Showing papers on "Thunderstorm published in 1989"
TL;DR: In this article, the authors investigated the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning and found that IC activity is attributed to the updraft-driven accumulation of graupel particles in the central dipole region, and subsequent CG activity to the descent of ice particles beneath the height of the main negative charge.
Abstract: Thunderstorm case studies and earlier observations are described which illuminate the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning. A consistent temporal evolution starting with peak IC activity changing to predominant CG activity and concluding with strong outflow (microburst) suggests that ice is responsible for both the electrical (i.e., lightning) and dynamical (i.e., microburst) phenomena. The IC activity is attributed to the updraft-driven accumulation of graupel particles in the central dipole region, and the subsequent CG activity to the descent of ice particles beneath the height of the main negative charge. The subsequent descent and melting of ice particles beneath the height of the 0°C isotherm are associated with the acceleration of the downdraft and outflow. The IC lightning precursor can provide a valuable short-term (5–10 min) warning for microburst hazard at ground level.
274 citations
TL;DR: In this article, lightning and Doppler radar data for two tornadic storms in Oklahoma on May 22, 1981 are used to analyze ground flash rates relative to the time of tornadoes.
Abstract: Lightning and Doppler radar data for two tornadic storms in Oklahoma on May 22, 1981 are used to analyze ground flash rates relative to the time of tornadoes. It is found that the ground flash rates had no obvious relationship with the tornado times, although the stroke rate in both storms was greatest after the tornadic stage ended. The variations in the cyclone shear and the intracloud flash rates within 10 km of the mesocyclone region are examined. The results suggest that most tornadic storms have an increase in total flash rates near the time of the tornado and that this increase is often dominated by intracloud flashes.
221 citations
TL;DR: Aircraft, radar, and surface observations were used to study the relationship between precipitation development and the onset of electrification in thunderstorms which formed near or over the Magdalena Mountains of New Mexico.
Abstract: Aircraft, radar, and surface observations were used to study the relationship between precipitation development and the onset of electrification in thunderstorms which formed near or over the Magdalena Mountains of New Mexico. The study included storms which were electrically active as well as ones in which no electrical enhancement was observed. Electric fields inside these clouds showed negligible enhancement and did not exceed 1 kV m−1 until reflectivities at 6 km above mean sea level (msl) (about −10°C) exceeded approximately 40 dBZ and cloud tops exceeded 8 km. The onset of electrification occurred during or immediately after convective growth within the cloud.
125 citations
TL;DR: In this article, the air conductivity and the vertical electric field were measured over thunderstorms from a high-altitude U-2 airplane during the summer of 1986, and the results indicated that the relative efficiency of a thunderstorm to supply current to the global electric circuit has an inverse relationship to the strength of the cloud electrical generator.
Abstract: During the summer of 1986, the air conductivity and the vertical electric field were measured over thunderstorms from a high-altitude U-2 airplane. The conductivity near 20 km was found to be relatively steady above storms with variations less than ±15%. In addition, the positive and negative conductivities were at all times approximately equal and displayed a very similar temporal/spatial behavior. While traversing thunderstorms, sustained upward directed electric fields were maintained along the flight path over distances of 20 to 40 km, and electric fields in excess of 5 kV/m were often observed while passing over very intense storm centers. Estimates of the storm generator current and the Wilson conduction current were obtained for 15 storm over-flights using the current densities derived from the conductivity and field measurements and integrated over area. Area-integrated Maxwell currents of 0.09 to 5.9 A were observed with an average of 2.2 A. The U-2 data show that this measure of storm current varies linearly with flash rate, which suggest that the average charge transfer per discharge is independent of storm development or activity. The Wilson conduction current, which contributes to the global circuit, varied between 0.09 and 3.7 A above storms with an average of 1.7 A. The conduction current was not found to be linearly related to the total flash rate but, instead, leveled off with increasing flash rate. This result may indicate that the relative efficiency of a thunderstorm to supply current to the global electric circuit has an inverse relationship to the strength of the cloud electrical generator.
106 citations
TL;DR: In this paper, a physical model of initiation of lightning flashes by aircraft in thunderstorms, based on the "bi-directional, uncharged leader" concept of Kasemir, is verified with airborne data from lightning strikes to instrumented airplanes (NASA F-106B and FAA CV-580).
