Hurricane Katrina (2005), Part II: Evolution and hemispheric impacts

The landfall of Hurricane Katrina (2005) near New Orleans, Louisiana on 29 August 2005 will be remembered as one of the worst natural disasters in the history of the United States. By comparison, the extratropical transition (ET) of the system as it accelerates poleward over the following days is innocuous and the system continually weakens until its eventual demise off the coast of Greenland. The extent of Katrina's perturbation of the midlatitude flow would appear to be limited given the lack of reintensification or downstream development during ET; however, the slow progression of a strong upper-tropospheric warm pool across the North Atlantic ocean in the week following Katrina's landfall prompts the question of whether even a non-reintensifying ET event can lead to significant modification of the midlatitude flow.

Analysis of Hurricane Katrina's outflow layer after landfall suggests that it does not comprise the long-lived midlatitude warm pool. Instead, it curls anticyclonically and re-enters the tropical warm source region east of Florida. However, the interaction between Katrina's anticyclonic outflow and an approaching baroclinic trough is shown to establish an anomalous southwesterly conduit that injects a pre-existing warm pool over the southwestern United States into the midlatitudes. This warm pool reduces predictability in medium-range forecasts over the North Atlantic and Europe while simultaneously aiding in the development of Hurricanes Maria and Nate.

The origin of the warm pool is shown to be the combination of anticyclonic upper-level features generated by Eastern Pacific Hurricane Hilary and the South Asian Anticyclone. The global nature of the connections involved with the development of the warm pool and its injection into the extratropics has an impact on forecasting since the predictability issue associated with ET in this case involves far more than the potential reintensification of the transitioning system itself.