The same system responsible for six tornadoes in southeastern Minnesota and a 75 mph hurricane-force wind gust in northern Iowa last night is racing eastward. The upper-level trough behind the violent storm is an assault by autumn against the subtropical air-masses of summer. Behind the storms, the coolest air since May will gradually overspread the Northeast just in time for the first day of Autumn. But the crisp, cozy weather will come at a steep cost. Powerful wind gusts are expected to rip through the Lower Great Lakes region, with a few tornadoes to boot.
The last of summer’s subtropical high pressure systems is clinging onto life. A weakening upper-level ridge centered centered over the lower Mid-Atlantic coast will have no defenses against the incoming upper-level trough. The surface low-pressure system associated with the trough will travel well to the north of the border through central Ontario and Québec Friday. Though the low may seem distant, its cold front and parent trough will be in ideal proximity to the upper-great lakes to spark trouble.
There will be several triggers to Friday’s thunderstorms. The jet streak associated with the parent trough will be anomalously vigorous at all levels of the troposphere. This will serve to draw air upward near its southeastern flank, enhancing upward motion from the cold front. Unseasonably warm temperatures in the lower and middle 80s in the inland northeast underlies the region of mos intense transport of warm air, which also encourages upward motion. The warm air will be accompanied by dew points in the lower and middle 70s, humidity levels high for any time of year let alone the third week of September. The final and most obvious trigger, the cold front will also serve as a trigger for upward motion.
Storms will organize into a squall-line just ahead of the cold front with haste Friday afternoon. This will occur soon as peak daytime heating destabilizes the surface, around mid-afternoon in the Ohio Valley. The squall line will stretch from Québec all the way southwestward through Arkansas. A few strong storms could form as far south as the confluence of the Ohio and Mississippi Rivers in southern Illinois, Indiana and western Kentucky. However, storms will generally be more intense farther to the north and east, in greater proximity to the trough and the cold air.
The storms will be most intense early this evening in northeastern Ohio, western Pennsylvania and western New York. This is where the strongest instability will overlap the most robust low-level wind shear. Wind sheer is the change in wind speed and direction with increasing altitude. Friday this change with altitude will be particularly rapid in the lower atmosphere. The instability will support strong thunderstorm updrafts. The wind shear will organize the storms and keep the updrafts separated from rain and dry air in the downdrafts.
The wind shear will also be sufficient to generate rotation, especially in storms that develop ahead of the main line due to local destabilization of the surface. These discrete supercells are most likely to develop late in the afternoon or early in the evening between northeastern Ohio and the western fringes of the Finger Lakes. Powerful winds are the most likely threat with these supercells, but some of them could produce a tornado.
The storms will have just crossed the shores of both Lake Erie and Lake Ontario by sunset. Akron, Erie, and Buffalo, and Rochester will be confronting storms with wind gusts powerful enough to topple trees, powerlines, and rip the sidings from walls and roofs. Weak tornadoes are possible within these squall lines, especially in Central New York and the North Country of New York west of I-81, where the low level wind shear will be most intense.
The squall line will likely not approach until dusk along the I-81 corridor, but robust jet streak will support the storms despite the loss of daytime heating. East of this region, a different fate awaits. A sharp instability gradient will develop near the I81 corridor. East of hear, shear will remain robust but instability, the fuel for storms, will vanish. The storms will weaken and light rain will build behind the storms as they cross the eastern halves of New York and Pennsylvania. The jet streak will slow the dissipation such that a few fizzling storms could reach the coast late in the overnight hours.
Cities most at risk for damaging wind gusts and tornadoes include Akron, Cleveland, Erie, Buffalo, Rochester and Syracuse. Storms are most likely to leave a trail of widespread wind damage here. Storms will quickly fall apart east of the maxium threat zone, but Pittsburgh, Elmira, and Utica could still experience scattered reports of damaging gusts. As the storms weaken, there is a marginal chance that downdrafts could pull strong winds from the low level jet streak to produce damaging gusts as far east along the cold front as Burlington, Scranton, and Louisville.
Behind the storms, temperatures will embark on a cooling trend. Temperatures will not leave the 50s in Upstate New York, all of Northern New England, northern Ohio, and southeastern Michigan. Temperatures will plummet into the 30s across this area Saturday night, possibly giving some elevated locations their first frost of the season just a the fall equinox is underway.