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Robert A. Maddox and Kenneth W. Howard

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David O. Blanchard and Kenneth W. Howard

A brief overview of the 13 June 1984 Denver hailstorm is presented. This storm produced very large hail in a few locations and copious amounts of small hail over a large area. Documentation of the storm includes data from a surface mesonetwork, cooperative observers and storm spotters, dual Doppler radar, profiler winds, radiosonde, lightning detectors, and photographs of smoke tracers. Integration of these data sets provides an interesting and informative look at this destructive storm.

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Steven V. Vasiloff and Kenneth W. Howard

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A Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) was deployed near Phoenix, Arizona, during the summer of 2004. The goal was to capture a severe microburst at close range to understand the low-altitude wind structure and evolution. During the evening of 27 July, a severe storm formed along the Estrella Mountains south of Phoenix and moved south of the SMART-R as well as the National Weather Service’s (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) in Phoenix (KIWA). Several microburst–downburst pulses were observed by radar and a surface wind gust of 67 mi h−1 was reported. The radar data illustrate the finescale structure of the microburst pulses, with the SMART-R’s higher-resolution data showing Doppler velocities 3–4 m s−1 greater than the KIWA radar. SMART-R wind shear values were 2–3 times greater with the finer resolution of the SMART-R revealing smaller features in the surface outflow wind structure. Asymmetric outflow may have been a factor as well in the different divergence values. The evolution of the outflow was very rapid with the 5-min KIWA scan intervals being too course to sample the detailed evolution. The SMART-R scans were at 3–5-min intervals and also had difficulty resolving the event. The storm environment displayed characteristics of both moderate-to-high-reflectivity microbursts, typical of the high plains of Colorado.

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John A. Augustine and Kenneth W. Howard

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Infrared imagery from GOES was used to document mesoscale convective complexes (MCCs) over the United States during 1986 and 1987. A near-record 58 MCCs occurred in 1986, and 44 occurred in 1987. Although these totals were above average relative to MCC numbers of the 7 years prior to 1985, seasonal distributions for both years were atypical. Particularly, each had an extended period (∼3 weeks) when no MCCs occurred in late spring and early summer, a time when the mean MCC seasonal distribution peaks. This peculiarity was investigated by comparing mean large-scale surface and upper-air environments of null- and active-MCC periods of both years. Results confirmed the primary importance to MCC development of strong low-level thermal forcing, as well as proper vertical phasing of favorable lower- and midtropospheric environments.

A cursory survey of MCCs documented outside of the United States reveals that MCCs, or MCC-type storms, are a warm-season phenomenon of midlatitude, subtropical, and low-latitude regions around the globe. They have been documented in South America, Mexico, Europe, West Africa, and China. These storm systems are similar to United States MCCs in that they are nocturnal, persist for over 10 h, tend to develop within weak synoptic-scale dynamics in response to strong low-level thermal forcing and conditional instability, and occur typically downwind (midlevel) of elevated terrain. It is surmised that MCCs probably occur over other parts of the midlatitudes, subtropics, and low latitudes that have yet to be surveyed.

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John A. Augustine and Kenneth W. Howard

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Digital GOES infrared imagery is used to document mesoscale convective complexes (MCCs) over the United States during 1985. The introduction of digital imagery to this process, which has been carried out since 1978, has made possible a partial automation of the MCC documentation procedure and subsequently expanded opportunities for research. In conjunction with these improvements, the definition of an MCC has been slightly modified from that proposed by Maddox in 1980. The warmer threshold area measurement (⩽−32°C) of Maddox's original criteria has been dropped from consideration because its measurement was too subjective, and also was determined to be unnecessary. In 1985, 59 MCCs were identified; this total is approximately 20 to 40 more than in any year since 1978, when these annual summaries began. The monthly distribution and seasonal progression of MCCs in 1985 are similar to those of prior years. The enhanced MCC activity in June 1985 is associated with a persistent favorable quasi-geostrophic forcing during that period. Significant MCC research conducted in 1985 included a prototype large-scale field program (0.-K. PRE-STORM) in May and June dedicated solely to the investigation of middle-latitude mesoscale convective systems.

