Dust storms are one of the principal hazardous weather phenomena in El Paso, Texas, U.S.A., and Ciudad Juarez, Chihuahua, Mexico (collectively the “Paso del Norte” metropolitan area, with a population of over 2 million). From 1932 through 2005, dust events lasting at least 2 hours were recorded at the El Paso International Airport (ELP) an average of 15 days per year. During the 2001- 2005 period, blowing dust, blowing sand, and related phenomena (i.e., dust events) occurred at ELP on 219 days (12% of all days); 55% of the events were synoptically driven, while 45% of the events were associated with thunderstorm outflows (haboobs). This dust has effects on the local economy and infrastructure, and may adversely affect the health of the Paso del Norte's residents.

HYSPLIT back trajectories into El Paso were run for all days in 2001 – 2005, including for those days with dust events. Contour plots of the HYSPLIT trajectory endpoints and residence time, source contribution function, incremental probability, and conditional probability showed different airflow patterns during dusty days as compared to overall trajectories into El Paso. After analyzing the overall residence time and source contribution function plots, air masses that arrive in El Paso were seen to be more or less symmetrically distributed around the city. However, for days with synoptically-driven dust events, the source contribution function and incremental probability- a measure of the areas most likely to have been traversed by air masses arriving at El Paso - were strongly positively associated with the region west-southwest of the city, a zone of known dust source areas. The pattern of incremental probability plots of air mass trajectories during synoptic dust days was consistent with air parcels moving toward cyclones crossing or developing directly NE of the region (“Albuquerque Lows”). For days with convective dust events, trajectories were positively associated with air masses arriving from the south and southeast, consistent with monsoon moisture surges that often bring strong thunderstorms. Trajectory conditional probabilities on dust days strongly favored trajectories originating ≥ ~600 km upwind of El Paso 24 hours back from the west for synoptically-driven events, and from the south and east for convectively-driven events. The recognition of these characteristic air mass trajectories can aid in forecasting dust storms in the Paso del Norte metropolitan area.