It will feel more like March than May this weekend as the Polar Vortex dislodges from the Arctic and passes directly over the Northeast. Record cold and even a rare May snowfall will make for a stark contrast to the recent mild winter.
Low pressure from the Ohio Valley will inaugurate the arrival of the Polar Vortex with rain changing to snow across much of the interior. The rain will work its way northeastward throughout the afternoon and evening as the low darts toward the Gulf of Maine. A powerful cold front will sweep across the Northeast behind the low’s passage. It will change rain to snow across most of northern and central Pennsylvania, southern and eastern New York State, and northern New Jersey in the evening and across New England overnight.
High elevations in Pennsylvania and southern New York, especially those over 1500 feet in the Poconos and the Catskills, will likely pick up a few inches of snow before dry air behind the cold front ends precipitation. The highest elevations could pick up 2-4″. Lower elevations, however, will confront marginal surface temperatures so little snow accumulation is expected beyond a slushy coating. Still, a white coating in May will still be surprising for cities like Scranton, Binghamton, and Albany.
In New England, the bulk of the precipitation will arrive after dusk. The cooler surface temperatures at the onset of precipitation will facilitate snow accumulation, even in lower elevations of northern Connecticut, western and central Massachusetts, southeast New Hampshire, and eastern Maine, where 1-2″ is possible before the snow clears Saturday morning.
Coastal areas, too, have strong chances of witnessing late season flakes fly. New York City, Long Island, and coastal Connecticut could all see wet snow flakes mix in with the rain late Friday night before the precipitation clears. A full transition to snow could occur over coastal areas further north, including Boston and Portland, where a coating to possibly an inch could accumulate on grassy surfaces.
While accumulating May snow in coastal New England is incredibly rare, it is not without precedent. A powerful Mother’s Day Nor’Easter on May 9-10 1977 dumped up to 20″ of snow over portions of southern New England. Boston Logan Intl’ Airport picked up a half inch of snow on the morning of May 10, marking the latest calendar day accumulating snow had ever fallen.
While snow records are unlikely to be broken, temperature records that have stood for decades could be smashed. The Polar Vortex will pass over central Pennsylvania and Upstate New York before accelerating to the Canadian Maritimes during the evening. Unseasonable cold will grip the entire Northeast Saturday but it’s here, beneath its core, where the cold will be most extreme for May. As depicted in the table below, forecast highs across most of the region will be in the 40s but under the core of the Polar Vortex they may even fail to climb out of the 30s, including in cities like Scranton and Syracuse. These represent temperature departures of 20-25°F below normal.
Friday and Saturday night, temperatures will fall into the 30s region-wide, with many interior areas dropping into the 20s. The hard freeze will indubitably damage crops that have already been planted across Pennsylvania, New York, and northern New England. Fortunately, a strong northwest wind will preclude frost from forming on plants. But the strong winds will also help generate another early winter-time threat: Lake Effect Snow.
Lake Effect Snow is most common in late fall and early winter when arctic air blows over the relatively warm lake surfaces. Lakes Ontario and Erie have warmed into the 40s and lower 50s. Against an airmass of temperatures in the 30s in a convective environment, the temperature difference will be sufficient to generate snow showers.
The warmer Lake Erie will be the predominant source of snow showers, with southern New York and northern Pennsylvania undergoing several bouts of snow showers. The Polar Vortex above will help add additional upward momentum such there may even be a few rumbles of thunder associated with the snow squalls. It will even help generate snow showers away from the lakes. However, cities along the Lake Erie coastline like Cleveland, Erie, and Buffalo, will feature the most persistent snow showers. The heaviest squalls will dramatically reduce visibility and briefly whiten the ground. The strong May sun and above-freezing temperatures will quickly melt whatever snow manages to stick here, however.
The Polar Vortex will be quick to exit Saturday night. Temperatures will be at least 10°F warmer region wide, but even with highs in the 50s and low 60s temperatures will still be well below normal. Average highs for May 10 range from 72°F in Philadelphia to 64°F in Boston. Below normal temperatures are expected to persist through at least the middle of the month.