Thunderstorms will develop over the interior Northeast late Wednesday afternoon into the evening ahead of a robust shortwave trough and its attendant cold front. Storms will be most widespread in parts of New York and Pennsylvania, where large hail and possibly a weak tornado or two will be possible along with the main threat of damaging straight-line and downward rushing winds.
A warm front has delivered a hot, sticky airmass to the Northeast, where a ripe environment for thunderstorms provided ample sunshine. A weak shortwave trough associated with an overnight thunderstorm complex in the Midwest will ride across the frontal boundary through early Wednesday afternoon, bringing light to moderate showers with it as it treks northeastward into New England. Cloud debris associated with this system will limit the extent to how strong thunderstorms become Wednesday afternoon.
Showers will come to an end by early afternoon in the thunderstorm risk zone and areas to the south. Partial clearing will ensue throughout the afternoon. The most clearing will occur over the western threat zone, so the most widespread thunderstorms will generally develop west of I-81 in Pennsylvania and west of I-87 in Upstate New York. With afternoon sun, scattered thunderstorms will develop before the arrival of the cold front. These initial rounds of storms are unlikely to be severe. The primary threat will occur later with the approach of the cold front and shortwave.
Large quantities of instability should develop south of the Adirondacks with more modest amounts to the north and west in the North Country of New York. However, this instability is spread throughout almost the entire troposphere. Air accelerates upwards faster when the same amount of instability is confined to a thinner layer. Hence, the strongest storms will develop closer to the jet streak of the shortwave trough over northern Pennsylvania and parts of Upstate New York. The jet streak will provide ample lift and wind shear for supercell thunderstorms.
The main line of thunderstorms will first develop ahead of the cold front over communities in southern and eastern Ohio and along the shores of Lake Erie in Pennsylvania and New York late in the afternoon. The line will initially be comprised of discrete ordinary cells and supercell thunderstorms, so some areas may not impacted. Additional discrete thunderstorms may develop just ahead of this main line of storms, especially over the west, central and North Country regions of New York with fewer isolated storms over central Pennsylvania and New York’s Mohawk Valley and Capital District.
The storms will intensify until roughly 9pm, after which they will begin to weaken with the loss of heat from the surface and with the approach of the Adirondack, Green, and Taconic mountains. Some strong storms capable of producing strong wind gusts may still threaten cities like Albany and Burlington, especially if the sun were to shine for a few hours in the afternoon. The weaker line of ordinary thunderstorms will reach Albany by 10pm and Burlington and the Hudson Valley by 11pm.
Straight-line winds will be the main threat with these storms, though some microbursts, large hail, and weak tornadoes are also possible with some cells. Microbursts are intense, downward rushes of wind caused by heavy rain and evaporation. The hail and tornado threat will mainly be confined to north-central Pennsylvania northeastward to the North Country of New York, west of the Adirondacks, with tornado chances increasing further north in this risk zone. Cities like Elmira, Syracuse, and Utica are at risk to all three severe threats. Cool southwesterly flow from the lakes may spare cities like Buffalo and Watertown from the worst storms.
The Mohawk Valley and western Capital Region need to be monitored closely into the evening as uncertainty exists about the strength of storms. If the sky clears early enough in the afternoon, the tornado and threat may extend further southeastward. Otherwise, ordinary storms will dominate.
Behind the storms Thursday, gusty winds may disrupt outdoor activities in the Mohawk Valley, Eastern New York, and the lee side of the Berkshires and Green Mountains in New England. Thursday will be cool for these areas, but a big warm up is in store for the weekend with the arrival of a more summer-like pattern.