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Edwin Kessler National Severe Storms Laboratory National Oceanic and Atmospheric Administration Norman, Okla

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Edwin Kessler National Severe Storms Laboratory National Oceanic and Atmospheric Administration Norman, Okla Edwin Kessler National Severe Storms Laboratory National Oceanic and Atmospheric Administration Norman, Okla. Abstract sure within and carried off by the wind as their con- The prominent characteristics of tornadoes, their socio- nections are weakened. Only buildings of reinforced logical and meteorological importance, aspects of the concrete and rigidly connected structural steel character- national weather service that pertain to storm forecast- istically escape serious structural damage from violent ing and warning, observational and theoretical studies tornadoes, and windows, roofs, and sidings are always of tornadoes, and some prospects for modifying tor- vulnerable (Brooks, 1951; Flora, 1954; Melaragno, 1968; nadoes, are briefly surveyed. Some paths for future de- Somes et al., 1970). velopment of the warning service and of scientific in- During the last 15 years, about 125 persons have been vestigations are indicated. killed by tornadoes annually, and the annual property damage has averaged about $75 million. These figures 1. General description and significance may be compared with estimated losses caused by light- Tornadoes are among the smallest in horizontal extent ning, hail, and hurricanes as shown in Table 1. of the atmosphere's whirling winds, but they are the Although tornadoes hardly rank as a prominent cause most locally destructive. Although occasionally reported of death in the United States, the number of tornado from Australia, western Europe, India and Japan, it is deaths is highest in relation to property damage among only in the United States (Fig. 1) that very intense tor- the listed natural phenomena. This is attributable in nadoes occur frequently. A tornado typical of highly part to our inability to effectively warn everyone endan- destructive storms in the United States is marked by a gered by this very destructive phenomenon—a tornado characteristic funnel cloud, accompanies an otherwise usually comes and goes suddenly and affects only a thou- severe thunderstorm, is on the ground about 20 min, sandth part of a region covered by tornado-spawning and damages an area 1/4 mile wide along a path toward thunderstorms. Extreme variability is characteristic of the northeast about 10 mi long. While much damage is tornadoes and most tornado losses are associated with a probably caused by winds of about 125 mph, the maxi- few storms (Cressman, 1969) that utterly detroy the struc- mum winds of tornadoes (never accurately measured) tures in large parts of urban areas, or entire small com- are probably between 175 and 250 mph. Damage has munities (Fig. 2). Allen Pearson, Director of the ESSA also been attributed to the sudden drop of pressure ac- National Severe Storms Forecast Center, has noted that companying tornadoes. Values exceeding 0.1 of the total 85% of tornado fatalities from 1960 to May 1970 were atmospheric pressure or 200 lb ft-2 have been cited in produced by 1-1/2% of reported tornadoes. A severe tor- the literature, but the extreme observations made during nado event leaves a community momentarily stunned very unusual conditions appear questionable. During tor- nadoes, especially when structures are poorly vented, TABLE 1. U. S. losses attributed to some weather phenomena. roofs and walls may be moved outward by a higher pres- Average annual Average annual property Type of storm deaths in U. S. damage in U. S. Tornado1 125 $ 75 million Lightning 1502 100 million3 Hail — 284 million4 Hurricane1 75 500 million 1 Based on data from the Environmental Data Service, ESSA; applicable to period 1955-1969. 2 Estimate based on data from National Center for Health Statistics applicable to period 1959-1965. See Zegel (1967). 3 Includes property damage by lightning-caused building fires, $30,600,000 in 1967, according to Accident Facts, National Safety Council, Chicago, 111. (1968 ed., 96 pp). Other property loss in- cludes forest fires, aircraft damage, disruption of electro-magnetic transmissions and casualties to livestock. See ESSA (1969). 4 Estimate for period 1958-1967 provided by Stanley Chagnon, FIG. 1. Distribution of tornadoes in the United States (taken Illinois State Water Survey, Urbana, 111.; about 10% of Illinois from Pautz, 1969). losses represents property, remainder is crop damage. 