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Charles A. Doswell III, Harold E. Brooks, and Michael P. Kay

Abstract

The probability of nontornadic severe weather event reports near any location in the United States for any day of the year has been estimated. Gaussian smoothers in space and time have been applied to the observed record of severe thunderstorm occurrence from 1980 to 1994 to produce daily maps and annual cycles at any point. Many aspects of this climatology have been identified in previous work, but the method allows for the consideration of the record in several new ways. A review of the raw data, broken down in various ways, reveals that numerous nonmeteorological artifacts are present in the raw data. These are predominantly associated with the marginal nontornadic severe thunderstorm events, including an enormous growth in the number of severe weather reports since the mid-1950s. Much of this growth may be associated with a drive to improve warning verification scores. The smoothed spatial and temporal distributions of the probability of nontornadic severe thunderstorm events are presented in several ways. The distribution of significant nontornadic severe thunderstorm reports (wind speeds ≥ 65 kt and/or hailstone diameters ≥ 2 in.) is consistent with the hypothesis that supercells are responsible for the majority of such reports.

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Charles A. Doswell III, Robert Davies-Jones, and David L. Keller

Abstract

The so-called True Skill Statistic (TSS) and the Heidke Skill Score (S), as used in the context of the contingency, table approach to forecast verification, are compared. It is shown that the TSS approaches the Probability of Detection (POD) whenever the forecasting is dominated by correct forecasts of non-occurrence, i.e., forecasting rare events like severe local storms. This means that the TSS is vulnerable to “hedging” in rare event forecasting. The S-statistic is shown to be superior to the TSS in this situation, accounting for correct forecasts of null events in a controlled fashion. It turns out that the TSS and S values are related in a subtle way, becoming identical when the expected values (due to chance in a k × k contingency table) remain unchanged when comparing the actual forecast table to that of a hypothetical perfect set of forecasts. Examples of the behavior of the TSS and S values in different situations are provided which support the recommendation that S be used in preference to TSS for rare event forecasting. A geometrical interpretation is also given for certain aspects of the 2 × 2 contingency table and this is generalized to the k × l case. Using this geometrical interpretation, it is shown to be possible to apply dichotomous verification techniques in polychotomous situations, thus allowing a direct comparison between dichotomous and polychotomous forecasting.

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Charles A. Doswell III, Clemente Ramis, Romualdo Romero, and Sergio Alonso

Abstract

A diagnostic evaluation of three project ANOMALIA case studies involving heavy precipitation in the western Mediterranean region has been carried out. The evaluation shows the unique characteristics of each event, as well as some limited similarities. Heavy precipitation events in the western Mediterranean region typically occur downstream of a significant cyclone aloft (often, but not always, exhibiting “cutoff” cyclone characteristics), but important structural and evolutionary differences are found among these cases. At low levels, a long fetch of flow over the Mediterranean Sea frequently interacts with terrain features to produce persistent heavy precipitation. Although most heavy precipitation events occur during the fall season, they can develop at other times. In the first case, the synoptic-scale environment produced low static stability and substantial storm-relative environmental helicity, thereby supporting both heavy rain in the vicinity of Valencia on mainland Spain and on Ibiza in the Balearic Islands, as well as a tornado at Menorca in the Balearic Islands on 7–9 October 1992. The second case involved a slow-moving cyclone that destabilized the stratification and produced several days of heavy precipitation over the period 31 January–6 February 1993. In the third case, in the Italian Piedmont region on 5–6 November 1994, the heavy precipitation included a nonconvective component, with moist but relatively stable air impinging on steep terrain gradients.

A set of basic diagnostic tools is applied to the cases, and it is shown that anything but a superficial diagnosis of each case requires flexibility in selecting diagnostic tools. The ways by which heavy precipitation is created can vary substantially from case to case and in different parts of the world; however, there is a common thread. Heavy precipitation is the result of moist, low-level air ascending rapidly, so any diagnosis aimed at forecasting heavy precipitation needs to address the following: vertical motion, static stability, moisture supply, and orographic effects (when appropriate). Forecasting implications of the cases are discussed, with the emphasis on considering these physically relevant processes.

