Extreme Events Across New Mexico During the 2018 North American Monsoon

The 2018 North American Monsoon season was characterized by localized severe storm and flash flood events in New Mexico. Notable events included flash flooding in Belen and parts of Rio Rancho on 5 July; inundation of San Antonio on 15 July; the 1,000-year flood event in and around Santa Fe on 23 July; and a severe hail event in the Albuquerque area on 30 July. Analysis of these events was conducted to better understand the synoptic, mesoscale, and local conditions that contributed to their occurrence with the goal of helping forecasters better predict similar monsoon-driven severe storm and flash flood events in the future. Additionally, analysis of 464 reporting sites (to include 12 ASOS/AWOS, 105 COOP, and 347 CoCoRaHS) across New Mexico revealed that most of the state experienced below average rainfall, with pockets of above average rainfall along and to the east of the central mountain chain during the 2018 North American Monsoon season. The rainfall distribution pattern developed as the prevalent monsoon pattern evolved from the Type II "reverse" monsoon pattern in July and early August to the “classic” Type I pattern in late August. The likely causes of the three flash flooding events in July 2018 stem from orographic uplift and outflow boundary interactions, and the diversion of arroyos and streams from their natural watercourses by human development. An amplified jet stream with embedded shortwave troughs that skirt across northern New Mexico is favored for severe thunderstorm activity across the state during the North American Monsoon season.