The Paso del Norte metropolitan area (population ~ 2 million) is comprised of two cities (El Paso, Texas, USA and Ciudad Juárez, Chihuahua, Mexico)- a single metropolis in two nations separated by a river (the Rio Grande/Rio Bravo del Norte). The two cities share the same meteorology, but a different socioeconomic status. Located in the Chihuahuan Desert, average annual precipitation is 22 cm, most of which falls during the North American Monsoon in late summer, often in a short time, making the metropolitan area prone to flash flooding. The topographic and socioeconomic complexity of the area creates complexities in planning for, forecasting, and responding to these flood events.

The risk for a disaster is the product of a population's exposure to a natural hazard and its socio-economic vulnerability, i.e. the ability to anticipate, respond to, and recover from the hazard. This is illustrated in the Paso del Norte with the cases of the catastrophic flooding of early August 2006, and to a lesser extent with flash flooding associated with tropical cyclone Dolly's remnants in July 2008. Exposure would be reduced by discouraging residential construction in flood-prone areas, but urbanization of floodplains has happened in both cities. The urban environmental impacts of the floods are more severe in Mexico than in the USA; the severity of damage in Ciudad Juárez is amplified by a reduced capacity for social protection. Monetary damages from the 2006 flooding in Ciudad Juárez exceeded twice the city's annual budget, reducing the ability for governmental assistance and requiring assistance from NGOs for recovery. Conversely, while 2006 flood impacts in El Paso justified a federal disaster declaration, the relative severity of the simultaneously-experienced disaster was an order of magnitude less than in Mexico. Within El Paso County, damage and prospects for recovery were variable and related to the socio-economic status of different neighborhoods, especially between incorporated areas and unincorporated colonias.

Meteorological and topographic factors also impact the flood risk, especially in El Paso, a city bisected by the Franklin Mountains. In both cities, neighborhoods on or below slopes where orography increases precipitation and runoff are more vulnerable to flooding; engineering failures and the lack of consideration of orographic effects in the development of flood intensity/ duration/ frequency curves (which in itself has been hindered by the limited number of rain gauges in the metroplex) have led to underassessment of flash flood risk in some portions of El Paso and exacerbated uninsured monetary losses. For example, “official” short-term (≤ 12 hour) rainfall totals at El Paso International Airport in the 2006 and 2008 events would be associated with return periods of a few decades (2006) or a few years (2008) from those data, but rainfall totals in orographically-enhanced slopes of the Franklin Mountains would be associated with apparent return periods of a century or more. Average recurrence interval curves based on airport data are not appropriate for flood hazard planning in neighborhoods <10 km to the southwest.

The Paso del Norte illustrates how both socioeconomic and physical factors can create highly variable risks for, and capacities to respond to, severe weather hazards within a single metropolitan area. This example supports the development of impact-based, rather than purely hazard-based, severe weather forecasting and planning, which is the envisioned platform of the future for the National Weather Service. Such a methodology would be especially effective for urban areas with high spatial variabilities in meteorology and/or economic development. Assessment metrics incorporating GIS and spatially variable risks and impacts, now used to evaluate other natural hazards (tornadoes, earthquakes, winter storms), could be appropriate for urban flooding.