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Harold E. Brooks and Charles A. Doswell III

Abstract

The authors have carried out verification of 590 12–24-h high-temperature forecasts from numerical guidance products and human forecasters for Oklahoma City, Oklahoma, using both a measures-oriented verification scheme and a distributions-oriented scheme. The latter captures the richness associated with the relationship of forecasts and observations, providing insight into strengths and weaknesses of the forecasting systems, and showing areas in which improvement in accuracy can be obtained.

The analysis of this single forecast element at one lead time shows the amount of information available from a distributions-oriented verification scheme. In order to obtain a complete picture of the overall state of forecasting, it would be necessary to verify all elements at all lead times. The authors urge the development of such a national verification scheme as soon as possible, since without it, it will be impossible to monitor changes in the quality of forecasts and forecasting systems in the future.

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Charles A. Doswell III and Harold E. Brooks

Abstract

After the tornadoes of 3 May 1999, the Federal Emergency Management Agency formed a Building Performance Assessment Team (BPAT) to examine the main tornado paths during the outbreak and to make recommendations based on the damage they saw. This is the first time a tornado disaster has been subjected to a BPAT investigation. Some aspects of the BPAT final report are reviewed and considered in the context of tornado preparedness in Kansas and Oklahoma. Although the preparedness efforts of many public and private institutions apparently played a large role in reducing casualties from the storm, a number of building deficiencies were found during the BPAT's evaluation. Especially in public facilities, there are several aspects of tornado preparedness that could be improved. Moreover, there is clear evidence that a nonnegligible fraction of the damage associated with these storms could have been mitigated with some relatively simple and inexpensive construction enhancements. Widespread implementation of these enhancements would reduce projectile loading and its associated threats to both life and property.

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R. Jeffrey Trapp and Charles A. Doswell III

Abstract

The spherical geometry of weather radar scans results in a data distribution wherein datapoint separation in one coordinate direction and/or in one part of the analysis domain can differ widely from that in another. Objective analysis of the nonuniform radar data to a uniform Cartesian grid is desirable for many diagnostic purposes. For the benefit of the diagnostic data analyst as well as of users of these analyses, the authors evaluate properties of techniques typically used for such objective analysis. This is done partly through theoretical consideration of the properties of the schemes, but mostly by empirical testing. In terms of preservation of the phase and amplitude of the input data, predictability of the degree of smoothing and filtering, and relative insensitivity to input data unsteadiness or spatial characteristic, the isotropic Gaussian or Barnes-type weight function with constant smoothing parameter appears to be the most desirable of the schemes considered. Modification of this scheme so that the weight function varies spatially, with the datapoint spacing, results in an improved analysis, according to some commonly used measures of error. Interpretation of analyses based on such a modified scheme can be affected, however. For example, analyses of unsteady input fields suffer from a convolution of the temporal evolution of the data with spatial variations of the weight function. As a consequence, unambiguous assessment of physical evolution is precluded.

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Charles A. Doswell III and Harold E. Brooks

Abstract

The authors discuss the relationship between budget-cutting exercises and knowledge of the value of weather services. The complex interaction between quality (accuracy) and value of weather forecasts prevents theoretical approaches from contributing much to the discussion, except perhaps to indicate some of the sources for its complexity. The absence of comprehensive theoretical answers indicates the importance of empirical determinations of forecast value; as it stands, the United States is poorly equipped to make intelligent decisions in the current and future budget situations. To obtain credible empirical answers, forecasters will need to develop closer working relationships with their users than ever before, seeking specific information regarding economic value of forecasts. Some suggestions for developing plausible value estimates are offered, based largely on limited studies already in the literature. Efforts to create closer ties between forecasters and users can yield diverse benefits, including the desired credible estimates of the value of forecasts, as well as estimates of the sensitivity of that value to changes in accuracy of the forecasts. The authors argue for the development of an infrastructure to make these empirical value estimates, as a critical need within weather forecasting agencies, public and private, in view of continuing budget pressures.

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Matthew J. Bunkers and Charles A. Doswell III
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Jeffry S. Evans and Charles A. Doswell III

Abstract

Observed upper air soundings that occurred within 2 h and 167 km of derechos were collected and analyzed to document atmospheric stability and wind shear conditions associated with long-lived convective windstorms. Sixty-seven derechos, accompanied by 113 proximity soundings, were identified during the years 1983–93. Owing to the large variability of the synoptic-scale environments associated with derechos, each derecho was further divided into categories based on the strength of synoptic-scale forcing associated with each event.