Abstract: A physical model of initiation of lightning flashes by aircraft in thunderstorms, based on the “bi-directional, uncharged leader” concept of Kasemir, is verified with airborne data from lightning strikes to instrumented airplanes (NASA F-106B and FAA CV-580). Characteristics of electromagnetic processes during lightning attachment to aircraft are identified with those in negative stepped leaders and positive leaders in natural lightning, in flashes triggered by wire-trailing rockets, and in laboratory discharges. It is shown that (1) a triggered flash starts on aircraft with either a negative corona or a positive leader that depends on the ambient electric field vector and the vehicle form factors; (2) the positive leader with continuous current that increases with time to the level of several hundred amperes is followed in a few milliseconds by the negative stepped leader with current pulses of a few kiloamperes; and (3) the two leaders develop in space simultaneously and bi-directionally from the oppositely charged extremities of the airplane.
90 citations
TL;DR: In this article, the microphysical evolution of an isolated, intense storm observed on July 20, 1988 during the Microburst and Severe Thunderstorm experiment is investigated using radar observations and model results.
Abstract: Radar observations and model results are used to investigate the microphysical evolution of an isolated, intense storm observed on July 20, 1988 during the Microburst and Severe Thunderstorm experiment. The storm grew to a height of 14 km and upon collapsing, produced heavy rain, pea-sized hail, and a microburst at the surface. The radar observations indicate that the initial precipitation development was by collision-coalescence. As the storm intensified, accretional growth became dominant leading to rapid precipitation development. Radar-derived rainfall rates peaked around 150 to 190 mm/h. Each morning during the experiment, a two-dimensional, time-dependent cloud model, initialized with the morning sounding, was run. The model results from the July 20 sounding are compared to the radar observations. Good agreement is shown in some aspects of the storm development, although the numerical simulation predicted a more vigorous storm than actually developed.
87 citations
TL;DR: In this article, a computer algorithm for deriving accurate values of lightning-caused changes in cloud electric fields under active storm conditions was developed and applied to data obtained during two thunderstorms from a network of ground-based electric field mills at the NASA Kennedy Space Center and the Cape Canaveral Air Force Station.
Abstract: A computer algorithm for deriving accurate values of lightning-caused changes in cloud electric fields under active storm conditions was developed and applied to data obtained during two thunderstorms from a network of ground-based electric field mills at the NASA Kennedy Space Center and the Cape Canaveral Air Force Station. The resulting field changes were analyzed using a least-squares optimization procedure and point-charge (Q) and point-dipole (P) models. The values and the time variations of the Q-model parameters under active storm conditions were found to be similar to those reported by Maier and Krider (1986) for small storms, when the computations were carried out with the same analysis criteria and comparable biases. The parameters of the P solutions were found to vary with time within the storm interval and from storm to storm.
86 citations
TL;DR: A note of inquiry published in a magazine read by airplane pilots has elicited 15 new observations of a rare form of lightning that comes out of the cloud top, goes up vertically, and terminates in the clear air above as mentioned in this paper.
Abstract: A note of inquiry published in a magazine read by airplane pilots has elicited 15 new observations of a rare form of lightning that comes out of the cloud top, goes up vertically, and terminates in the clear air above. These confirm previous observations showing that this phenomenon usually occurs above very large and energetic thunderclouds. However, small clouds with tops at 15,000 ft mean sea level (5000 m) have been observed to exhibit this rare form of lightning also. One of the more spectacular observations was made over the severe storm system that produced the devastating tornadoes of April 11, 1965.
77 citations
TL;DR: The present understanding of both natural and artificially initiated (triggered) lightning is reviewed, and suggestions are given for future research that can improve both of the physics of lightning and the parameters that are important for protection.
Abstract: Recent research on lightning has been motivated, in part, by the desire to prevent spectacular accidents, such as occurred in 1969 during the launch of Apollo 12 and in 1987 during the launch of Atlas-Centaur 67, and by the need to protect advanced ground-based and airborne systems that utilize low voltage, solid-state electronics. The present understanding of both natural and artificially initiated (triggered) lightning is reviewed, and suggestions are given for future research that can improve our understanding both of the physics of lightning and the parameters that are important for protection.
72 citations
TL;DR: The Island Thunderstorm Experiment (ITEX) as mentioned in this paper is a field and modeling study of the tropical thunderstorms that form regularly over Bathurst and Melville Islands north of Darwin, Northern Territory, Australia, during the transition season and breaks in the summer monsoon season.
