At the surface, a strong area of low pressure was centered over the mid-Mississippi River Valley on the morning of the 18th. A stationary front extended from the low into the Ohio Valley. A strong cold front trailed from the low in Missouri southward into Eastern Texas and into the Rio Grande Valley. Ahead of the cold front, winds were south to southwest, advecting warm, moist Gulf of Mexico moisture into the southeastern states. Throughout the day, the strong area of low pressure advanced along the Ohio River to near the tri-state region of Illinois, Indiana, and Kentucky by early evening. The cold front continued to advance east and was positioned from near Paducah, Kentucky to near Memphis, Tennessee to Monroe, Louisiana to near Lake Charles, LA by early evening. Strong southwest flow continued ahead of the front, pumping moisture into the Tennessee Valley region.
Aloft, a 500mb trough axis extended along the Mississippi River Valley, with jet stream maximums noted over the northeast United States and from Arizona eastward into Texas. A strong 500mb vorticity maximum passed north of the Tennessee Valley during the day. Also, strong instability was noted across the region. Evening analyses indicated Lifted indices of as low as -8, Total Totals indices around 53, Sweat indices in the 300s, and helicities between 300 and 500. The evening sounding at Birmingham (see image at right) indicated strong low-level shear, dry air in the mid levels of the atmosphere, and impressive instability.