During the afternoon and evening of 4 October 1998, 26 tornadoes formed over central and eastern Oklahoma, causing widespread damage to the region. As deep convection evolved during the late evening, portions of the region experienced excessive rainfall with precipitation amounts up to 175 mm (7 inches) causing widespread flooding and related damage. The environment for this event was characterized by a very moist unstable air mass that included mixed layer CAPE values up to 3000 J/kg and precipitable water amounts of 41 mm (1.6 inches). The surface to 3 km storm relative helicity was between 300 to 500 m2/s2 indicating the combination of instability and wind shear were favorable for supercells. However, the presence of a 20 to 23 m/s (40-46 kt) low level jet also resulted in a wind shear profile conducive for upstream development of newer cells during the evening. Storms also redeveloped along surface outflow boundaries originating from earlier convection. These factors, combined with the high moisture content of the air mass, contributed to very heavy rainfall across the region.