926 Vol. 51, No. 10, October 1970 Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Bulletin A?nerican Meteorological Society rather than to a description of the expected behavior of entities already developed. Forecasts of tornadoes are closely linked to forecasts of severe thunderstorms, and like other weather fore- casts, these must start from a description of the present state of the atmosphere. They are less specific than we would like, partly because of a lack of understanding, but partly because our observations are too sparse to describe atmospheric variability on the scale productive of the tornado or thunderstorm phenomena. Thus, the extent of a severe thunderstorm may be 10 to 25 mi, and the lifetime of a storm system can be considered about 6 hr. On the other hand, the distance between primary surface weather stations is about 100 mi, and between upper air stations over 200 mi. Observations are made hourly at the surface stations (more often under special conditions) but usually at only 12-hr intervals at the upper air stations. Therefore, even if our knowledge were otherwise adequate to the task, the weather observ- ing system we now have would limit us to indicating the probability of thunderstorms and accompanying tor- FIG. 2. Effects of a tornado at Udall, Kansas, 2235 CST, nadoes in regions much larger than the storms. 25 May 1955. Photo taken at 1600 on 26 May by Melvin Briefly stated, the tornado forecasting parameters are Haynes. warmth and moisture in a layer about 5000 ft deep near the Earth's surface, with a cool dry region at interme- and disorganized and draws a response of the magnitude diate levels, strong winds in the upper atmosphere, and demanded in war (Moore, 1958; Fritz, 1961). a trend toward intensification of related conditions. The The beneficial rains and relief from heat often brought prediction of both thunderstorms and tornadoes is de- by severe storms, as well as the deaths and property dam- rived from years of accumulated observations, evaluated age they inflict, justify man's rational interest in them. by statistical and dynamical methods, and controlled in But public attitudes probably express as well some psy- practice by the judgment of experienced forecasters chological and emotional traits instilled during the mil- (Winston, 1956). lennia of man's development while he was less insulated Present tornado forecasts usually refer to developments from wind and rain than he is in technologically ad- expected to begin from 1 to 7 hr after the forecast is vanced countries today. Certainly many of us regard a issued in regions of about 25,000 mi2. About one-third of severe thunderstorm with a mingled fear and fascination tornado forecasts are issued as the immediate conse- akin to that which probably gripped our ancestors as quence of a radar observation of an existing thunder- they fled the elements to the shelter of primitive dwell- storm in a suspicious area. About 40% of affirmative ings. predictions are correct, i.e., are followed by tornadoes Of course, the meteorologist's special interest in tor- somewhere in the forecast box during the forecast pe- nadoes is related to their intriguing dynamical properties riod. The incorrect affirmative forecasts divide about and to the idea that understanding of them should con- evenly between cases without tornadoes and cases with tribute to advancement of weather knowledge on a broad tornadoes outside but near the predicted regions. Since front. Some of the most important aspects of hourly and the climatological expectancy of tornadoes during 6 hr daily weather variations are represented in the great in a randomly selected 25,000 mi2 area in eastern and vertical transports of heat, moisture, and momentum in central United States is only about one in 400,1 it is thunderstorms and tornadoes, the conversion there of plain that the affirmative forecasts give evidence of con- large amounts of potential to kinetic energy, and the siderable skill in identifying the meteorological parame- dissipation of storm kinetic energy in the boundary layer. ters associated with development of severe storms and The atmosphere's larger scale features must also be in- tornadoes. fluenced by these storm processes, because of their in- Forecasts of severe storms and tornadoes 1 to 7 hr volvement with the vertical and global redistribution in advance are called "watches." In view of the wide area of energy originally absorbed as solar radiance at the covered by the forecast in relation to the area likely to Earth's surface. be affected, the public is only encouraged by a "watch" to remain alert to further advisories. The forecasts are 2. Tornado prediction i The expectancy of a tornado in a 25,000 mi2 area during As used here, a "prediction" or "forecast" refers to phe- six hours is about 1/20 near the seasonal and geographical nomena foreshadowed in advance of their development, maxima of tornadoes. 927 Unauthenticated | Downloaded 10/04/21 08:26 AM UTC Vol. 51, No. 10, October 1970 sightings and damage reports and facilitates issuance of timely warnings to communities lying in the projected paths of storms. Warnings of several minutes to an hour or more have been provided in connection with practically all major tornadoes during the last 5 years. Usually, however, there are some endangered persons who cannot be reached by the warning system, or whose response to the warnings does not increase their safety (Fritz, 1961). 4. Tornado research Morton (1966) and some others listed at the end of this paper have additional references and comprehensive re- views of tornado research, and the reader is referred to FIG.
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