Abstract
Abstract. During 9–16 September 2013, the Front Range region of Colorado experienced heavy rainfall that resulted in severe flooding. Precipitation totals for the event exceeded 450 mm, damages to public and private properties were estimated to be over USD 2 billion, and nine lives were lost. This study analyzes the characteristics of precipitable water (PW) surrounding the event using 10 years of high-resolution GPS PW data in Boulder, Colorado, which was located within the region of maximum rainfall. PW in Boulder is dominated by seasonal variability with an average summertime maximum of 36 mm. In 2013, the seasonal PW maximum extended into early September and the September monthly mean PW exceeded the 99th percentile of climatology with a value 25 % higher than the 40-year climatology. Prior to the flood, around 18:00 UTC on 8 September, PW rapidly increased from 22 to 32 mm and remained around 30 mm for the entire event as a result of the nearly saturated atmosphere. The frequency distribution of September PW for Boulder is typically normal, but in 2013 the distribution was bimodal due to a combination of above-average PW values from 1 to 15 September and much drier conditions from 16 to 30 September. The above-normal, near-saturation PW values during the flood were the result of large-scale moisture transport into Colorado from the Tropical Eastern Pacific and the Gulf of Mexico. This moisture transport was the product of a stagnating cutoff low over the southwestern United States working in conjunction with an anticyclone located over the southeastern United States. A blocking ridge located over the Canadian Rocky Mountains kept both of the synoptic features in place over the course of several days, which helped to provide continuous moisture to the storm, thus enhancing the accumulated precipitation totals.
Highlights
During 9–16 September 2013, multiple local and state precipitation records were broken when low-level easterly flow interacted with an anomalous moisture pool over the Front Range region of Colorado to produce one of the largest floods in state history (Colorado Climate Center, 2013)
The moisture transport was dependent on the strength and location of the dominant synoptic features, and based on the analysis shown in Fig. 8 the moisture has been transported into Colorado from both the Tropical Eastern Pacific and the Gulf of Mexico
Monthly-averaged precipitable water (PW) values in the GPS dataset for September 2013 were above the 99th percentile when compared to the climatological data as well as around 25 % higher than the monthly-averaged climatological mean value for September
Summary
During 9–16 September 2013, multiple local and state precipitation records were broken when low-level easterly flow interacted with an anomalous moisture pool over the Front Range region of Colorado to produce one of the largest floods in state history (Colorado Climate Center, 2013). In another study which examined the climatology of rainfall events in Colorado, Mahoney et al (2015) found that the region of Colorado east of the Continental Divide does not generally experience heavy precipitation events in the fall because it is during this time of year that the region experiences seasonal atmospheric drying They did note that there was enhanced climatological variability in September and October, making it difficult to place these months into the same category as the drier months (November–February). The amount of precipitation that fell during the September 2013 event required a large amount of moisture at a time of year when atmospheric moisture climatologically begins to decrease from higher summer values to lower winter values (Mahoney et al, 2015) This uncharacteristic increase in moisture implies moisture was transported into the region. Where did the moisture for the 2013 event originate? To answer this question, synoptic-scale dynamics and pre-existing conditions that led to large-scale, continuous moisture transport were evaluated
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