Abstract: The Island Thunderstorm Experiment (ITEX) is a field and modeling study of the tropical thunderstorms that form regularly over Bathurst and Melville Islands north of Darwin, Northern Territory, Australia, during the transition season and breaks in the summer monsoon season. Such thunderstorms are of widespread occurrence in the tropics and they play an important role in tropical dynamics. ITEX is a joint project of the Bureau of Meteorology Research Centre and Monash University's Centre for Dynamical Meteorology. Preliminary studies have been used to plan an intensive period of observations that was carried out from 20 November to 10 December 1988. The resulting data will provide the basis for a series of analytical and numerical studies of tropical island thunderstorms.
61 citations
Journal Article•
TL;DR: In this article, three cases of squall lines composed of strong to severe thunderstorms that formed over Arizona, and Sonora (Mexico) on July, 16-17 and 17-18, 1984, and August 2-3, 1986, are examined.
Abstract: Three cases of squall lines composed of strong to severe thunderstorms that formed over Arizona, and Sonora (Mexico) on July, 16-17 and 17-18, 1984, and August 2-3, 1986, are examined. Data, which included satellite imagery, VISSR-derived fields, surface data, and records or cloud-to-ground lightning strikes, indicate that the initiation, growth, and dissipation of all three squall lines were very similar. Results indicate that these mesoscale convective systems developed in an environment of relatively strong low-level shear with very weak shear aloft and that they possessed almost all the properties of a typical tropical squall line.
TL;DR: In this article, the results of electric field measurements made within a severe thunderstorm in central Oklahoma on June 13, 1983 have been presented, showing the electrical structure of a severe storm anvil to be more complex than the simple structure of positive interior and thin, negative screening layers.
Abstract: This paper presents the results of electric field measurements made within a severe thunderstorm in central Oklahoma on June 13, 1983. The observations show the electrical structure of a severe storm anvil to be more complex than the simple structure of a positive interior and thin, negative screening layers. Evidence is found of extensive regions of both positive and negative charge in the interior of the anvil, and the thicknesses of screening layers are found to be about an order of magnitude greater than calculated in previous models. A conceptual model for the development of the electrical structure of thunderstorm anvils is proposed in which anvil dynamics plays an important role.
TL;DR: In this article, the correctness of the polarity assignments as determined by the LLP hardware used in the SUNYA lightning detection network was investigated. But the authors concluded that present acceptance criteria are adequate, provided that distances from strokes to stations do not exceed 600 km.
Abstract: Lightning stroke occurrences often appear to have been sorted polarity-wise in both space and time, especially in winter storms. This phenomenon raises a number of important questions related to charge separation in meteorological environments which may favor one polarity over the other. If the lightning locating hardware is correctly identifying ground stroke polarities, then additional considerations need be given to charging processes that are operative when the main convective phase of the thunderstorm is over. Clearly, a check on the validity of the network polarity determinations is warranted. We have studied the correctness of the polarity assignments as determined by the LLP hardware used in the SUNYA lightning detection network. From a very limited sample of waveforms recorded during the winters of 1986–1987 and 1987–1988, we conclude that present acceptance criteria are adequate, provided that distances from strokes to stations do not exceed 600 km. Polarity determinations with present algorithms can often be incorrect for strokes beyond 700 km as a result of the misidentification of the ionospheric reflection for the strongly attenuated ground wave.
TL;DR: In this article, the vertical component of the precipitation particle velocity was accurately measured, in conjunction with the vertical air velocity, obtained by integration of the continuity equation, the particle terminal velocity field was computed.
Abstract: Observations were made of a thunderstorm over Langmuir Laboratory for Atmospheric Research on 2 August 1984. Four Doppler radars were closely spaced, allowing observations to be taken at high elevation angles. As a result, the vertical component of the precipitation particle velocity was accurately measured. In conjunction with the vertical air velocity, obtained by integration of the continuity equation, the particle terminal velocity field was computed.
Backward integration of particle trajectories showed that most precipitation embryos originated near the tops of thunderstorm precursor clouds when these cloud tops were 7–8 km above sea level. Early and mature phase precipitation grew by accretion at levels below 7 km, and resulted in relatively high density graupel or hail. Late phase precipitation underwent growth mainly above 8 km, and was sufficiently small in diameter and low in density to undergo rapid melting at the freezing level.
