The Northeast has baked in the brunt of the heat over the last several days. Unfortunately, Wednesday will be more of the same. Temperatures will reach the low 90s as far north as Burlington, VT and Montréal, QC. The humidity will make it feel even hotter. Dew points in the low 70s will push heat indices over 100 degrees as far north as Albany. For the North Country of New York and Northern New England, Wednesday will be the last day of the extreme heat associated with this heatwave. But with antecedent moisture yielding modest instability ahead of a cold front and vigorous jet stream, the heat will exit with a bang. Powerful tree-toppling wind gusts will be most prevalent threat but isolated tornadoes will also be possible.
This particular cold front has a violent history. Incredibly, it is the same frontal system that ravished the Midwest with three days of severe weather. Storms that formed ahead of it produced widespread damage, with hundreds of reports consisting of toppled trees, downed power lines, collapsed roofs, golf-ball sized hail and tornadoes. Fortunately, conditions are not favorable for this caliber of severe weather.
The cold front and jet streak provoking Wednesday’s severe weather are associated with a broad upper-level trough of low pressure. The trough was centered over Lake Superior early Wednesday morning as it swiftly slid east-southeast. However, a subtropical ridge of high pressure over the Southeast and Mid-Atlantic will impede the trough from penetrating too far into the Northeast. This ridge has been the culprit of the recent heat and humidity, as its circulation has pumped Gulf of Mexico heat and humidity into the eastern half of the US.
The collision of the trough into the ridge will immediately end this heatwave in Northern New England and upstate New York. It will gradually suppress the heat in the rest of the Northeast through the weekend. The cost of this immediate relief will be strong thunderstorms in the vicinity of this collision.
Just ahead of the cold front this afternoon, a robust low-level jet streak will lift through the inland Northeast. It will intensify as a result of ample ascent and daytime heating. This low-level jet streak will intensify transport of warm air from the south, amplifying lift in a positive feedback loop. It will also supply abundant wind shear, allowing storms’ updrafts to remain separated from their downdrafts for an extended period of time. This will permit a few rotating updrafts so that a few supercells may develop early in a regime that otherwise favors linear organization.
Isolated storms will initially fire as daytime heating breaks the “cap” on convection by the middle of the afternoon in northern Ohio, western and northern New York, and northern New England. Storms will increase in coverage throughout the afternoon, with some of the discrete storms becoming supercellular. By Early evening, several linear systems will exist ahead of the cold front from the Ohio River Valley to Lake Erie and from the North Country of New York to Maine. The latter will be more of a broken line of storms as it will be farther from the front. It will reach Massachusets and the Hudson Valley just after sunset before dying out.
A second round of storms will approach the North County and Northern New England just after sunset. These storms may produce damaging wind gusts, but they are not likely to be as strong as those that preceded them. Behind this second round of storms, the cold front will finally usher in relief. This relief will not reach the Atlantic Coast until Thursday evening.
The corridor of wind shear will be confined along the south shores of Lake Erie from northern Ohio to Buffalo and from the North Country of New York east to northern and western Maine. Storms here have the best chance of becoming severe. Cleveland, Buffalo, Watertown, Plattsburg, Burlington, and Caribou are all at risk of seeing powerful wind gusts that may topple trees, damage sides of buildings, and cut power. With strong low-level shear, brief tornadoes cannot be ruled out. Torrential rain will also indubitably result from the storms given the humidity, but the low-level jet will carry storms too quickly to prompt flash flooding.
Marginal shear will exist south of the corridor of more robust wind shear, where storms will not arrive until after sunset. Thus, the severe threat will be lower here. Syracuse, Albany, Pittsfield, and Concord may face these weaker storms. A second round of storms is not expected here as it is too far north.