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Ivan Tsonevsky, Charles A. Doswell III, and Harold E. Brooks

Abstract

ECMWF provides the ensemble-based extreme forecast index (EFI) and shift of tails (SOT) products to facilitate forecasting severe weather in the medium range. Exploiting the ingredients-based method of forecasting deep moist convection, two parameters, convective available potential energy (CAPE) and a composite CAPE–shear parameter, have been recently added to the EFI/SOT, targeting severe convective weather. Verification results based on the area under the relative operating characteristic curve (ROCA) show high skill of both EFIs at discriminating between severe and nonsevere convection in the medium range over Europe and the United States. In the first 7 days of the forecast ROCA values show significant skill, staying well above the no-skill threshold of 0.5. Two case studies are presented to give some practical considerations and discuss certain limitations of the EFI/SOT forecasts and how they could be overcome. In particular, both convective EFI/SOT products are good at providing guidance for where and when severe convection is possible if there is sufficient lift for convective initiation. Probability of precipitation is suggested as a suitable ensemble product for assessing whether convection is likely to be initiated. The model climate should also be considered when determining whether severe convection is possible; EFI and SOT values are related to the climatological frequency of occurrence of deep, moist convection over a given place and time of year.

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Charles A. Doswell III, Alan R. Moller, and Harold E. Brooks

Abstract

The history of storm spotting and public awareness of the tornado threat is reviewed. It is shown that a downward trend in fatalities apparently began after the famous “Tri-State” tornado of 1925. Storm spotting’s history begins in World War II as an effort to protect the nation’s military installations, but became a public service with the resumption of public tornado forecasting, pioneered in 1948 by the Air Force’s Fawbush and Miller and begun in the public sector in 1952. The current spotter program, known generally as SKYWARN, is a civilian-based volunteer organization. Responsibility for spotter training has rested with the national forecasting services (originally, the Weather Bureau and now the National Weather Service). That training has evolved with (a) the proliferation of widespread film and (recently) video footage of severe storms; (b) growth in the scientific knowledge about tornadoes and tornadic storms, as well as a better understanding of how tornadoes produce damage; and (c) the inception and growth of scientific and hobbyist storm chasing.

The concept of an integrated warning system is presented in detail, and considered in light of past and present accomplishments and what needs to be done in the future to maintain the downward trend in fatalities. As the integrated warning system has evolved over its history, it has become clear that volunteer spotters and the public forecasting services need to be closely tied. Further, public information dissemination is a major factor in an integrated warning service; warnings and forecasts that do not reach the users and produce appropriate responses are not very valuable, even if they are accurate and timely. The history of the integration has been somewhat checkered, but compelling evidence of the overall efficacy of the watch–warning program can be found in the maintenance of the downward trend in annual fatalities that began in 1925.

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Harold E. Brooks, Charles A. Doswell III, and Jeremy Cooper

Abstract

The authors investigated differences in the environments associated with tornadic and nontornadic mesocyclones are investigated using proximity soundings. Questions about the definition of proximity are raised. As the environments of severe storms with high spatial and temporal resolution are observed, the operational meaning of proximity becomes less clear. Thus the exploration of the proximity dataset is subject to certain caveats that are presented in some detail.

Results from this relatively small proximity dataset support a recently developed conceptual model of the development and maintenance of low-level mesocyclones within supercells. Three regimes of low-level mesocyclonic behavior are predicted by the conceptual model: (i) low-level mesocyclones are slow to develop, if at all, (ii) low-level mesocyclones form quickly but are short lived, and (iii) low-level mesocyclones develop slowly but have the potential to persist for hours. The model suggests that a balance is needed between the midtropospheric storm-relative winds, storm-relative environmental helicity, and low-level absolute humidity to develop long-lived tornadic mesocyclones. In the absence of that balance, such storms should be rare. The failure of earlier forecast efforts to discriminate between tornadic and nontornadic severe storms is discussed in the context of a physical understanding of supercell tornadogenesis. Finally, it is shown that attempts to gather large datasets of proximity soundings associated with rare weather events are likely to take many years.

