John Homenuk

PM Update: One storm out, another one in

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It is often said in the meteorological world that “dry begets dry”. In this case, snow begets snow. Amidst a hemispheric pattern which has historically been unfavorable for snow, multiple snowstorms have driven through the Northeast United States over the past two weeks. Most recently, for obvious reasons, is the snowstorm which occurred just this past Saturday Night into Sunday and left a significant snowfall accumulation in many areas, especially Northern New Jersey, New York City and Southeast New York into Connecticut. But even prior to that, a rare significant December snowfall over Philadelphia and Central New Jersey got winter off to a surprisingly fast start.

Saturday and Sunday’s event was certainly the most significant so far on this young winter’s season, with a significant snowfall and then icing across the interior. It was also the most interesting from a mesoscale perspective, with a dramatic coastal front and mid level warm air advection changing areas over to sleet and freezing rain despite surface temperatures in the 20’s. Regardless, after several inches of snow most areas changed over to rain. The cold air held on longer inland, and was much quicker to scour out near the coast. You can access our official verification map, with plotted totals over our forecast, right here. Attention now turns to a clipper type storm system, which will swing through the area on Tuesday — and forecast models are beginning to hone in on the potential for another light snowfall.

High resolution NAM model showing a period of moderate snow throughout the area on Tuesday morning.

High resolution NAM model showing a period of moderate snow throughout the area on Tuesday morning.

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John Homenuk

Live: Significant winter storm set to impact the area

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Link Hub: Latest Watches and Warnings |  Snowfall Map | Snowfall Probabilities

The National Weather Service issued Winter Storm Warnings and Winter Weather Advisories which were valid from Saturday through Sunday, in advance of a major winter storm which is forecast to impact the area. Beginning later Saturday, snow is expected to spread throughout the region from southwest to northeast. The snow will likely be light initially, as it battles with drier air. But as a disturbance moves northward and eventually develops a stronger surface low near the Mid-Atlantic coast, heavy precipitation will move towards the area on Saturday afternoon and evening.

Temperatures, both aloft and at the surface, remain a major challenge for forecasters and will have a significant impact on how the event shakes down. Warmer temperatures aloft are expected to begin to move into Southern and Central New Jersey by later Saturday afternoon, and progress northward through the evening. At the coast, this will spell the end for frozen precipitation, as the warm layer’s depth expands and surface winds turn southeast off the warmer ocean waters. But inland, the battle will just be beginning, as the warmer air moves in aloft but is forced over the top of a shallow cold layer which remains stubborn near the surface. The combination could lead to snow changing to sleet and freezing rain across parts of the area suburbs, resulting in not only a moderate snowstorm, but significant icing thereafter.

Latest storm total snowfall forecast, including what has already fallen. Updated 11am 12/14/13.

Latest storm total snowfall forecast, including what has already fallen. Updated 11am 12/14/13.

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John Homenuk

Weekend winter storm to bring myriad of threats

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The National Weather Service issued Winter Storm Watches on Friday, in advance of a winter storm which is expected to impact the area this weekend. Forecast models have come into better agreement on the evolution of the storm system, which begins in the Central United States on Friday. The storm will track northeastward, as an initial disturbance moves towards the Mississippi Valley. The primary surface low will track towards the Ohio Valley by Saturday morning, before a compressed height field to the north and high pressure over New England force a secondary surface low to redevelop off of the Mid Atlantic coast.

Precipitation is expected to move into the area by late morning on Saturday, with snow beginning to pick up in intensity by early Saturday afternoon. Moderate to heavy snow is expected to continue through Saturday Night. But as warm air advection (the push of warm air towards the area in the mid and low levels via southerly winds) begins to pick up, snow will change over to sleet, freezing rain and quickly rain in areas like Southern New Jersey and Southeast Long Island. Southeast winds off the warmer area waters will not help to keep cold air locked in place along the coastal plain. But inland, deeper cold at the surface will hold on for a longer period of time – meaning a prolonged period of snow. With time, in the area suburbs, the warm air advection will push over the top of the cold air at the surface, changing precipitation to sleet and eventually freezing rain.

4km NAM simulated radar, showing a period of heavy snow throughout the area on Saturday before a transition to freezing rain and rain.

4km NAM simulated radar, showing a period of heavy snow throughout the area on Saturday before a transition to freezing rain and rain.

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John Homenuk

Weekend winter storm likely to bring various impacts

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A developing mid and upper level disturbance over the Central United States will slide eastward towards the Ohio Valley, and then redevelop off the Mid-Atlantic coast this weekend, providing a myriad of winter weather threats in our area When combined with the deep cold arctic air entrenched in the area upon its arrival, the storm seems likely to produce a fairly significant amount of snow and ice, especially away from the coast. But warmer air moving in aloft, and eventually at the surface, presents a forecast headache — as precipitation type will be difficult to predict until the event draws nearer.

Forecast model at this range are beginning to come into better agreement on the eventual track of the surface low associated with the mid and upper level disturbance. The initial (or primary) surface low seems likely to be driven from the Mississippi Valley into the Northern Mid Atlantic or Ohio Valley. But cold air, reinforced to the north, and associated with a strong high pressure system, and compressing heights aloft will force the storm to redevelop to the south and east off the Mid Atlantic coast before eventually scooting northeastward out to sea.

Canadian GGEM model showing the low pressure system off the Mid Atlantic coast with precipitation in our area, and a sharp gradient from snow to sleet and rain.

Canadian GGEM model showing the low pressure system off the Mid Atlantic coast with precipitation in our area, and a sharp gradient from snow to sleet and rain.

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