| 09 Apr 2026
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.
Manuscript from Humphreys et al investigates the evaporative moisture sources associated with the exceptionally wet May–July 2023 season in Colorado’s Front Range using WAM2layers driven by ERA5 reanalysis. The topic is timely and regionally important, and the manuscript provides a useful climatological context for the 2023 event. The combination of moisture tracking, clustering, and EOF analysis is potentially valuable. However, several methodological choices and interpretations require further clarification and stronger justification before the conclusions can be considered robust. In particular, the manuscript would benefit from a more rigorous uncertainty assessment, clearer physical interpretation of the statistical analyses, and stronger discussion of model limitations. I would recommend major revision.
Major comments
1.The manuscript acknowledges that ERA5 has difficulties representing complex terrain and convective precipitation over the Front Range region and provides comparison with PRISM in Appendix A.1. However, several important issues remain insufficiently addressed.
Figure A2 indicates substantial spatial discrepancies between ERA5 and PRISM precipitation. It is unclear how these biases may propagate into the WAM2layers backtracking results. Have the authors considered any sensitivity tests (e.g., weighting or bias-correcting precipitation using PRISM) to assess the robustness of the diagnosed moisture sources?
In addition, since WAM2layers is driven by ERA5 evaporation fields, potential biases in ERA5 evapotranspiration over mountainous regions in the western US, where ERA5 is known to exhibit wet biases, may systematically influence the estimated land-surface recycling contributions. At minimum, this limitation should be discussed more explicitly.
2. I think the manuscript requires significant restructuring. Currently, the Appendix contains a very large number of figures, many of which are either not referenced in the main text or are only briefly mentioned without sufficient accompanying discussion or interpretation. Several figures appear disconnected from the core paper.
In my opinion, materials that are not central to the primary conclusions should instead be moved to a Supplementary Information file rather than remaining in the Appendix of the main manuscript. Even for supplementary materials, the figures should still be explicitly cited, interpreted, and connected to the scientific arguments presented in the paper.
For example, Line 87 states that the Appendix contains results from other regions, but these results are not accompanied by additional analysis or discussion explaining their relevance to the study objectives.
3. The paper currently contains unnecessary descriptive text. For example, the Introduction does not need subdivision into two separate subsections. In most manuscripts, the final paragraph of the Introduction already serves to state the study objectives. In the Results section, several introductory sentences (e.g., Lines 230–232) are not necessary and could instead be incorporated directly into subsection titles.
4. The statistical robustness of the EOF decomposition and k-means clustering analysis is somewhat concerning given the relatively limited sample size (24 seasons). With such a small sample size, it is difficult to assess whether the identified EOF modes and clusters are statistically stable and physically meaningful. At present, the EOF discussion feels largely descriptive rather than mechanistically interpretable. It should provide stronger justification for the EOF and clustering methodology, discussion of robustness and sensitivity, and clearer physical interpretation of the dominant modes and clusters.
5. Overall writing style resembles a project report rather than a scientific manuscript. For example, the manuscript repeatedly refers to “the project,” which should be avoid in formal scientific writing. In addition, the paper presents a large amount of figures and analysis but does not always clearly discuss the key scientific findings and implications. I encourage the authors to place greater emphasis on synthesizing the primary scientific insights rather than exhaustively documenting all analyses.
Minor comments
Line 83: Remove “which partially funded this project.” Such acknowledgements are more appropriately included in the Acknowledgements section rather than here.
Line 140: Please provide a clearer justification for conducting the analysis at monthly timescales. Extreme precipitation events are often strongly associated with higher-frequency variability (e.g., daily scales), and the implications of temporal aggregation should therefore be discussed more carefully.
Line 167: Consider replacing “For this project” with “For this work” or “For this study.” Similar wording should be corrected throughout the manuscript.
Figure 3 caption: The caption states that maps defining the regions are provided elsewhere. Please explicitly cite the corresponding supplementary or appendix figure number (e.g., Figure A5) within the caption or main text.
Line 463: Typo error, “Finaly” to “Finally”.
Line 563: The phrase “Due to project time-constraints” is not appropriate for scientific writing. I also think this kind of wording should generally be avoided in scientific manuscripts. Consider rephrasing more objectively, for example, “In this study, we only focus on …”