This study examines the radar-indicated structures and other features of extreme rain events in the United States over a 3-yr period. A rainfall event is defined as "extreme" when the 24-h precipitation total at one or more stations surpasses the 50-yr recurrence interval amount for that location. This definition yields 116 such cases from 1999 to 2001 in the area east of the Rocky Mountains, excluding Florida. Two-kilometer national composite radar reflectivity data are then used to examine the structure and evolution of each extreme rain event. Sixty-five percent of the total number of events are associated with mesoscale convective systems (MCSs). While a wide variety of organizational structures (as indicated by radar reflectivity data) are seen among the MCS cases, two patterns of organization are observed most frequently. The first type has a line, often oriented east-west, with "training" convective elements. It also has a region of adjoining stratiform rain that is displaced to the north of the line. The second type has a back-building or quasi-stationary area of convection that produces a region of stratiform rain downstream. Surface observations and composite analysis of Rapid Update Cycle Version 2 (RUC-2) model data reveal that training line/adjoining stratiform (TL/AS) systems typically form in a very moist, unstable environment on the cool side of a preexisting slow-moving surface boundary. On the other hand, back-building/quasistationary (BB) MCSs are more dependent on mesoscale and storm-scale processes, particularly lifting provided by storm-generated cold pools, than on preexisting synoptic boundaries.

The PECAN field campaign assembled a rich array of observations from lower-tropospheric profiling systems, mobile radars and mesonets, and aircraft over the Great Plains during June-July 2015 to better understand nocturnal mesoscale convective systems and their relationship with the stable boundary layer, the low-level jet, and atmospheric bores.

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.

This study examines the characteristics of a large number of extreme rain events over the eastern two-thirds of the United States. Over a 5-yr period, 184 events are identified where the 24-h precipitation total at one or more stations exceeds the 50-yr recurrence amount for that location. Over the entire region of study, these events are most common in July. In the northern United States, extreme rain events are confined almost exclusively to the warm season; in the southern part of the country, these events are distributed more evenly throughout the year. National composite radar reflectivity data are used to classify each event as a mesoscale convective system (MCS), a synoptic system, or a tropical system, and then to classify the MCS and synoptic events into subclassifications based on their organizational structures. This analysis shows that 66% of all the events and 74% of the warm-season events are associated with MCSs; nearly all of the cool-season events are caused by storms with strong synoptic forcing. Similarly, nearly all of the extreme rain events in the northern part of the country are caused by MCSs; synoptic and tropical systems play a larger role in the South and East. MCS-related events are found to most commonly begin at around 1800 local standard time (LST), produce their peak rainfall between 2100 and 2300 LST, and dissipate or move out of the affected area by 0300 LST.

Information about the location and magnitudes of natural gas (NG) leaks from urban distribution pipelines is important for minimizing greenhouse gas emissions and optimizing investment in pipeline management. To enable rapid collection of such data, we developed a relatively simple method using high-precision methane analyzers in Google Street View cars. Our data indicate that this automated leak survey system can document patterns in leak location and magnitude within and among cities, even without wind data. We found that urban areas with prevalent corrosion-prone distribution lines (Boston, MA, Staten Island, NY, and Syracuse, NY), leaked approximately 25-fold more methane than cities with more modern pipeline materials (Burlington, VT, and Indianapolis, IN). Although this mobile monitoring method produces conservative estimates of leak rates and leak counts, it can still help prioritize both leak repairs and replacement of leak-prone sections of distribution lines, thus minimizing methane emissions over short and long terms.

Twenty-eight predecessor rain events (PREs) that occurred over the United States east of the Rockies during 1995-2008 are examined from a synoptic climatology and case study perspective. PREs are coherent mesoscale regions of heavy rainfall, with rainfall rates $100 mm (24 h) 21 , that can occur approximately 1000 km poleward of recurving tropical cyclones (TCs). PREs occur most commonly in August and September, and approximately 36 h prior to the arrival of the main rain shield associated with the TC. A distinguishing feature of PREs is that they are sustained by deep tropical moisture that is transported poleward directly from the TC. PREs are high-impact weather events that can often result in significant inland flooding, either from the PRE itself or from the subsequent arrival of the main rain shield associated with the TC that falls onto soils already saturated by the PRE.The composite analysis shows that on the synoptic-scale, PREs form in the equatorward jet-entrance region of a 200-hPa jet on the western flank of a 925-hPa equivalent potential temperature ridge located east of a 700-hPa trough. On the mesoscale, PREs occur in conjunction with low-level frontogenetical forcing along a baroclinic zone where heavy rainfall is focused. A case study analysis was conducted of a PRE ahead of TC Erin (2007) that produced record-breaking rainfall (.250 mm) from southern Minnesota to Lake Michigan. This analysis highlighted the importance of frontogenetical forcing along a low-level baroclinic zone in the presence of deep tropical moisture from TC Erin in producing a long-lived, quasi-stationary mesoscale convective system.

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Observations and numerical simulations are used to investigate the atmospheric processes that led to extreme rainfall and resultant destructive flash flooding in eastern Missouri on 6-7 May 2000. In this event, a quasi-stationary mesoscale convective system (MCS) developed near a preexisting mesoscale convective vortex (MCV) in a very moist environment that included a strong low-level jet (LLJ). This nocturnal MCS produced in excess of 300 mm of rain in a small area to the southwest of St. Louis, Missouri. Operational model forecasts and simulations using a convective parameterization scheme failed to produce the observed rainfall totals for this event. However, convection-permitting simulations using the Weather Research and Forecasting Model were successful in reproducing the quasi-stationary organization and evolution of this MCS. In both observations and simulations, scattered elevated convective cells were repeatedly initiated 50-75 km upstream before merging into the mature MCS and contributing to the heavy rainfall. Lifting provided by the interaction between the LLJ and the MCV assisted in initiating and maintaining the convection. Simulations indicate that the MCS was long lived despite the lack of a convectively generated cold pool at the surface. Instead, a nearly stationary low-level gravity wave helped to organize the convection into a quasi-linear system that was conducive to extreme local rainfall amounts. Idealized simulations of convection in a similar environment show that such a low-level gravity wave is a response to diabatic heating and that the vertical wind profile featuring a strong reversal of the wind shear with height is responsible for keeping the wave nearly stationary. In addition, the convective system acted to reintensify the midlevel MCV and also caused a distinct surface low pressure center to develop in both the observed and simulated system.