A dangerous situation is evolving over parts of the Mid-South and Ohio River Valley as conditions continue to ripen for severe thunderstorms throughout the day. Wind is the most prominent threat, but large hail and tornadoes are also possible, especially along the Mississippi River basin between northern Alabama and western Kentucky.
On Monday morning, an unseasonably strong surface cyclone continued to slowly drift northeastward across northern Missouri. Caught at the center of its parent upper-level low, winds aloft will be too weak Monday to clear the system from the Midwest before Tuesday, when it will be picked up by the upstream jet streak of the upper level trough. The upstream jet will weaken the surface low, but until then it will have quite the impact across much of the Central US.
Morning convection will linger in the vicinity of the cyclone’s warm front over much of the Mississippi River and Ohio River basins Monday morning, gradually clearing from west to east. These initial showers and thunderstorms were generated by an eastward-creeping warm frontal boundary. Behind the warm front, an exceptionally juicy atmosphere will encapsulate the region until an upper-level low and its associated cold front plow through Monday afternoon and evening, instigating explosive development of thunderstorms.
The thunderstorms will have an abundant tap of potential energy to draw from Monday afternoon. Moderate to extreme instability will develop where swampy dew points climb in the upper 70s to near 80. In a normal dynamic environment, the lack of cold air aloft would apply a speed limit on how fast upward ascending air can condense and form thunderstorms. Air only rises by convection if it is warmer than its surrounding environment. But an unseasonably vigorous mid-level jet stream will energize the environment, helping accelerate upward motion and allowing storms to thrive in the instability.
The mid-level jet stream will slide northeastward across Ark-La-Tex through the Ohio River Valley. Model consensus suggests the axis of best lift will straddle the Mississippi River and southwestern terminus of the Ohio River. Concurrently, the strongest wind shear will develop during the afternoon over this same corridor. Wind shear is the change in wind speed and/or direction with height. It is pivotal in separating storm updrafts from downdrafts.
Initially, discrete thunderstorms will develop ahead of the upper-low’s associated cold front Monday afternoon. These thunderstorms will first develop from southeastern Texas northeastward to southeastern Missouri. Deep shear will be robust enough for some of these storms to develop rotation and become supercellular, especially within 100 miles of the Mississippi River. This region extends from Vicksburg, MS to the terminus of the Ohio River between Illinois, Kentucky, and Missouri. These storms will be capable of producing powerful straight-line winds, hail with diameters in excess of one inch, and several tornadoes. Cities like Memphis, Jonesboro, and Southaven near the Mississippi River basin within the vicinity of the borders are most at risk for these violent storms.
East of the Mississippi River, the storms will arrive mainly during the second half of rush hour. By this time, the storms’ outflow will have resulted in the storms merging into linear structures, some of which may become bow echoes given the abundant shear. The hail and tornado threat will decrease throughout the evening, but the wind threat will increase. Bow echoes are linear structures of storms that are bent forward in the center, allowing the strong inflow of winds to push the storm ahead. These systems will be capable of producing widespread wind damage through dusk.
Storms will also intensify Monday afternoon along the surface cyclone’s warm front, which will be draped across the I-80 corridor from northeastern Indiana to Des Moines, IA. Instability will be lower than areas further south. With clouds and showers lingering much of the day, dynamics along the front will be the main source of intensification. Low level winds may be strong enough for storms to become intense enough to result in isolated damaging winds.
A dangerous situation will soon be underway in parts of the Mid-South. Residents are encouraged to pay attention to alerts from the National Weather Service and to seek shelter in the lowest room without windows in their residence during severe thunderstorm and tornado warnings. The upper-level low will swing northeastward Tuesday, bringing the severe threat to eastern Ohio as well as from western Pennsylvania southward to central Virginia.