the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-796', Anonymous Referee #1, 20 May 2026
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AC1: 'Reply on RC1', Katherine Humphreys, 27 Aug 2026
Dear Anonymous Reviewer 1,
We appreciate your feedback and constructive suggestions on our manuscript. Please find an attached PDF detailing our responses to each of your comments in the supplemental file. Thank you again for your time.
Best regards,
Katherine Humphreys, on behalf of all authors
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AC1: 'Reply on RC1', Katherine Humphreys, 27 Aug 2026
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RC2: 'Comment on egusphere-2026-796', Anonymous Referee #2, 29 Jun 2026
This paper evaluates the role of moisture source in contributing to a severe wet period in eastern Colorado during May-June-July 2023. The analysis uses moisture tracking methods to identify where the precipitation over Colorado originated from (“tracked evaporation”) and to compare source distributions in 2023 to previous years. K-means clustering and PCA are used to show that while moisture source contributions were anomalously high during 2023, the pattern of moisture sources was similar to other normal-precipitation years. Overall, I appreciated the thoroughness of the analysis, including various sensitivity tests throughout, and the clear, detailed presentation. The paper sparked a few curiosity-based questions in my mind, which I am sharing below, even though I acknowledge that answering these may be beyond the scope of the study. A list of minor comments follows these.
Questions:
- It seems that the patterns that emerge in MJJ 2023 mirror known biases in precipitation from models and reanalysis. Is there anything to be learned from this similarity?
- Are you able to say how much the EOF1 pattern explains the anomalous precipitation observed during 2023? Even though 2023 received most of its moisture from its typical sources, there were a few minor source regions whose percent contributions nevertheless changed considerably (Fig. 5). It makes me wonder whether a few minor PC modes actually contributed quite significantly to the 2023 precipitation patterns.
Comments:
In Line 89, Appendix B is mentioned before A, which feels odd. Moreover, B contains no text contextualization and feels more supplementary to the article (rather than essential for interpreting the results). Would it be worth considering moving B material to SI instead?
Figure 3 - please describe the pink outline in the caption.
I recognize that the source region map (Fig A5) is provided in the appendix, but I actually think the map is critical for communicating the paper’s main results. At the least, please refer readers the map from the main text, not just from the figure captions.
Line 289 states that Jun ’23 received the most anomalous precipitation. I could not find this shown anywhere in the figures or tables, and I also wondered if this wouldn’t fit better in Section 3.2, which discusses how anomalous 2023 was compared to other years.
Line 353 indicates that there are three regions responsible for most of the anomalous precipitation. I understand that these regions provided the most precipitation; however, according to Fig 5, they showed some of the least anomalous percent changes. A little care with wording here would be beneficial for your readers.
Section 3.3 talks about clustering “tracked evaporation anomalies.” It took me a while to understand exactly what this meant. Consider unpacking this phrase for your reader. It is relevant for readers to understand how the anomalies are calculated.
There are a few other places where the text is difficult to follow. They are: Lines 142- 144, and Lines 249-250
Line 389: the same…sources, add “as other years.”
Like other sections, the discussion is well written and quite thorough. However, near Line 480, I noticed a strong emphasis on possible Southwest moisture source variability and little discussion of “western” sources, which seem to be quite important for 2023.
Figure A1: it is difficult to make out the 2x and 0.5x lines. Also, caption says “source regions” but main text calls them “sink regions.” Is this a typo?
Figure A6 “percent error overtime” should be “over time”?
Citation: https://doi.org/10.5194/egusphere-2026-796-RC2 -
AC2: 'Reply on RC2', Katherine Humphreys, 27 Aug 2026
Dear Anonymous Reviewer 2,
We appreciate your feedback and constructive suggestions on our manuscript. Please find an attached PDF detailing our responses to each of your questions and comments in the supplemental file. Thank you again for your time.
Best regards,
Katherine Humphreys, on behalf of all authors
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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 …”