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.

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Organization and Environmental Properties of Extreme-Rain-Producing Mesoscale Convective Systems

This paper surveys the current state of knowledge regarding large-scale meteorological patterns (LSMPs) associated with short-duration (less than 1 week) extreme precipitation events over North America. In contrast to teleconnections, which are typically defined based on the characteristic spatial variations of a meteorological field or on the remote circulation response to a known forcing, LSMPs are defined relative to the occurrence of a specific phenomenon—here, extreme precipitation—and with an emphasis on the synoptic scales that have a primary influence in individual events, have medium-range weather predictability, and are well-resolved in both weather and climate models. For the LSMP relationship with extreme precipitation, we consider the previous literature with respect to definitions and data, dynamical mechanisms, model representation, and climate change trends. There is considerable uncertainty in identifying extremes based on existing observational precipitation data and some limitations in analyzing the associated LSMPs in reanalysis data. Many different definitions of “extreme” are in use, making it difficult to directly compare different studies. Dynamically, several types of meteorological systems—extratropical cyclones, tropical cyclones, mesoscale convective systems, and mesohighs—and several mechanisms—fronts, atmospheric rivers, and orographic ascent—have been shown to be important aspects of extreme precipitation LSMPs. The extreme precipitation is often realized through mesoscale processes organized, enhanced, or triggered by the LSMP. Understanding of model representation, trends, and projections for LSMPs is at an early stage, although some promising analysis techniques have been identified and the LSMP perspective is useful for evaluating the model dynamics associated with extremes.

/pdf/north-american-extreme-temperature-events-and-related-large-1mzz6ojjpv.pdf

North American extreme temperature events and related large scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

The Extraordinary California Drought of 2013-2014: Character, Context, and the Role of Climate Change

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.

http://schumacher.atmos.colostate.edu/research/schumacher_johnson_2006_waf.pdf

Characteristics of U.S. extreme rain events during 1999-2003

Moisture flux convergence (MFC) is a term in the conservation of water vapor equation and was first calculated in the 1950s and 1960s as a vertically integrated quantity to predict rainfall associated with synoptic-scale systems. Vertically integrated MFC was also incorporated into the Kuo cumulus parameterization scheme for the Tropics. MFC was eventually suggested for use in forecasting convective initiation in the midlatitudes in 1970, but practical MFC usage quickly evolved to include only surface data, owing to the higher spatial and temporal resolution of surface observations. Since then, surface MFC has been widely applied as a short-term (0–3 h) prognostic quantity for forecasting convective initiation, with an emphasis on determining the favorable spatial location(s) for such development. A scale analysis shows that surface MFC is directly proportional to the horizontal mass convergence field, allowing MFC to be highly effective in highlighting mesoscale boundaries between different air masses near the earth’s surface that can be resolved by surface data and appropriate grid spacing in gridded analyses and numerical models. However, the effectiveness of boundaries in generating deep moist convection is influenced by many factors, including the depth of the vertical circulation along the boundary and the presence of convective available potential energy (CAPE) and convective inhibition (CIN) near the boundary. Moreover, lower- and upper-tropospheric jets, frontogenesis, and other forcing mechanisms may produce horizontal mass convergence above the surface, providing the necessary lift to bring elevated parcels to their level of free convection without connection to the boundary layer. Case examples elucidate these points as a context for applying horizontal mass convergence for convective initiation. Because horizontal mass convergence is a more appropriate diagnostic in an ingredients-based methodology for forecasting convective initiation, its use is recommended over MFC.

FORECASTER'S FORUM The Use of Moisture Flux Convergence in Forecasting Convective Initiation: Historical and Operational Perspectives

Twenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly lowlevel jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 1408) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature ue advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surfacebased stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximumue CAPE values of more than 1000 J kg21 in the vicinity of the MCS site and higher values upstream. Maximumue convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg 21) while to the south convection is limited by large values of CIN (greater than 60 J kg 21). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.

/pdf/the-environment-of-warm-season-elevated-thunderstorms-34oordhoj7.pdf

The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States

A 30-yr climatology of the snow-to-liquid-equivalent ratio (SLR) using the National Weather Service (NWS) Cooperative Summary of the Day (COOP) data is presented. Descriptive statistics are presented for 96 NWS county warning areas (CWAs), along with a discussion of selected histograms of interest. The results of the climatology indicate that a mean SLR value of 13 appears more appropriate for much of the country rather than the often-assumed value of 10, although considerable spatial variation in the mean exists. The distribution for the entire dataset exhibits positive skewness. Histograms for individual CWAs are both positively and negatively skewed, depending upon the variability of the in-cloud, subcloud, and ground conditions.

