Fall is finally here. Yet summer’s warmth and humidity still linger too close for comfort. Reinforcing cold shots from Canada are becoming more successful at keeping summer at bay, but not without producing with each passage a flash, rumble, and a splash.

An upper-level trough of low pressure will traverse the eastern two-thirds of the US between Tuesday and Wednesday night, dragging a strong cold front with it. This cold front will generate a line of severe thunderstorms that will push its way across the Midwest, Mid-Atlantic and New England. The line will produce a swath of wind damage across a span of over one-thousand miles. In some areas, supercells may develop ahead of the main line, adding a small threat of tornadoes and large hail to the mix.




The upper-level trough driving the severe weather will have the backing of cooling across northern Canada to support its intensity as it moves eastward. As transport of cold air at the surface increases over time, the trough will intensify and stand its ground against a sub-tropical ridge of high pressure over the southwestern Atlantic, impeding its forward motion. Sub-tropical southerly flow around the ridge will increase temperatures and humidity ahead of the trough and its associated cold front across the East. Temperatures will climb into the low 80s alongside dew points in the low 70s. This transport of warm, humid air will contribute to rising motion and the development of modest instability. The summer-like air mass will first spread into the Midwest Tuesday before being pushed eastward by the trough and cold front Wednesday.

A dynamic consequence of the two contrasting air masses will be a strong jet stream in the middle and upper levels of the atmosphere. A strong upper-level jet involves upward motion, thus creating wind shear just along and ahead of the cold front. The upward motion will create towering thunderstorms tops, aiding in the production of strong downdrafts and wind gusts. The wind shear will ensure the storms remain organized. Ahead of the cold front, a robust low to mid-level jet stream will transport rich moisture content and generate low-level wind shear. The low-level wind shear could generate rotation in any thunderstorms that develop ahead of the line. The timing of the cold front places Chicago and regions south and east (to roughly Cincinnati) under the most intense jet streak, where isolated tornadoes will be possible ahead of the main line of storms.




The Northeast will be under fire from the storms Wednesday. The same unstable air mass that will provide fuel for severe storms in the Midwest Tuesday will have shifted to the Mid-Atlantic and New England. Transport of warm, humid air, will be most robust over New England, upstate New York, and central Pennsylvania. Wind shear will be very strong here, organizing storms into a mostly continuous squall line. Passage of the cold front near the peak of day-time heating will decrease the chances that supercells develop ahead of the squall line compared to the Midwest. However, any that do form could produce weak tornadoes. Along I-95, wind shear will be too meager until the approach of the storms late in the evening to admit the possibility of supercell development.

Storms will be ongoing through Monday night and Tuesday morning in the Ohio Valley due to a warm frontal passage. How fast skies clear in the afternoon will dictate how widespread the severe threat will be in the Midwest Tuesday afternoon and evening. If the sun peaks through early, then the surface will have more time to destabilize, resulting in a stronger line of storms and more supercells developing ahead of it.




The storms in the Midwest will gradually organize along a broken line ahead of the cold front Tuesday afternoon. The outbreak will originate as a large cluster of storms over Iowa , northern Missouri, and southwestern Wisconsin early Tuesday afternoon. As the cluster moves southeast late in the afternoon, it will encounter more favorable dynamics for severe weather. In northern Illinois and Indiana, clusters of supercells may develop well in advance of the cold frontal passage. These violent storms could produce a tornado or large hail in close proximity of big cities like Chicago, Indianapolis, and Cincinnati. By early evening, additional storms will have developed ahead of the cold front. These storms will center along a broken line to the southwest. Tulsa, St. Louis, Louisville, and Detroit could face the wrath of tree-toppling gusts ahead of bowing segments within this line of severe storms.

Thunderstorm activity will continue well into the overnight hours in Missouri, southern Illinois, Indiana, Kentucky, lower Michigan and Ohio. As the night continues, the severe weather threat will gradually subside. However, the line of storms will continue into the morning hours, escaping ahead of the cold front into Pennsylvania and New York, where it will dissipate late Wednesday morning.

Skies may briefly clear Wednesday ahead of the cold front in western New York and Pennsylvania, where the cold front will approach early in the afternoon. Wind shear will quickly organize the storms into a continuous squall line, but it will be too early for the surface to destabilize. Therefore, cities like Erie, Buffalo and Rochester have a lower risk of severe weather than areas further east.




By mid-afternoon Wednesday, isolated thunderstorms will have popped up along and west of the Appalachian mountains. The front will slow in approach of the mountains, giving the surface more time to destabilize. The squall line will stretch from Montréal, QB southwestward to roughly Knoxville, TN by early evening. Syracuse, Binghamton, Pittsburgh, and Charleston could be targeted by tree-toppling wind gusts from the squall line. A few storms may develop ahead of the squall line due to outflow. Thee storms could develop rotation and become supercellular, especially in northern New Jersey  southwestward through the DC metropolitan area. This area is where instability will be greatest. Some of these storms could migrate eastward toward I-95 and reach cities like New York, Trenton, and Philadelphia.

Besides strong wind gusts produced by a few pop-up afternoon and evening thunderstorms, the I-95 corridor will be too far east to experience the worst of the squall line. It will weaken upon arrival late in the evening due to the loss of daytime heating, but the strong jet streak could still permit a few strong wind gusts. Southerly winds ahead of the cold front will bring stable marine air to Long Island, Rhode Island, southern Massachusetts, and southern Maine. This oceanic air mass at the surface will gnaw apart at any strong storms that do manage to survive the trek to these coastal areas.

This is an evolving severe weather event. WeatherOpics will continue to provide updates as the threat draws closer.




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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