The changes in the structure occuring during the landfall of Hurricane Nora are discussed. Nora originated over the Eastern Pacific Ocean, south of the coast of Mexico, during the period 16-18 September 1997, moved northward and maintained her strength over the anomalously warm water west of Baja California. Eventually, the storm made landfall, crossing the Gulf of California and moving into western Arizona on 25 September 1997.
Satellite imagery showed that the storm weakened while crossing the mountains along the northern half of Baja California. There is also evidence of discontinuous propagation and formation of a secondary circulation in the lee of the mountains while the primary circulation was still upstream. The result was a deep convective system imbedded in the mid-level circulation which then moved northward into Arizona. This deep and relatively small convective system moved over southern Arizona and left a narrow swatch of moderately heavy rainfall in western Arizona and southeastern California. This pattern of precipitation was in contrast to the generally heavier and more widespread coverage that occurs when hurricanes make landfall along coastlines with level terrain.
The focus of this study is on the interaction of the low-level circulation with the topography of Baja California during the period of the primary landfall. The blocking effect of the mountains and its role in both shutting down the primary low-level circulation and in the formation of the secondary circulation downwind are examined with numerical simulations using MM5. These simulations are initialized with gridded data from the ETA analysis