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Robert A. Maddox and Kenneth W. Howard

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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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Clinton E. Wallace, Robert A. Maddox, and Kenneth W. Howard

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The daily evolution of local surface conditions at Phoenix, Arizona, and the characteristics of the 1200 UTC sounding at Tucson, Arizona, have been examined to determine important meteorological features that lead to thunderstorm occurrence over the low deserts of central Arizona. Each day of July and August during the period 1990–95 has been stratified based upon daily mean, surface moisture conditions at Phoenix, Arizona, and the occurrence of afternoon and evening convective activity in the Phoenix metropolitan area. The nearest operational sounding, taken 160 km to the southeast at Tucson, is shown to be not representative of low-level thermodynamic conditions in central Arizona. Thus, Phoenix forecasters’ ability to identify precursor conditions for the development of thunderstorms is impaired. On days that convective storms occur in the Phoenix area, there is a decrease in the diurnal amplitude of surface dewpoint changes, signifying increased/deeper boundary layer moisture. This signal is very subtle and may not have much forecast utility. Additionally, it is found that surges of moist air from the Gulf of California do not occur frequently during the 36–48 h immediately prior to thunderstorm events in the Phoenix area. It is shown that the 1200 UTC Tucson wind profile has a significant northerly flow in low levels on moist days when storms do not occur in the Phoenix area. The forecaster needs information on the local temperature and moisture profile to assess the potential for thunderstorms in the Phoenix area. However, routine upper-air observations are unavailable. Steps are being taken to obtain morning soundings in Phoenix, and the improving capabilities of satellite-derived thermodynamic data and mesoscale models may also provide the forecaster critical information in the future. The findings, although specifically developed for the Phoenix area, may be relevant to thunderstorm forecasting in many regions of the interior West.

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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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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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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, Jian Zhang, Jonathan J. Gourley, and Kenneth W. Howard

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Terrain and radar beam-elevation data are used to examine the spatial coverage provided by the national operational network of Doppler weather radars. This information is of importance to a wide variety of users, and potential users, of radar data from the national network. Charts generated for radar coverage at 3 and 5 km above mean sea level show that radar surveillance near 700 and 500 hPa is very limited for some portions of the contiguous United States. Radar coverage charts at heights of 1, 2, and 3 km above ground level illustrate the extent of low-level radar data gathered above the actual land surface. These maps indicate how restricted the national radar network coverage is at low levels, which limits the usefulness of the radar data, especially for quantitative precipitation estimation. The analyses also identify several regions of the contiguous United States in which weather phenomena are sampled by many adjacent radars. Thus, these regions are characterized by very comprehensive radar information that could be used in many kinds of research studies.

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Katherine M. Willingham, Elizabeth J. Thompson, Kenneth W. Howard, and Charles L. Dempsey

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During the 2008 North American monsoon season, 140 microburst events were identified in Phoenix, Arizona, and the surrounding Sonoran Desert. The Sonoran microbursts were studied and examined for their frequency and characteristics, as observed from data collected from three Doppler radars and electrical power infrastructure damage reports. Sonoran microburst events were wet microbursts and occurred most frequently in the evening hours (1900–2100 local time). Stronger maximum differential velocities (20–25 m s−1) were observed more frequently in Sonoran microbursts than in many previously documented microbursts. Alignment of Doppler radar data to reports of wind-related damage to electrical power infrastructure in Phoenix allowed a comparison of microburst wind damage versus gust-front wind damage. For these damage reports, microburst winds caused more significant damage than gust-front winds.

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