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Forecast models have trended dramatically in favor of a significant Nor’Easter, developing off the East Coast of the United States on Monday Night into Tuesday. The pattern features a very complex evolution of events, but favors a higher than normal likelihood of a significant low pressure system and precipitation developing along the East Coast, with higher than normal confidence in significant snowfall especially along the coastal plain.
Aloft, a significant upper level trough will eject southeastward toward the Ohio Valley and eventually to a position in the Southeast States. With a closed 540dm isopleth, this mid level trough is quite amplified. Mid and upper level heights will respond to this amplification along the US East Coast, with a pulse of the Western Atlantic Ridge. This will allow for mid level winds to become oriented south to north along the Eastern Seaboard.
Meanwhile, tremendous mid and upper level jet streak will surge south and eastward across the top of and eventually down the eastern periphery of a West Coast ridge on late Monday into Tuesday. This jet streak, and associated mid level shortwave, will become the spark in a powder-keg type scenario on the East Coast. As it moves southeastward and phases with the initial mid and upper level trough, it will force the development of a significant low pressure system off the East Coast on Monday Night.
From this point forward forecast models continue to display some divergence in handling precipitation features — i.e, where the heaviest precipitation will fall and how much of it will fall there. There is a 75 mile or so spread between the GFS and the European Model, for example, with the track of the storm and associated precipitation shield.
The reason for this? Even at this range, the two forecast models are handling the phase quite differently. The European is more aggressive with the phase, and thus tugs the low due northward more quickly and even bends it back slightly west of due north to a position south and slightly east of Montauk, Long Island. More importantly for historic snowfall, it closes off the storm at 500mb well to the south of our area and it crawls northward, but generally stays to our south. This allows for extreme lift to develop in the mid levels of the atmosphere — translating to widespread extreme precipitation values and heavy snow.
The question now becomes, for forecasters: How do you weight the European model into the forecast when compared to the GFS, which is farther east and noticeably less amplified? The answer: The Euro is superior, and should be weighted as such. Still, with other global models leaning slightly east of the Euro, it becomes critical to at least blend some of their more moderated solutions into the forecast. Our latest forecast package leans 70% toward the European, and 30% toward the less amplified guidance (i.e GFS, RGEM, CMC). This brings us to a very snowy scenario — but one not quite as extreme or mind-blowing as the European.
While the other non-European guidance is further east with the storm itself and the precipitation, precipitation shields are not always linear. The non-Euro guidance still eventually closes off at 500mb just to our southeast, and closed 500mb and 850mb lows southeast of Montauk arestill textbook for very heavy snow to make it into NJ, NYC and LI. So, in this situation it’s important not to necessarily pay too much attention to exact precipitation outputs from forecast models.
Additionally, the jet streak orientation is ideal for significant phasing and a large comma head of precipitation to develop well to the west of the storm system itself. Guidance, additionally, is great agreement on a significant TROWAL (trough of warm air advection) developing and backing into our area Monday night into Tuesday morning, which should help to provide plenty of mid-level forcing and lift for 2-4” per hour snowfall rates at times.
Our forecast evolution of the storm features the following:
A significant Nor’Easter is likely to develop from Monday Night into Tuesday, initially off the coast of the Carolinas and eventually to a position near or just inside the 40/70 Benchmark southeast of Long Island. Significant precipitation will expand northwestward from the surface low as a result of favorable mid level jet orientation and dynamics.
As the surface low deepens and a cold conveyor belt develops, intense atmospheric dynamics and lift for precipitation will occur. Very heavy snow will spread northwestward from the Western Atlantic to the NJ Coasts and Long Island by late Monday Night into the wee hours of Tuesday Morning. The westward extent of this precipitation shield is currently extremely uncertain — and will determine whether the storm is significant to major, or historic. In this heavy banding, snowfall rates of 2-4” per hour at times are likely.
Heavy snow will take on a more south to north orientation during the day on Tuesday as the cold conveyor belt pivots (again, position uncertain). At this point, strong winds from the developing low pressure system will bring blizzard conditions to coastal locations. Eventually, by later Tuesday afternoon or evening, precipitation will begin to lighten in intensity as mid level dynamics lessen. Still, snow is supported with our blend forecast along the coast and could continue into the overnight hours on Tuesday as the low pressure system remains offshore.
Confidence in 12” of more of snow is high for the entire forecast area. Confidence in 18” or more of snow from New York City and east is moderate to high, whereas confidence in 18” or more of snow west of New York City is low to moderate. In general, our forecast would lead to 18-24”, with locally higher amounts, from NYC and points east and northeast, with 12-18”, with locally higher amounts west of NYC.
The most advantageous thing to do, at this point, is to stay tuned to the latest forecasts. Newer forecast model cycles will have improved data and will give forecasters the ability to gain confidence in the track, intensity, and eventual impacts of the storm.