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Robert A. Maddox, Kenneth W. Howard, and Charles L. Dempsey

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On 20/21 August 1993, deep convective storms occurred across much of Arizona, except for the southwestern quarter of the state. Several storms were quite severe, producing downbursts and extensive wind damage in the greater Phoenix area during the late afternoon and evening. The most severe convective storms occurred from 0000 to 0230 UTC 21 August and were noteworthy in that, except for the first reported severe thunderstorm, there was almost no cloud-to-ground (CG) lightning observed during their life cycles. Other intense storms on this day, particularly early storms to the south of Phoenix and those occurring over mountainous terrain to the north and east of Phoenix, were prolific producers of CG lightning. Radar data for an 8-h period (2000 UTC 20 August–0400 UTC 21 August) indicated that 88 convective cells having maximum reflectivities greater than 55 dBZ and persisting longer than 25 min occurred within a 200-km range of Phoenix. Of these cells, 30 were identified as “low-lightning” storms, that is, cells having three or fewer detected CG strikes during their entire radar-detected life cycle. The region within which the low-lightning storms were occurring spread to the north and east during the analysis period.

Examination of the reflectivity structure of the storms using operational Doppler radar data from Phoenix, and of the supportive environment using upper-air sounding data taken at Luke Air Force Base just northwest of Phoenix, revealed no apparent physical reasons for the distinct difference in observed cloud-to-ground lightning character between the storms in and to the west of the immediate Phoenix area versus those to the north, east, and south. However, the radar data do reveal that several extensive clouds of chaff initiated over flight-restricted military ranges to the southwest of Phoenix. The prevailing flow advected the chaff clouds to the north and east. Convective storms that occurred in the area likely affected by the dispersing chaff clouds were characterized by little or no CG lightning.

Field studies in the 1970s demonstrated that chaff injected into building thunderstorms markedly decreased CG lightning strikes. There are no data available regarding either the in-cloud lightning character of storms on this day or the technical specifications of the chaff being used in military aircraft anti–electronic warfare systems. However, it is hypothesized that this case of severe, but low-lightning, convective storms resulted from inadvertent lightning suppression over south-central Arizona due to an extended period of numerous chaff releases over the military ranges.

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Steven V. Vasiloff, Kenneth W. Howard, and Jian Zhang

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The principal source of information for operational flash flood monitoring and warning issuance is weather radar–based quantitative estimates of precipitation. Rain gauges are considered truth for the purposes of validating and calibrating real-time radar-derived precipitation data, both in a real-time sense and climatologically. This paper examines various uncertainties and challenges involved with using radar and rain gauge data in a severe local storm environment. A series of severe thunderstorm systems that occurred across northeastern Montana illustrates various problems with comparing radar precipitation estimates and real-time gauge data, including extreme wind effects, hail, missing gauge data, and radar quality control. Ten radar–gauge time series pairs were analyzed with most found to be not useful for real-time radar calibration. These issues must be carefully considered within the context of ongoing efforts to develop robust real-time tools for evaluating radar–gauge uncertainties. Recommendations are made for radar and gauge data quality control efforts that would benefit the operational use of gauge data.

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Kenneth W. Howard, Jonathan J. Gourley, and Robert A. Maddox

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Radar measurement uncertainties associated with storm top, cloud top, and other height measurements are well recognized; however, the authors feel the resulting impacts on the trends of storm features are not as well documented or understood by some users of the WSR-88D system. Detailed examination of radar-measured life cycles of thunderstorms occurring in Arizona indicates substantial limitations in the WSR-88D’s capability to depict certain aspects of storm-height attribute evolution (i.e., life cycle) accurately. These inherent limitations are illustrated using a vertical reflectivity structure model for the life cycle of a simple, “single-pulse” thunderstorm. The life cycle of this simple storm is “scanned” at varying ranges and translation speeds. The results show that radar-determined trends are often substantially different from those of the model storm and that in extreme cases the radar-detected storm and the model storm can have trends in storm-top height of opposite sign. Caution is clearly required by both the operational and research users of some products derived from operational WSR-88D data.

