Understanding the Dominant Sources and Tracks of Moisture for Summer Rainfall in the Southwest United States

Abstract We investigated the moisture sources and tracks that enable summer rainfall over the four‐state southwestern U.S. region of Arizona, New Mexico, Colorado, and Utah by employing a high‐resolution Lagrangian particle tracking model. Six locations were selected—Cedar City (Utah), Grand Junction (Colorado), Eastonville (Colorado), Laveen (Arizona), Redrock (New Mexico), and Melrose (New Mexico)—together, they represent six spatial regions of summer precipitation for the Southwest. Moisture tracks were generated for all the rainy days at these stations for the historical period 1979–2013. Tracks were generated for a 3‐day period ending with the day of rainfall, which were then used to identify the source of moisture, pathway or trajectory, and the modulation along the track. The four major sources of moisture—Gulf of California, Gulf of Mexico (GoM), land, and the Pacific Ocean—were identified as responsible for summer rainfall over southwestern United States. The two dominant moisture sources at Laveen, Cedar City, Redrock, Grand Junction, and Eastonville were Gulf of California and land; at Melrose GoM and land were the dominant sources. The leading source of moisture at each location contributed to most of the extreme rainfall events. Tracks from GoM traveled the fastest and those from land sources were the slowest. Large‐scale circulation features–pressure, convergence, and specific humidity–were consistent with the moisture tracks and were found to be strong throughout the 3‐day period. This detailed and comprehensive generation of rainfall tracks offers unique insights into the moisture source and delivery for summer rainfall over southwestern United States.