 12z European Model at the 500mb level valid for Tuesday evening shows several key pieces which could help produce a big snowstorm. Image credit goes to the WSI Model Lab.

Today’s 12z European Model at the 500mb level valid for Tuesday evening shows several key pieces which could help produce a big snowstorm. Image credit goes to the WSI Model Lab.

The image above shows today’s 12z European Model at the 500mb level, valid for Tuesday evening. There are several key pieces labeled which indicate the potential for a big storm. Although the storm itself is still a few days away, the fact that this setup is in place within 36 hours gives us a bit higher confidence in a snowstorm of some kind.

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Cold air returns ahead of potential storm

Behind a coastal system which brought widespread rainfall totals over 1″ on Sunday, colder air is filtering into the area and will settle in by the beginning of the new week. In fact, arctic air will pool to the north of our area over New England and much of Eastern Canada, as the Polar Vortex elongates and stretches from Central to Eastern Canada by mid week. The result, in our area, will be colder than normal temperatures with single digit and teen low temperatures possible over the interior.

The newer forecast models bring temperatures into the teens even in New York City and much of New Jersey by Tuesday morning. A few weak disturbances will pass by to the north of our area during the first half of the week, but aren’t expected to provide any significant precipitation. Other than a few snow showers, the first part of the week will simply be quiet and quite cold. But by weeks end, forecast models continue to signal a potential storm system which could provide the area with winter weather. The details, however, remain uncertain and likely won’t come into better focus for a few more days.

NAM model forecasting low temperatures in the single digits and teens throughout the area on Tuesday morning.

NAM model forecasting low temperatures in the single digits and teens throughout the area on Tuesday morning.

Late week snow potential remains, but situation is complicated: Yesterday we thoroughly discussed the potential for a winter storm in our area during the latter part of the upcoming week. Since then, not much has changed. Forecast models consistently have shown the potential for an event. However, the eventual breakdown of the more detailed pieces of the pattern remain highly uncertain — and likely won’t be fully understood until we draw a few days closer to the event itself.

The main culprit behind the storm system is the northern stream disturbance, which slides southeastward from the Pacific during the middle of the week. But the pattern evolving around that disturbance, which supports the potential for winter weather, is far more intricate. The beginnings of the change can be traced back to a high latitude blocking ridge which forms over Greenland during midweek. As transient and temporary as this block may be, it acts to slow down the pattern ever-so-slightly. To the south of this ridge, the Polar Vortex elongates over Eastern Canada to support an ample supply of cold air to the north.

GFS model showing the potential winter weather event shifting offshore with minimal impacts.

GFS model showing the potential winter weather event shifting offshore with minimal impacts.

So while the aforementioned northern stream disturbance swings into the Tennessee Valley, it will be entering an environment which could potentially favor a winter storm in the Northeast US. The issue, at this point, is attempting to understand just how the pattern will shake down. The variables mentioned above will have a tremendous impact on the sensible weather outcome. A weaker block, and the Polar Vortex shifts northward allowing the initial surface low to move northward as well. This would be a significantly less wintry outcome for our area. A stronger block, or better positioned Polar Vortex, and the pattern can slow down slightly more — allowing for further amplification.

Forecast guidance at this range should be used, as titled, for guidance only. After breaking down the pattern it becomes clear that slight changes in the forecast will create tremendous variance. So while confidence is increasing in a winter weather event somewhere in the Northeast, the details are likely not going to be understood with confidence until a few days from now.

Coastal storm to bring periods of heavy rain Sunday

A developing coastal storm system is expected to bring periods of heavy rain to the area later on Sunday, with a low pressure system tracking from the Mid Atlantic states to a position just off the coasts of New Jersey and eventually Long Island. Despite the historically good track for wintry precipitation, a lack of cold air in the antecedent airmass will lead to mostly if not all rain throughout the forecast area — with no wintry precipitation concerns.

Instead, high clouds will begin filtering into the area on Sunday and rain will stream into New Jersey from southwest to northeast. Increasing moisture and warming temperatures both in the mid and low levels of the atmosphere will mark an environment which will become favorable for heavy rain by later Sunday evening. As the surface low passes off the coast of New Jersey early on Sunday evening, enhanced lift and dynamics will create a period of very heavy rain — which will shift through New Jersey and New York City , eventually moving northeast into New England. Rainfall totals are likely to exceed 1″ throughout a vast majority of the area.

NAM model showing Sunday's forester at many different levels of the atmosphere, producing heavy rain in our area Sunday evening.

NAM model showing Sunday’s forester at many different levels of the atmosphere, producing heavy rain in our area Sunday evening.

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