A wave of low pressure associated with a rather disorganized weather pattern over the United States will fuel the risk for severe thunderstorms across portions of the Ohio River Valley and the Southeast Thursday into Friday. While the atmospheric setup for severe storms will not be perfect, it will be conducive enough for strong to severe thunderstorms to develop.
There are several ingredients needed for the formation of severe weather. That includes wind shear, lift, instability, and moisture. Wind shear levels will be moderate as somewhat strong upper-level winds flow in from the south and west. In terms of lift, there will be a surface low which will be the main driver for this event, but the lack of a cold front will keep the strong storms from becoming widespread. There will also be moderate amounts of instability with CAPE (Convective Available Potential Energy Values) approaching or exceeding 3000 J/kg in some locations. Moisture levels will be high with dew points in the 60s and 70s. Storms need moisture to form and grow, and that crucial ingredient will be present.
Now while the most risk for severe thunderstorms will be in the western Ohio River Valley, the threat will also exist in the Tennessee River Valley as well as far east as the Appalachian Mountains. These storms will pose the primary risk for large hail in excess of one inch in diameter, but damaging winds of at least 58 mph and an isolated tornado may also be in the cards.
This morning, the radar shows a rather disorganized picture with numerous clusters of showers and thunderstorms present the Ohio and Tennessee River Valleys. Now in the afternoon, we’ll see these storms evolve into different lines. One of those lines will work across the Appalachians and into the Mid-Atlantic, posing the risk for flash flooding. There may also be an area of heavy thunderstorms that affect the southern Appalachians or the eastern Tennessee River Valley. In terms of the strongest of the storms, however, that will be associated with a developing line of storms moving south and east across the western Ohio River Valley. These storms will peak in intensity during the evening hours following its development in the mid-afternoon. Given the tropical moisture in place, thanks in part to Alberto, there will be the risk of flash flooding. The soil in saturated and the ground may not be able to handle the additional rainfall from these storms. Some locations can expect over two to three inches of rainfall through tonight. By the early-morning hours of Friday, the storms will then fall apart and break up, leading to just a few scattered showers and thunderstorms across the region Friday morning.
Speaking of Friday, isolated strong to severe thunderstorms may be possible from the southern Mid-Atlantic region through the southern Appalachian Mountains and into the Tennessee River Valley. Downburst winds and small hail will be the main risks of these scattered storms that are expected to develop in the afternoon. Meanwhile back toward the Northern Plains, an outbreak of severe thunderstorms may be possible.