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Monday’s snow threat trends north, still a concern

The winter weather potential for late this weekend into early next week has been well advertised. A low pressure system is forecast to develop from the Ohio Valley into the Northeast states late this weekend, along a thermal gradient otherwise known as a baroclinic zone, and force the development of precipitation in the Northeast States. Forecast models, however, had been displaying tremendous uncertainty with the positioning of that thermal gradient — uncertainties which were well documented. The past 48 hours have seen forecast models trend farther north with the thermal gradient, changing the complexion of the storm threat completely.

What this means for our area, is that precipitation type and amounts become less certain. The thermal gradient, essentially, is a boundary where there is a large temperature gradient over a short distance. Along and just north of this zone, enhanced lift for precipitation can occur as the development of a low pressure system is promoted near or just south of it. With this gradient trending north, forecast models have introduced warmer temperatures in the mid levels, and have shifted precipitation northward into New England.

Still, the exact outcome of the storm system remains up in the air. Some forecast models (like the more recent runs of the NAM, for example) are far enough north that winter weather concerns would be alleviated in our area, save for the high elevations of the Hudson Valley. But other models, like the GFS and ECMWF, would introduce a prolonged period of light or moderate snow, sleet and freezing rain to many areas from New York City northward. Such an event would likely cause some travel impacts and difficulties, as well as the potential for light accumulations of all winter weather precipitation types.

What has caused the event to trend north? The atmosphere isn’t modeled to behave as it once was. A large upper level low in Southeast Canada had been modeled compress the mid and upper level atmospheric flow over New England this weekend. The result of this would be a thermal gradient pushed farther south, and any precipitation that developed would’ve fallen as snow for the majority of our forecast area. But a more progressive pattern in the higher latitudes has allowed for that upper level low to shift farther north, in turn allowing the thermal gradient to shift northward as well.

GFS model forecast at 500mb this weekend. Notice the upper level low over Southeast Canada and the gradient over the Northeast.

GFS model forecast at 500mb this weekend. Notice the upper level low over Southeast Canada and the gradient over the Northeast.

For forecasters, the issue now becomes precipitation type, and the answer to that magic question won’t come until we can pin down the area where this thermal gradient will set up. Newer forecast models seem to be converging toward a solution which will keep most of New Jersey and New York City from seeing significant winter weather. Our concerns, moving forward, stem from stubborn surface cold as a result of a stout and impressive high pressure to our north over Southeast Canada.

As a storm system develops over the Ohio Valley, even if the thermal gradient is too far north for snow, cold air entrenched in the low levels of the atmosphere could cause precipitation to fall as either sleet or freezing rain. We saw this scenario earlier this week where mid level temperatures killed our chances for snow, but cold air in the low levels led to widespread freezing rain in many areas. While surface temperatures look a bit warmer this time, we’re carefully monitoring the potential for wintry precipitation other than snow, especially in the area suburbs.

GFSFZRA

GFS model showing precipitation falling as snow over the far interior, and freezing rain in the suburbs on Monday, with rain near the coast.

The general theme over the next day or two, for forecasters, will be trying to pin down the thermal gradient and the sensible weather effects its positioning will have. Newer forecast model data will provide us with increased confidence, and the more intricate details of the forecast should be ironed out over the weekend.

As always, stay tuned here and on our social media outlets for more up to the minute information on the storm as we get it.

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