Evaporative Moisture Sources of Colorado’s Front Range: A Case Study of the Exceptionally Wet May–July Season of 2023

Abstract. In 2023, parts of eastern Colorado experienced their wettest three-month period (May–July) out of 129 years of record. This extreme precipitation led to flash flooding, road washouts, and significant property damage among Colorado communities along the Front Range including Denver, Boulder, and Fort Collins. Although the Front Range is the most populated region in Colorado, few studies have explored the evaporative origin of Front Range precipitation. To better anticipate and understand extreme precipitation events across the region, we focus our investigation on the evaporative origin of the extreme precipitation in May, June, and July (MJJ) of 2023 and how it compares to moisture sources during the previous two decades. This study uses the Water Accounting Model 2 Layers (WAM2layers) and hourly ERA5 reanalyses to quantify the evaporative sources of precipitation in Colorado’s Front Range during MJJ of 2023 and over the past 23 years (2000–2022). Our moisture source analysis reveals that for the Front Range region in May–July of 2023: (1) the three primary moisture sources are the Pacific Ocean, the western United States, and Colorado itself, contributing just over 66.2 % of total precipitation; (2) historically, those same regions dominate evaporative contributions in MJJ, but terrestrial contributions and local moisture recycling (i.e., precipitation that recently evaporated from within the Front Range) in May–July of 2023 accounted for a slightly greater proportion of precipitation than on average; (3) moisture sources in May–July 2023 were a statistical outlier in terms of the magnitude of moisture contributed to the Front Range; and (4) anomalous evaporative contributions in May–July 2023 originated from a broad range of regions, more consistent with the leading mode of moisture source variability, which reflects widespread anomalies from Colorado and the Pacific Ocean, rather than the second leading mode characterized by a north–south dipole in anomalous contributions from regions north and south of the Front Range. This research provides new insights into the origins of extreme rainfall in the summer of 2023 as well as the historical moisture sources of warm-season precipitation in the Front Range.