/pdf/a-climatology-of-snow-to-liquid-ratio-for-the-contiguous-2aqop4p5tk.pdf

A Climatology of Snow-to-Liquid Ratio for the Contiguous United States

A case study of a long, narrow band of heavy snowfall is presented that illustrates those processes that force and focus the precipitation in a unique linear fashion. System-relative flow on isentropic surfaces shows how the trough of warm air aloft (trowal) formed to the north-northwest of a weak synoptic-scale surface cyclone. To the north of the trowal, midtropospheric frontogenesis formed as the warm, moist, high-e air in the trowal canyon became confluent with cold, dry air to the northwest of a closed midlevel circulation. Within the trowal airstream, isentropic uplsope is shown to contribute to vertical motion, while transverse to this flow, mesoscale lift is enhanced on the warm side of a frontogenetical zone in the presence of weak symmetric stability and conditional symmetric instability. Further, it is shown that a sloping zone of small positive to negative equivalent potential vorticity forms to the southeast of the midtropospheric system-relative closed circulation as low-e air associated with the dry conveyor belt, seen in water vapor imagery, overruns warm, moist high-e air associated with the warm conveyor belt. In this way cold season instability forms due to differential moisture advection on the warm side of the frontogenesis axis. Finally, a conceptual model is shown that encapsulates the key processes that contributed to the extensive, narrow band of heavy snow in the presence of a weak synoptic-scale surface cyclone.

http://www.eas.slu.edu/CIPS/PUBLICATIONS/Moore2005c.pdf

A Process-Oriented Methodology Toward Understanding the Organization of an Extensive Mesoscale Snowband: A Diagnostic Case Study of 4–5 December 1999

A 30-yr (1979–2009) climatology of major ice storms with 19.05 mm (0.75 in) ice accumulation or greater across the central United States is presented. An examination of the United States Army Cold Regions Research and Engineering Laboratory Ice Storm Database, in conjunction with National Climatic Data Center Storm Data, revealed 20 (out of 51) ice storms during the period which satisfied the selection criteria herein. Using the General Meteorological Package with the North American Regional Reanalysis dataset, system-relative composites using both traditional (i.e., mean) and probabilistic (i.e., frequency analysis of median, quartiles, and threshold exceedance) composite-analysis measures for the major ice storms were created. The composite analysis was computed at start, maximum coverage, and end times to depict the evolution of features used to diagnose the synoptic and mesoscale environment potentially favorable for major ice storms within the central United States. At the maximum-coverage time, the composite analysis indicates the presence of sub-freezing surface temperatures to the north of a southwest-to-northeast oriented quasi-stationary front below a strong 850-hPa jet streak. The low-level jet streak bisects the quasi-stationary front and is enhanced by the direct thermal circulation associated with an upper-level jet streak. These features are responsible for supplying warm and moist air into the elevated warm layer, which provides an environment to completely melt frozen precipitation. An upper-level long-wave trough is anchored over the southwestern United States and there is some evidence that it leads to short-wave troughs ejecting into the plains. These troughs may be responsible for multiple rounds of enhanced vertical motion, precipitation, and freezing rain. Finally, an analysis of probabilistic measures of the 300- and 850-hPa jet streaks and elevated warm-layer characteristics indicates the importance of these fields in major ice storms in this region.

/pdf/characteristics-of-major-ice-storms-in-the-central-united-5d2lmxd93z.pdf

Characteristics of Major Ice Storms in the Central United States

AUGUST 2003 | n the afternoon of 10 November 1998, much of southeastern South Dakota and the surrounding area was digging itself out from under a foot of snow. The snowfall was spread over several states; however, the heaviest totals were found in pockets around Sioux Falls, South Dakota (FSD; Fig. 1). The impressive snowfall near Sioux Falls was accompanied by nearly five hours of thunder and lightning during the most intense period of snowfall (1100–1600 UTC). The extratropical cyclone (ETC) responsible for creating the heavy snow and blizzard conditions in parts of South Dakota and Minnesota was of historic magnitude with a central mean sea level pressure (MSLP) of 964 hPa at its peak over Duluth International Airport at 2035 UTC on 10 November [National Climatic Data Center (NCDC) 1998]. The clarity of the features in this early season snowstorm makes it a good candidate for illustrating the common characteristics of synoptic-scale processes often found with heavy banded snowfall in the central United States. To examine the main physical processes attending this deep system, both observed surface data and initialized upper-air data from the Rapid Update Cycle (RUC) II model are used. The major synoptic features at the time of most intense snowfall (1500 UTC) are the focus of this study; however, some analyses from nine hours earlier (0600 UTC) are shown to illustrate the dynamic development of this exceptionally strong ETC. The system initially formed to the lee side of the Rocky Mountains and moved eastward across Colorado and Kansas early on 9 November. It then turned northeastward and intensified rapidly as it moved through eastern Nebraska into Minnesota. At 0600 UTC 10 November, the ETC was located in eastern Nebraska and had a central MSLP of 984 hPa (Fig. 2a). The surface analysis reveals a warm front extending eastward ahead of the system through Missouri and two cold fronts pinwheeling southward from the center of low pressure. The dual cold-front structure at this time is supported by the presence of two wind shift lines (not shown) and two distinct thermal gradients, as illustrated by the packing of the isentropes in Fig. 2a. By 1500 UTC the system had moved into southern Minnesota and the central MSLP had dropped to 967 hPa (Fig. 2b); most of this 17-hPa pressure fall occurred in the last six hours. It was during this rapid intensification phase that snowfall rates became sigTHE MAP ROOM

Charles E. Graves

Papers

The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States

A Climatology of Snow-to-Liquid Ratio for the Contiguous United States

A Process-Oriented Methodology Toward Understanding the Organization of an Extensive Mesoscale Snowband: A Diagnostic Case Study of 4–5 December 1999

Characteristics of Major Ice Storms in the Central United States

Band on the Run - Chasing the Physical Processes Associated with Heavy Snowfall.