TL;DR: The evolution of criteria for nontornadic severe thunderstorms from undefined categories and general terms to the present day definition for this class of thunderstorm is presented in this article, where major historical events in the development of the Severe Local Storms Unit (SELS) which directly or indirectly influenced changes in the criteria are included.
Abstract: The evolution of criteria for nontornadic severe thunderstorms from undefined categories and general terms to the present day definition for this class of thunderstorm is presented. Major historical events in the development of the Severe Local Storms Unit (SELS) which directly or indirectly influenced changes in the criteria are included.
TL;DR: In this paper, the vector electric field and polar conductivities were measured by zero-pressure balloon-borne payloads launched from Wallops Island, Virgina during the summers of 1987 and 1988.
Abstract: The vector electric field and polar conductivities were measured by zero-pressure balloon-borne payloads launched from Wallops Island, Virgina during the summers of 1987 and 1988. Data were collected over thunderstorms (or electrified clouds) during 6-hour flights at altitudes near 30 km. The vector electric field measurements were made with the double Langmuir probe high-impedance method, and the direct conductivity measurements were obtained with the relaxation technique. Evidence is presented for conductivity variations over thunderstorms (or electrified clouds). It is found that both positive and negative polar conductivity data do show variations of up to a factor of 2 from ambient values associated with the disturbed periods. Some ideas for possible physical mechanisms which may be responsible for the conductivity variations over thunderstorms are also discussed in this paper.
TL;DR: In this paper, the amount of precipitation recorded during individual thunderstorms at 220 stations in the contiguous United States of America for a 30-year period was determined by use of the incomplete gamma distribution.
Abstract: The amount of precipitation recorded during individual thunderstorms was determined at 220 stations in the contiguous United States of America for a 30-year period. The probability distribution of thunderstorm rainfall amounts at each station during each season was summarized by use of the incomplete gamma distribution. The incomplete gamma distribution is completely described by two parameters, which were used in a clustering routine to determine whether regionalizing tendencies exist in the probability distributions. Large-scale regions were found when the results of the clustering were mapped. In general, these regions corresponded well with the areas of dominance of many of the proposed mechanisms for thunderstorm development.
TL;DR: In this article, a boundary-layer solitary wave sensed with the National Severe Storms Laboratory's Doppler weather radar and a 444m-tall instrumented tower suggest that solitary and other nonlinear waves are a source of significant wind shear hazard to safe flight.
Abstract: Observations of a boundary-layer solitary wave sensed with the National Severe Storms Laboratory's Doppler weather radar and a 444-m-tall instrumented tower suggest that solitary and other nonlinear waves are a source of significant wind shear hazard to safe flight and thus should be studied both experimentally and theoretically. Wave transport of the horizontal momentum of the vertically sheared ambient air contributed much to the observed wind perturbations and horizontal wind shear. Observations are compared with, and shown to agree fairly closely with, the waveform predicted by steady state, weakly nonlinear, internal wave theory.
TL;DR: In this paper, a sounding released into the updraft of a developing cumulonimbus south of Gruver, Texas by a University of Oklahoma storn-intercept team indicated a nearly pseudo-moist-adiabatic lapse rate, a temperature excess of 4°6°C over the environment at 500 mb, and updraft speeds of 35-40 m s−1 between 6 and 7 km AGL, in good agreement with parcel theory.
Abstract: During the afternoon of 25 May 1987, thunderstorms, some of which were tornadic and produced large hail, formed near a dryline in the Texas Panhandle. This note discusses a sounding released into the updraft of a developing cumulonimbus south of Gruver, Texas by a University of Oklahoma storn-intercept team. The sounding indicated a nearly pseudo-moist-adiabatic lapse rate, a temperature excess of 4°–6°C over the environment at 500 mb, and updraft speeds of 35–40 m s−1between 6 and 7 km AGL, in good agreement with parcel theory. A radar-observed fine line appeared along the dryline and retreated westward, as was also noted in two earlier case studies.
TL;DR: In this article, data characterizing the updraft structures and hydrometers observed in moderate thunderstorms by the armored T-28 aircraft during the 1986 Cooperative Huntsville Meteorological Experiment (COHMEX) are presented.
TL;DR: In this article, electric field measurements were made by a sailplane inside thunderstorms near Langmuir Laboratory and the Magdalena Mountains in central New Mexico, where the dominant charge was negative and associated with a local reflectivity core.
