The second in a series of two weekend low pressure systems is set to depart from the Mid-Atlantic coast bound for Newfoundland Sunday night. The system is predominantly a rain maker for coastal areas, isolated from any source of cold air. However, elevated portions of New York and New England will soon find themselves basking in a stew of assorted wintry precipitation as the storm intensifies during its voyage. 

Sunday afternoon, the low brought scattered precipitation across most of the Northeast in its approach toward the coast. Precipitation was generally light rain, with some pockets of steadier showers embedded within its swath of precipitation. Some cold air was starting to intrude, as there were some swaths of heavy mixed precipitation in New York’s Adirondacks and in northern New England. Rapid intensification after reaching the coast Sunday night will produce cold air in the mid-levels of the atmosphere by means of diffluence, evaporation, and melting snow

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The cold will intensify behind the low’s center in Pennsylvania, Upstate New York, and Northern New England. Strong low level frontogenesis, the tightening of temperature gradients, will produce heavy precipitation rates over southeastern New York and New England. Meanwhile,
diffluence (wind spreading apart) between the upper-level shortwave supporting the surface low and an amplifying trough from Canada will support rising motion north and west of the main area of precipitation. The evaporation in New York and Pennsylvania in tandem with melting of snow on the warm side of the temperature gradient will cool the atmosphere enough to contribute to the intensification of the cold pool. As the cold air spreads southeastward behind the low, precipitation will gradually change from rain to a wintry mix of freezing rain, sleet, and snow in elevated areas away from the coast. 

Forecasting mixed precipitation types is always a tricky procedure. Slight changes to the temperature and moisture profile at the lower levels of the atmosphere could mean a complete difference in precipitation type. The inland Northeast’s mountainous topography complicates the determination further, as the tops of the mountains will likely experience different precipitation types than their bases. Additionally, they act as a barrier against air to rise and descend.

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That the cold air production will originate above the surface due to melting precipitation, evaporation, and adiabatic lift (rising motion with no release or intake of heat) means terrain will greatly influence precipitation types. Above the cold pool, temperatures will generally be warmer, resulting in a temperature inversion. Momentum and gravity will encourage the cold pool to descend to the surface with the inversion trailing above it.

Mountain peaks will be closer to the inversion than the valleys, meaning liquid droplets have less time to freeze, remaining supercooled (below freezing) as they reach the surface of the slopes. These supercooled rain drops, colloquially known as freezing rain, will accumulate on trees, power lines, and untreated surfaces in the Green, Berkshire, Adirondack, and Catskill Mountains. Cities in these mountain ranges like Pittsfield, Manchester, and Hunter could receive up to a tenth of an inch of ice early Sunday evening before transitioning to sleet and snow. This is not enough ice to bring power-related issues, but it could be plenty to make for unsuspectingly slick roads.

Areas lower in elevation will be farther from the inversion and thus will encounter more sleet than freezing rain given their colder profile. Rain will changeover to sleet from Northeastern Pennsylvania to southern New Hampshire from west to east between late Sunday afternoon and late Sunday evening. Cities that will undergo this transition from rain to sleet include Scranton, Poughkeepsie, Albany, Hartford, Worcester, and Concord.

Those nearest to the cold pool will also undergo a transition to snow. Where there is enough moisture overlapping upward motion above the temperature inversion, a brief period of moderate to heavy snow is likely. The temperature inversion will only just barely reach into above-freezing territory so it does not need to cool much for a transition to snow at the surface. If there is enough snow production in the so-called “dendritic growth zone” (the temperature and moisture range where snowflakes tend to quickly grow), then melting snowflakes can cool the atmosphere even further, allowing it to snow all the way to the surface. 

Elevated terrain will be the first to undergo the transition to snow as these locations will be closer in altitude to the snow growth zone and because they may eventually find themselves above the temperature inversion. When the inversion is eroded at the surface due to melting snow, or when the cold pool is low enough to support a lower-level dendritic snow growth zone, precipitation will transition to snow. 

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The snow will only last a few hours between Sunday evening and early Monday morning before precipitation ends. Eastern New Hampshire and Southern Maine are likely to receive 3-6″ of snow, with locally higher amounts. The precipitation will be heaviest here as the storm will have became more intense by the time it’s precipitation impacts this area.  Mountainous areas of Pennsylvania, New York, and New England could receive 2-4″ of snow, with the highest peaks accumulating 6″ or more. Valleys and foothills can expect a coating of 2″ of snow, with amounts decreasing with proximity to the coast. Wet snow may mix with rain during the last few hours of precipitation in southeastern New England, but the prospect of accumulating snow in cities like Providence or Boston Sunday night is low. The I-95 cities are unlikely to experience any other form of precipitation besides rain before precipitation ends behind the storm Sunday evening.

An amplifying trough emerging southeastward from Canada will usher in much colder air Monday afternoon through Wednesday. Enough moisture will be present to produce lake effect snow from Lakes Erie and Ontario, with the heaviest snow bands expected Monday afternoon in northwestern and central New York. But besides lake effect snow, the Northeast will be dry until the next system produces another widespread precipitation event Friday and Saturday. This storm looks to be predominantly rain, but there could be backend snow as with the current system. Be sure to stay tuned throughout the week as we know more. 


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Author

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.

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