Derechos are shown to develop and persist in a wide range of shear and instability conditions. Although this range of shear and instability narrows when derechos are grouped by synoptic-scale forcing strength, considerable variation of values remains, primarily with the shear. These results suggest that ambient shear and instability values alone are not sufficient to distinguish derecho environments from those associated with nonsevere mesoscale convective system (MCS) environments. Though the ground-relative (ambient) shear values vary, more consistency is found in the system-relative winds. It is found that midlevel system-relative winds are consistently weak, while low-level system-relative inflow is strong. This is especially valid for events associated with weak synoptic-scale forcing. Fast forward propagation is associated with weak system-relative midlevel winds, which may in turn be associated with outflow-dominated storms having strong cold pools at the ground. In addition, a comparison between weak forcing derecho and nonderecho MCSs suggests that it is the strength of the mean flow, and its possible effects on speed of movement, that enhance the potential for sustained severe wind gusts at the surface, given similar thermodynamic environments.

These results indicate that there is a larger range of shear and instability environments associated with derechos than has been suggested by some observational studies and numerical cloud simulations. In addition, little correlation is evident in operational observations between events with strong cold pools and those with correspondingly strong low-level (0–2/0–3 km) ambient shear. Owing to the large variance of convective available potential energy (CAPE)/shear values found in this observational study, forecasters should be aware of the potential for derecho formation within environments with weaker shear than suggested by numerical simulations. This is especially true in the absence of a deep, progressive midtropospheric trough and an associated surface cold front. When synoptic-scale forcing is strong, derechos can develop and persist within environments with almost no CAPE evident on nearby upper air soundings.

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Charles A. Doswell III and Erik N. Rasmussen

Abstract

A simple theoretical analysis of the impact of neglecting the virtual correction on calculation of CAPE is made. This theory suggests that while ignoring the virtual correction does not introduce much error for large CAPE values, the relative error can become substantial for small CAPE. A test of the theory is done by finding the error made by ignoring the virtual correction to CAPE for all the soundings in 1992 having positive CAPE (when the correction is made). Results of this empirical test confirm that the relative error made in ignoring the correction increases with decreasing CAPE. A number of other “corrections” to CAPE might be considered. In a discussion of the issues associated with the results of the analysis, it is recommended that CAPE calculations should include the virtual correction but that other complications should be avoided for most purposes, especially when making comparisons of CAPE values. A standardized CAPE calculation also is recommended.

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Harold E. Brooks and Charles A. Doswell III

Abstract

Historical records of damage from major tornadoes in the United States are taken and adjusted for inflation and wealth. Such adjustments provide a more reliable method to compare losses over time in the context of significant societal change. From 1890 to 1999, the costliest tornado on the record, adjusted for inflation, is the 3 May 1999 Oklahoma City tornado, with an adjusted $963 million in damage (constant 1997 dollars). Including an adjustment for growth in wealth, on the other hand, clearly shows the 27 May 1896 Saint Louis–East Saint Louis tornado to be the costliest on record. An extremely conservative adjustment for the 1896 tornado gives a value of $2.2 billion. A more realistic adjustment yields a figure of $2.9 billion. A comparison of the ratio of deaths to wealth-adjusted damage shows a clear break in 1953, at the beginning of the watch/warning/awareness program of the National Weather Service.

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Joseph T. Schaefer and Charles A. Doswell III

Abstract

By its very nature, interpolation in a vector field is ambiguous, owing to the somewhat arbitrary nature of the vector norm. Since a two-dimensional vector field cm be specified by two scalar quantities. which can be separately interpolated, the ambiguity can be resolved by forcing the interpolated wind field to preserve the vorticity and divergence fields associated with the raw data. A method to calculate divergence and vorticity directly from randomly spaced wind observations is developed and, using analytically generated data, shown to produce more accurate results than conventional computations. Two methods of retrieving the wind field from the analysed scalar fields are presented and also tested on the analytic field. Finally, total analysis, from wind observations to gridded wind fields, is demonstrated on real meteorological data.

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Charles A. Doswell III and Donald W. Burgess

Abstract

A continuing problem in dealing with climatology data concerning tornadoes in the United States is the validity of the quantitative information contained in the various available data bases. Two aspects of tornado data are discussed: the F-scale rating and the occurrence of very long path length events. The argument is advanced that the F-scale is more properly thought of as a damage scale than as an intensity scale. Failing to recognize this leads to confusion and controversy regarding the F-scale ratings assigned to events in the data base.

Changing perceptions of tornadoes have led to some questions concerning the actual frequency of very long path lengths, on the order of 100 statute miles (160.9 km) or more. Evidence is presented that at least some of the events classified as having long tracks are most likely the result of misinterpreting the results of a series of short-path tornadoes, produced by a single supercell thunderstorm.

Some discussion is presented concerning the implications of the problems with the data. Since the climatological record is of both meteorological and societal concern, some alternatives are considered, but no hard conclusions can be drawn without considerable further effort.

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