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Darren M. McCollum, Robert A. Maddox, and Kenneth W. Howard

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A mesoscale convective system (MCS) developed over central Arizona during the late evening and early morning of 23–24 July 1990 and produced widespread heavy rain, strong winds, and damage to buildings, vehicles, power poles, and trees across northern sections of the Phoenix metropolitan area. Although forecasters from both the National Weather Service and National Severe Storms Laboratory, working together in the 1990 SouthWest Area Monsoon Project (SWAMP), did not expect thunderstorms to develop, severe thunderstorm and flash flood warnings were issued for central Arizona between 0300 and 0500 local standard time. This study examines the precursor and supportive environment of the mesoscale convective system, drawing upon routine synoptic data and special observations gathered during SWAMP.

During the evening of 23 July and the early morning of 24 July, low-level southwesterly flow developed and advected moisture present over southwest Arizona across south-central Arizona into the foothills and mountains north and northeast of Phoenix. The increase in moisture produced substantial convective instability in an environment that had been quite stable during the late afternoon. Thunderstorms rapidly developed as this occurred. Outflow from these thunderstorms likely moved downslope into the lower deserts of central Arizona, helping to initiate additional convection. The most persistent convective activity developed within a region of low-level convergence between a pronounced mesoscale outflow boundary and the low-level southwesterly flow. The resultant MCS moved to the south-southeast and weakened just south of Phoenix, while its outflow apparently forced new thunderstorm development north of Tucson.

The operational sounding and surface observation network in Arizona failed to detect important mesoscale circulations and thermodynamic gradients that contributed to the occurrence of the severe weather over central Arizona. In this case, conditions favorable for severe thunderstorms developed rapidly, over a period of a few hours. Large-scale analyses provided little insight into the causes of this particular severe weather event. Higher time and space resolution observational data may be needed to improve forecasts of some severe weather events over the Phoenix area.

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Robert A. Maddox, Darren M. McCollum, and Kenneth W. Howard

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Severe thunderstorms are relatively rare over Arizona and occur most frequently during the summer monsoon period, that is, July, August, and early September. Forecasting in Arizona during the summertime is quite difficult and skill scores are low for both precipitation and severe thunderstorm watches and warnings. In the past, due to the sparse population of Arizona, severe thunderstorms usually impacted few people and were considered relatively insignificant events. However, over the last 20 years, the population of central Arizona has grown dramatically, and the impact of severe thunderstorm and flash flood occurrences has also increased.

Synoptic conditions associated with 27 severe thunderstorm events that occurred in central Arizona during the summer monsoon have been examined systematically and compared to long-term mean July conditions. The period of study covered 1978 to 1990, and cases selected were limited to the high population area of central Arizona. McCollum subjectively identified three distinct large-scale patterns (types I, II, and III) that were associated with the severe thunderstorm events. Significant large-scale departures from mean conditions are used to characterize the Arizona severe weather environment for these three pattern types. Significant pattern anomalies tend to be far removed from the state, typically by 1000 to 2000 km. Thus, even though the summertime environment may seem locally stagnant, a large-scale perspective is required to monitor the day to day evolution of the severe weather environment in the Southwest.

The key factor affecting convective instability at lower elevations, that is, in the deserts of central Arizona, is the amount of low-level moisture present. Severe storm conditions are distinctly more moist and unstable than average from the surface to 700 mb. The standard level charts for the severe weather patterns indicate that the Gulf of California plays an important role in providing a source for this moisture.

The summertime severe thunderstorm environment over the southwest United States is distinctly different than central and eastern United States storm settings, which are well known based upon years of study of substantial numbers of events. In general, the environment in which central Arizona severe monsoon thunderstorms occur is one of weak synoptic-scale flow, significant lower- to midtropospheric moisture, and moderate instability. The nature of subsynoptic circulations that initiate and support severe weather over central Arizona is difficult to infer. However, the existence of repetitive, large-scale patterns suggests that forecasting for the general threat of severe summertime thunderstorms can be improved.

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