Abstract: Electric field measurements were made by a sailplane inside thunderstorms near Langmuir Laboratory and the Magdalena Mountains in central New Mexico. The continuity of these measurements in-cloud allows us to deduce electric field growth rates in six cases ranging from initial electrification through the production of lightning. The electric field data are combined with radar reflectivity data to estimate charge locations and magnitudes. In each case, the dominant charge was negative, associated with a local reflectivity core, and demonstrated a cellular structure centered at altitudes ranging from 5.5 to 7.3 km mean sea level (−3 to −15° C). Three distinct phases characterize the electric field measurements during initial electrification: an early stage of slowly increasing fields that largely could be explained by the sailplane motion toward the estimated charges, followed by a rapid exponential growth period, and then a plateau in electric field growth rate (at about 300 V m−1 s−1) after the rapid growth period. These six cases extend earlier studies to further establish the character and magnitude of electric field growth during the early stages of thunderstorm electrification.
TL;DR: In this paper, numerical simulations based on a three-dimensional model for the electric fields in a thunderstorm are presented, where the authors solve problems with known analytical solutions in order to determine the relevant physical properties that must be incorporated in a storm model.
Abstract: Numerical simulations based on a three-dimensional model for the electric fields in a thunderstorm are presented. In some of the simulations, we solve problems with known analytical solutions in order to determine the relevant physical properties that must be incorporated in a thunderstorm model. We then examine the inverse problem: Given measurements for the electric fields in a thunderstorm what are the associated current generators? Fits based on an analytic formula that neglects conduction currents give approximations to the current generators while simulations based on the thunderstorm model yield refinements to the generators. As a specific illustration, we obtain estimates for current generators associated with a storm observed at the Kennedy Space Center on 11 July 1978. Finally, we explore qualitative properties of our method used to simulate lightning. It is observed that as the charged particles associated with the thunderstorm are spread over a larger and larger volume, the flesh rate...
TL;DR: In this paper, photographs have been taken at night from an airplane at an altitude of 20 km looking directly down on the tops of thunderclouds illuminated by lightning, and the hard, cauliflower-like appearance of the clouds gives evidence that strong convective activity is present.
Abstract: Photographs have been taken at night from an airplane at an altitude of 20 km looking directly down on the tops of thunderclouds illuminated by lightning. The hard, cauliflower-like appearance of the clouds gives evidence that strong convective activity is present. In one case a well-organized system of convective structures is evident whose deepest folds, apparently caused by downdrafts, are estimated to extend into the cloud for depths of as much as several kilometers. Often the whole cloud top, approximately 10 km across, is diffusely illuminated by lightning that is occurring lower in the cloud. In most of these cases no lightning channels can be seen, but occasionally a few segments of channels are visible bridging the folds between the convective protuberances. A few photographs show thin, weak, lightning channels that come out of the top of the cloud, proceed horizontally for several hundred meters, and then terminate in the clear air above the cloud. When such channels can be seen, the ba...
TL;DR: In this paper, a study of more than 200 tornado events was conducted and it was shown that Tornado intensity is best related to the magnitude of the vertical wind shear in the layer between the ground and 600 hPa.
Abstract: Although violent tornadoes comprise only 2.3% of tornado occurrences in the United States, they cause 68% of the fatalities attributed to tornadoes and severe thunderstorms. Despite these statistics, no attempt is made to forecast or warn of tornado intensity. A basis for this is provided in a study of more than 200 tornado events. Tornado intensity is best related to the magnitude of the vertical wind shear in the layer between the ground and 600 hPa. The results for this and other shear parameters are statistically highly significant when the data are grouped into two or three intensity classifications. Stability, as indicated by the “lifted index,” and midtropospheric relative humidity, correlate poorly with tornado intensity. The relationships established have application to tornado forecasting and nowcasting.
TL;DR: In this paper, the temporal and spatial relationships between thunderstorms and flashes were investigated using data for 1983-85 for 25 first-order stations (10 in the West and 15 along the East Coast).
