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During the second week of September 2013, a seasonally uncharacteristic weather pattern stalled over the Rocky Mountain Front Range region of northern Colorado bringing with it copious amounts of moisture from the Gulf of Mexico, Caribbean Sea, and the tropical eastern Pacific Ocean. This feed of moisture was funneled toward the east-facing mountain slopes through a series of mesoscale circulation features, resulting in several days of unusually widespread heavy rainfall over steep mountainous terrain. Catastrophic flooding ensued within several Front Range river systems that washed away highways, destroyed towns, isolated communities, necessitated days of airborne evacuations, and resulted in eight fatalities. The impacts from heavy rainfall and flooding were felt over a broad region of northern Colorado leading to 18 counties being designated as federal disaster areas and resulting in damages exceeding $2 billion (U.S. dollars). This study explores the meteorological and hydrological ingredients that led to this extreme event. After providing a basic timeline of events, synoptic and mesoscale circulation features of the event are discussed. Particular focus is placed on documenting how circulation features, embedded within the larger synoptic flow, served to funnel moist inflow into the mountain front driving several days of sustained orographic precipitation. Operational and research networks of polarimetric radar and surface instrumentation were used to evaluate the cloud structures and dominant hydrometeor characteristics. The performance of several quantitative precipitation estimates, quantitative precipitation forecasts, and hydrological forecast products are also analyzed with the intention of identifying what monitoring and prediction tools worked and where further improvements are needed.

Abstract. Thirty major storms that passed over Goodwin Creek, a small research watershed in northern Mississippi, were analyzed to assess the bias between radar rainfall estimates at rain gauge locations and the gauge amounts. These storms, each contributing at least 10 mm of storm total rainfall, accumulated approximately 785 mm of rain, which corresponds to about half the average annual rainfall amount for the area. The focus of this study was to demonstrate the importance of (1) bias adjustment of the radar rainfall estimates and (2) the quality control of the rain gauge data used for bias adjustment. The analyses are based on Memphis Weather Surveillance Radar--1988 Doppler radar data, tipping-bucket rain gauge data, and raindrop spectra information collected within the Goodwin Creek catchment. Because of measurement and rainfall estimation uncertainties, radar observations are often combined with rain gauge data to obtain the most accurate rainfall estimates. Rain gauge data, however, are subject to characteristic error sources: for Goodwin Creek, malfunctioning of the tipping-bucket rain gauges was frequently caused by biological and mechanical fouling, and human interference. Therefore careful quality control of the rain gauge data is crucial, and only good quality rain gauge information should be used for adjusting radar rainfall estimates. By using high-quality gauge data and storm-based bias adjustment, we achieved radar rainfall estimates with root-mean-square errors (RMSE) of approximately 10% for storm total rainfall accumulations of 30 mm or more. Differences resulting from radar data processing scenarios were found to be small compared to the effect caused by bias adjustment and using high-quality rain gauge data.

A storm system near the Blue Ridge Mountains of Virginia produced peak rainfall accumulations exceeding 600 mm in a 6‐hour period during the morning and early afternoon of June 27, 1995. The peak flood discharge of 3,000 m3 s−1 on the Rapidan River at a drainage area of 295 km2 places this event on the envelope curve of flood discharge for the United States east of the Mississippi River. Observations of radar reflectivity factor and Doppler velocity made by the Sterling, Virginia, WSR‐88D (Weather Surveillance Radar–1988 Doppler) are used for analyses of the storm. The temporal and spatial variability of rainfall are examined on a 1‐km grid scale and 6‐min timescale. Like many heavy rainfall events, storm motion played a key role in the production of heavy rainfall for the Rapidan storm. Storm motion and storm evolution for the Rapidan storm were closely linked to topographic features at the scale of the ridges which extend southward from the Blue Ridge and delineate the Rapidan basin. Key elements of the storm environment included strong boundary layer winds directed upslope toward the Blue Ridge, weak upper level winds, high precipitable water values, and a near‐saturated atmospheric column up to 6 km. An important element of storm structure was the low‐reflectivity centroid of the storm. This feature of the storm was related both to the exceptional rainfall rates of the storm and to the underestimation of storm total rainfall by the operational WS‐88D precipitation products. Components of the atmospheric and land surface water budgets are derived. The cumulative discharge from the Rapidan River was 0.87×108 m3 (296 mm over the 295‐km2 catchment). The storm total precipitation for the Rapidan basin was 1.01×108 m3 (344 mm over the catchment). The precipitation efficiency of the storm, that is, the ratio of storm total rainfall to atmospheric water vapor inflow, was approximately 90%.