Katrina developed in a region of persistent convection downshear of a low latitude trough (alternatively a potential voriticty tail) near Hispaniola. A pair of easterly waves are shown to have transited the region in the days prior to the storm's initiation, each leading to an enhancement of the synoptic-scale ascent forcing and increases in the extent and intensity of deep convection in the area. The incipient vortex emerges from the convective cluster on 23 August 2005, and begins to intensify as it enters an area of elevated potential intensity. After turning westward, Katrina strikes south Florida as a Category 1 storm on 26 August. Katrina shows little sign of weakening as it crosses the Florida Peninsual, a behavior that is shown to be consistent with the historical record based on an analog study that comprises part of this investigation.
Shortly after entering the Gulf of Mexico, Katrina begins the first of its two rapid intensification phases. The 18 h period between these stages (27 August) witnesses a doubling in the radius of maximum wind around the system, and the power dissipation of the storm is shown to increase nearly exponentially until Katrina reaches a superintense state late on 28 August. Increasing shear ahead of a broad trough over western North America induces a rapid extratropical transition (30 August) in which the lower and midlevel remnant circulation disconnects from the outflow layer. The decaying system tracks northeastward and produces heavy rains over the New England states and eastern Canada; however, the outflow layer stays nearly stationary over the Eastern Seaboard and helps to generate an anomalous southwesterly flow that is shown in a companion talk to perturb significantly the nature of the midlatitude circulation well into September 2005.