Norlun Troughs?

Going to get a little technical briefly. Going to put this pretty plainly: What’s coming Friday will more than likely not be forecasted very well. Why? We’re dealing with a phenomenon that’s pretty common in the Northeast in winter…it usually rears its ugly head at least once. We call this an inverted trough…Norlun trough…or instability trough. What is it? Well, typically, when you think of an atmospheric trough, you think of a “U” or “V” shaped pattern. In an inverted trough, this, well, inverts. It looks more like an upside down U. The lower pressure, or lower heights end up on the north side, rather than the south side. These sorts of setups can produce some nasty weather, heavy precipitation, and thunder.

Here’s more info on Norlun troughs (recognized in the 90s and named for the meteorologists who discovered it).
A technical paper on Norlun trough events.
And here’s more on inverted troughs.

So what’s it to you? Well, the models are beginning to hone in on how this storm on Friday and Saturday is going to behave. And they are indicating that an inverted/Norlun trough type setup may occur. The bottom line is that there will be snow. The question is twofold: Will the Norlun event occur, and if so, where will it setup? This morning’s models pegged it right over North Jersey and NYC. This evening it looks to be shifted a little further north. This is important, because outside of the Norlun trough there will be a generic sort of snowfall. But under the narrow Norlun trough, there should be significant snow, with the possibility of thundersnow and more.

Right now, the NAM model (a mesoscale, small scale, higher resolution model) is showing extremely heavy snow (and has on three straight runs today) for areas immediately north of NYC, Long Island, the Catskills, and the Western Mohawk Valley. The GFS model is suggesting this will be further north and east into New England and the Adirondacks.

Latest Snowfall Risk Assessment for Friday and Saturday

So my snow “assessment” has a broad area of potentially significant snow showing in red. I right now have this from about the Bronx, east-southeast into Long Island (yes, that places the division of snow vs. heavy snow within NYC). It stretches up into the western Hudson Valley, Catskills, western Mohawk Valley and southwestern Adirondacks. I also include much of southwest Connecticut as well, including Bridgeport and New Haven. The problem is that it’s next to impossible to pinpoint exactly where within this idealized zone the prolific snow will be. The yellow territory indicates the basic “cone of error” of sorts. I don’t foresee the risk of heavy snow shifting south of NYC much, if at all. The risk to me is clearly on the northern and eastern sides. This could shift further north, so the axis of the heavy snow stretches from Syracuse/Utica through Albany, the Berkshires, and Central Connecticut to the coast. And it could even shift further north too.

The heavy snow band will be awesome though, with extremely heavy rates of snow, likely some thunder, lightning, and whiteout conditions. I could see max amounts of 10-15″ in this band. You will only need to go a few miles from that core of heavy snow however to find amounts of 6″ or less…that’s the type of storm we’re looking at: Very narrow gradient, very sharp gradient, with extremely variable snowfall totals. Thus, this makes forecasting this system extremely difficult, if not impossible at this point. So if you live from NYC into Central New York and all of Southern New England, stay tuned. As I said, the forecasts for this will more than likely be challenging, vague, and possibly even wrong initially. But that’s the story.

Outside of the heavy snows, expect a brief period of maybe some heavy snow, with a coating or so, up to about 3″ in spots across much of NJ, PA and central New England. Closer to the heavy snow axis, there may be some 3-6″ amounts. But those will be isolated.

New model data indicating the possibility still of a major snowstorm Tuesday of next week. There are still a multitude of questions as to whether it will track in the sweet spot for a DC-Boston hit, as tonight’s GFS is showing, or if it will pass further south. More on that after this first event passes.

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Don’t forget, State of Occlusion is on Facebook! I’ll post some additional cool links and snow maps before they’re published here on that page. “Like” the blog by clicking here!

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