Abstract: Temporal and spatial relationships between thunderstorms (events) and flashes were investigated using data for 1983–85 for 25 first-order stations (10 in the West and 15 along the East Coast). Thunder events were compared with flashes within three ranges: 5 km, 10 km, and 20 km, around each station. Cluster analysis revealed six geographic regions: Florida, Southeast (South Carolina, Georgia), Mid-Atlantic (Virginia, Maryland Pennsylvania), Northeast (New York and New England), Rocky Mountains, and an intermontane area. Periods of multiple flashes not within thunder events and within 10 km of a point (most realistic for audibility), revealed that 10% to 20% (depending upon region) of all thunderstorms were missed. Also, 13% (Rockies) to 44% (Mid-Atlantic) of all thunderstorms have recorded durations too short (missed flashes before their reported start), and the average underestimated durations were from 55% (Northeast Mid-Atlantic) to 26% (Rockies). Flashes isolated in time and space, due to loc...
01 Sep 1989
TL;DR: In this article, a model was developed to identify the strength, size, and spacing of vortex arrays and to study the effects of severe wind hazards on operational safety for different types of aircraft.
Abstract: Advanced methods were developed to determine time varying winds and turbulence from digital flight data recorders carried aboard modern airliners. Analysis of several cases involving severe clear air turbulence encounters at cruise altitudes has shown that the aircraft encountered vortex arrays generated by destabilized wind shear layers above mountains or thunderstorms. A model was developed to identify the strength, size, and spacing of vortex arrays. This model is used to study the effects of severe wind hazards on operational safety for different types of aircraft. The study demonstrates that small remotely piloted vehicles and executive aircraft exhibit more violent behavior than do large airliners during encounters with high-altitude vortices. Analysis of digital flight data from the accident at Dallas/Ft. Worth in 1985 indicates that the aircraft encountered a microburst with rapidly changing winds embedded in a strong outflow near the ground. A multiple-vortex-ring model was developed to represent the microburst wind pattern. This model can be used in flight simulators to better understand the control problems in severe microburst encounters.
TL;DR: In this article, a radar climatology of convective precipitation for New Jersey during the summer season (May-September) was developed using manually digitized radar (MDR) data from four local radar sites.
Abstract: A radar climatology of convective precipitation for New Jersey during the summer season (May-September) was developed using manually digitized radar (MDR) data from four local radar sites. Convective precipitation frequencies were determined by tabulating occurrences of echo intensity levels equal to 2 or more for each of 63 grid boxes located in and around New Jersey. Convective precipitation frequencies over a 5-year period (1978–1982) were plotted by grid box, and isopleths of duration and intensity level were constructed. The distribution of the frequency of occurrence of convective activity was examined for thunderstorm (level 3 or more) occurrence and severe thunderstorm (level 5 or more) occurrence. The highest convective frequencies occurred over south-eastern Pennsylvania while the greatest frequency of more intense convection occurred over south-central New Jersey. An analysis of variance (ANOVA) was performed on the data set to determine the relative variability of convective precipitation in the grid. Year-to-year and week-to-week variability was high whilst intergrid-box variability was low. Duncan's multiple range test (DMRT) was applied to compare individual years to each other, and individual weeks to each other. When grid boxes were assigned to four climatological regions and DMRT applied, some regions were found to be significantly different from one another. No significant differences were found between grid boxes within each region. There was no interaction between grid regions and years as differences in mean convective activity between regions remained generally constant from year to year. The distribution of convective activity over New Jersey was found to be determined by local climatological factors while the total frequency from year to year was determined by the mean synoptic pattern.
TL;DR: The case of a tornadic thunderstorm on 26 July 1985 in northeastern Colorado is described from the synoptic to the thunderstorm scale utilizing a number of datasets some of which will become operational in the 1990s as discussed by the authors.
Abstract: The case of a tornadic thunderstorm on 26 July 1985 in northeastern Colorado is described from the synoptic to the thunderstorm scale utilizing a number of datasets some of which will become operational in the 1990s. The available data included profilers, Doppler radar, surface mesonet, satellite, and special soundings. Although the synoptic environment did not favor tornadic thunderstorms, strong thunderstorms formed in localized area during a 2-h period in the afternoon and produced an 18-min tornado. A number of events took place to produce the stronger then anticipated development, including interaction among mesoscale outflow and stationary boundaries. Of particular importance was the change in the local environment along a stationary boundary known as the Denver Convergence-Vorticity Zone. Special soundings taken near the stationary boundary revealed a deepening moist layer over time in association with the convergent wind field. Additional forcing from the collision of this boundary with o...