A nasty afternoon is in store for the Northeast Tuesday as thunderstorms bombard much of the region with blinding, torrential downpours and strong, damaging wind gusts as they march toward the Atlantic coast. These storms will end a second consecutive day of steamy temperatures along the Atlantic seaboard, where temperatures in the low 90s and dew points in the mid 70s will make it feel just shy of 100 degrees in urban areas before the storms’ arrival. Behind, however, is a trailing cold front, and much cooler, drier air will make for refreshing conditions through the remainder of the week.
As of early Tuesday morning, heavy complexes of showers and thunderstorms were already moving across upstate New York and western Pennsylvania ahead of a strong cold front and its parent upper-level trough. These showers were initiated by small kinks in the jet stream which acted to stretch air upward ahead of the trough. Extensive cloud cover and timing before peak daytime heating will spare areas of New York west of the Hudson Valley and areas of Pennsylvania west of the Susquehanna River from the severe weather that will threaten millions further south and east.
After an initial round of rain Tuesday morning, thunderstorms will develop ahead of the cold front and gradually intensify and increase in coverage throughout the day. The storms should clear areas north and west of I-88 in New York and west of I-81 in Pennsylvania by the middle of the afternoon as they barrel toward the major coastal cities. The storms will increase in coverage and intensity as they approach I-95.
While widespread downpours soak Pennsylvania and upstate New York, an immense degree of instability will build along the Atlantic Seaboard. The steamy environment of temperatures in the low 90s and dew points in the mid 70s will provide ample fuel for quick thunderstorm development. Robust mid-level winds and outflow from the thunderstorms should provide enough upward force to initiate ascent. The instability will allow air to accelerate as it moves upwards, quickly resulting in strong thunderstorms. Marginal wind shear will be a saving grace, limiting the severity of the storms. However, the shear will generally be insufficient to generate a single continuous line of storms. Instead, widespread clusters of heavy thunderstorms will organize ahead of the cold front, possibly forming into linear segments, and bring widespread, torrential downpours.
Evening rush hour will be a mess for the major cities along I-95 from Washington, DC to Portland, ME. The storms will reach maximum coverage during this time. Incessant, blinding downpours will make driving treacherous and could result in flash flooding under bridges, alongside small creeks and streams, and in other areas with poor drainage. Besides snarled traffic, downed trees and power lines will be likely, as some of the storms will be capable of producing vigorous wind gusts. The storms will maintain their potency all the way to the coast. Only eastern Long Island and Cape Cod are likely to evade the storms’ severity.
The trailing edge of the storms will reach the Mid-Atlantic coast shortly after sunset, but may not reach the New England coast until after midnight. Behind the cold frontal passage in the evening inland and overnight near the coast, the humidity will quickly tumble. Residents across the Northeast will wake up to crisp Canadian air Wednesday morning. Temperatures will only reach the mid 70s Wednesday afternoon in western New York, Pennsylvania, and northern New England, and the upper 70s to low 80s in the Hudson and Mohawk valleys and in southern New England. Air mass modification will only cool the high temperatures to the low to mid 80s along the Atlantic Seaboard. But with the low humidity, it will still feel comfortable compared to the steamy air mass in place since Sunday. This pleasant weather will last throughout the remainder of the week until the weekend, when another upper-level trough may spark another round of thunderstorms.