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Adrian M. Loftus, Daniel B. Weber, and Charles A. Doswell III

Abstract

Two methods designed to parameterize mesoscale ascent in a three-dimensional numerical cloud model via near-surface momentum and heat fluxes are presented and compared to the commonly used technique of an initial perturbation placed within the model initial condition. The flux techniques use a continuously reinforced thermal or convergent low-level wind field to produce upward vertical motion on the order of 10 cm s−1, by which deep, moist convection can be initiated. The sensitivity of the convective response to the type, strength, and size of the forcing is evaluated using numerical simulations of a conditionally unstable environment with weak unidirectional shear. Precipitation-free cloud processes are used to further simplify the model response to the forcing. The three methods tested produce an initial convective response, but only the momentum and heat flux methods are able to produce sustained deep convection that approximately resembles isolated multicellular convection. Cell regeneration periods, defined as the elapsed time between subsequent vertical velocity maxima passing through a constant level in the updraft region above the source, vary from 8 to 25 min, depending on the forcing type, magnitude, and geometry.

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John P. Monteverdi, Charles A. Doswell III, and Gary S. Lipari

Abstract

A study of 39 nontornadic and 30 tornadic thunderstorms (composing 25 tornado “events,” as defined in the text) that occurred in northern and central California during the period 1990–94 shows that stratification of the stronger tornadic events (associated with F1 or greater tornadoes) on the basis of 0–1- and 0–6-km positive and bulk shear magnitudes is justified statistically. Shear values for the weaker F0 events could not be distinguished statistically from the “background” values calculated for the nontornadic (null) thunderstorm events observed during the period. Shear magnitudes calculated for the F1/F2 events suggest that these tornadoes had developed in an environment supportive of supercell convection. Hindcasting the tornado events based upon shear thresholds produced a high probability of detection (POD) and low false alarm ratio (FAR), particularly for the stronger (F1/F2) events. Although the current sample size is limited and the conclusions drawn from it should be considered preliminary, it appears that California forecasters may be able to use shear profiles to distinguish days on which there is a higher threat of storms producing moderate and significant tornadoes. Buoyancy, as indicated by surface-based convective available potential energy (SBCAPE), was weak for each of the categories, and there were no statistically significant differences between SBCAPE values for each of the categories. Thus, as is true elsewhere, buoyancy magnitude alone appears to be of no value in forecasting whether California thunderstorms will be tornadic.

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David M. Schultz, Philip N. Schumacher, and Charles A. Doswell III

Abstract

In response to Sherwood’s comments and in an attempt to restore proper usage of terminology associated with moist instability, the early history of moist instability is reviewed. This review shows that many of Sherwood’s concerns about the terminology were understood at the time of their origination. Definitions of conditional instability include both the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist-adiabatic lapse rates) and the available-energy definition (i.e., a parcel possesses positive buoyant energy; also called latent instability), neither of which can be considered an instability in the classic sense. Furthermore, the lapse-rate definition is really a statement of uncertainty about instability. The uncertainty can be resolved by including the effects of moisture through a consideration of the available-energy definition (i.e., convective available potential energy) or potential instability. It is shown that such misunderstandings about conditional instability were likely due to the simplifications resulting from the substitution of lapse rates for buoyancy in the vertical acceleration equation. Despite these valid concerns about the value of the lapse-rate definition of conditional instability, consideration of the lapse rate and moisture separately can be useful in some contexts (e.g., the ingredients-based methodology for forecasting deep, moist convection). It is argued that the release of potential (or convective) instability through layer lifting may occur in association with fronts, rather than with isolated convection, the terminology “convective” being an unfortunate modifier. The merits and demerits of slantwise convective available potential energy are discussed, with the hope of improving diagnostic methodologies for assessing slantwise convection. Finally, it is argued that, when assessing precipitation events, undue emphasis may appear to be placed on instability, rather than the forcing for ascent, which should be of primary importance.

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Charles A. Doswell III, Leslie R. Lemon, and Robert A. Maddox

Several key events in the history of modern meteorology are reviewed and analyzed in light of the current state of forecasting. A common thread in much of the material reviewed is the need for greater interaction between research meteorologists and forecasters. Greatly hindering this desirable goal is the inadequate training system for forecast meteorologists. Some possible means for altering the structures within which forecaster training takes place are examined. Responses and commitment to improve the current situation are solicited.

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