We present the findings from an investigation of a long-lived convective system that persisted for two days over the central U.S. on 14-15 July 1998. At its peak intensity the storm developed into a bow echo producing damaging winds, local flooding, large hail and a couple of F0 tornadoes. While the severe weather was noteworthy, the most interesting aspect of the event was an abrupt 90 degree change in the storm's orientation and propagation vector that occurred about midway through its life.

The convection initiated over the higher terrain of southwestern Montana and became loosely organized into a N-S line perpendicular to the W-E oriented low level shear vector. A cold pool formed and raced ahead of the storm resulting in a succession of discrete propagation events across Montana and North Dakota. Upon entering northern Minnesota the convection intensified and assumed an E-W orientation in response to increased moisture, a strong low-level southerly flow and a N-S oriented shear vector. Following its reorientation, the line moved slowly southward initially, but in the highly unstable and favorably sheared environment, a strong rear-inflow jet developed and the system bowed southward into southern Minnesota and Iowa. The storm then decayed rapidly as it moved into dryer, more stable air due to subsidence aloft. There were no indications of any strong fronts that could have accounted for the abrupt changes or the longevity of the system. It is concluded that favorable cold pool - low level wind shear interactions and change in the lower tropospheric shear vector orientation explain the life cycle transformation.