PART V — SEVERE STORMS 



incorrect forecasts being nearly di- 

 vided between cases without tor- 

 nadoes and cases with tornadoes 

 outside, but near, the predicted re- 

 gions. It should be noted that the 

 climatological expectancy of torna- 

 does during six hours in a randomly 

 selected 25,000-square mile area in 

 eastern and central United States is 

 only about one in 400. Plainly, then, 

 present forecasts give evidence of 

 considerable skill in identifying the 

 meteorological parameters associated 

 with severe storms and tornadoes 

 and in correctly anticipating their 

 development. 



Briefly stated, the storm-forecast- 

 ing parameters are warmth and mois- 

 ture in a layer about 5,000 feet deep 

 near the earth's surface, with a cool 

 dry region at intermediate levels, 

 strong winds in the upper atmos- 

 phere, and a trend toward intensifi- 

 cation rather than diminution of these 

 conditions. The prediction of all the 

 necessary features is based on ob- 

 jective techniques, rooted in statistical 

 and dynamical evaluations and modi- 

 fied by the judgment of experienced 

 forecasters. 



Forecasts of severe storms and tor- 

 nadoes one to six hours in advance 

 are considered "watches." In view 

 of the wide area covered by the 

 forecast relative to the area likely to 

 be affected, the public is encouraged 

 by a "watch" merely to remain alert 

 to further advisories. The forecasts 

 are disseminated by teletype from 

 the National Severe Storm Forecast 

 Center in Kansas City, Missouri, to 

 local offices around the country. Oc- 

 casionally, a local National Weather 

 Service office may issue a modified 

 local forecast which takes special 

 account of peculiar local conditions. 

 Since subscribers to the teletype 

 service include most elements of the 

 communications media, storm indi- 

 cations are quickly brought to the 

 attention of the radio and TV public. 



Tornado Warning 



Severe storms are observed as they 

 develop by Weather Service offices, 



local government authorities, and 

 private persons. When the Weather 

 Service, through its own action or a 

 report by a private observer, becomes 

 aware that a severe storm or tornado 

 exists, a warning to communities in 

 the extrapolated path of the storm is 

 issued by teletype, or immediately by 

 radio and television if the situation 

 warrants. The public in the threat- 

 ened communities may be warned 

 by various actions of local authorities, 

 including the sounding of sirens. The 

 few minutes' warning thus provided 

 is credited with a twofold reduction 

 in loss of life. The greatest loss of 

 life from a tornado is often to be 

 found in the first community visited 

 by a storm, downstream locations 

 having the benefit of longer warning 

 time. 



These days, observer reports are 

 valuably augmented by radar ob- 

 servations. The primary radar net- 

 work of the National Weather Service 

 has stations spaced 200 to 250 miles 

 apart. When severe storms threaten, 

 the radar screens are monitored con- 

 tinuously. The more intense echoes 

 are associated with heavier precipita- 

 tion and a greater likelihood of hail, 

 strong straight-line winds, and tor- 

 nadoes. Severe tornadoes are often 

 associated with a hook-shaped ap- 

 pendage on the echo. Thus, the 

 forecaster's observation of the intense 

 radar echoes provides a continual 

 check on visual sightings and damage 

 reports, and provides for timely 

 warnings to communities lying in the 

 projected path of a storm. 



Tornado Research 



Observations — Accurate descrip- 

 tion of tornado vortices and of the 

 atmospheric conditions preceding and 

 accompanying tornadoes is essential 

 for improved understanding and pre- 

 diction of tornadoes, and for the 

 possible development of practical 

 means for influencing tornadoes ben- 

 eficially. But scientific observation 

 of tornadoes is made difficult because 

 of their random occurrence, brief 



duration, small size, and great vi- 

 olence. 



In an attempt to study tornado 

 vortices directly, the National Severe 

 Storms Laboratory has maintained a 

 network of 30 to 60 conventionally 

 equipped surface stations during the 

 past seven spring seasons in an area 

 where tornadoes are relatively fre- 

 quent. Only two of the stations, 

 however, have been directly affected 

 by the winds of a tornado vortex 

 during this period. The network den- 

 sity would have to be increased by 

 a factor of 100 to obtain detailed 

 data on the wind distribution in tor- 

 nado vortices. For detailed informa- 

 tion on the vortices, therefore, we 

 are forced to rely on chance observa- 

 tions, engineering analysis of dam- 

 aged areas, eyewitness accounts, and 

 on the results of efforts to obtain 

 data remotely by photography and 

 by indirect probes such as radar. 



Our information indicates that the 

 tornado is characterized by an inner 

 region where the winds decrease to- 

 ward the center, as in solid rotation, 

 and an outer region where the winds 

 fall off with increasing distance. 

 Many other tornado features are 

 highly variable. The tornado cloud, 

 presumed to be the surface of con- 

 stant reduced pressure at which the 

 well-mixed subcloud air is cooled to 

 saturation, varies in size and shape. 

 In some photographs it appears as un- 

 commonly smooth, suggesting lami- 

 nar flow, in others as highly irregular, 

 suggesting strong turbulence. Such 

 differences are quite important from 

 the point of view of tornado dynam- 

 ics. Since the less fierce waterspouts 

 are usually cylindrical and smooth- 

 walled, we are led to search for sig- 

 nificant variability in surface rough- 

 ness or atmospheric conditions over 

 land to account from the apparent 

 variability of turbulence and shape 

 of tornadoes. 



The electrical properties of the tor- 

 nadoes also appear highly variable. 

 Finley's report on 600 tornadoes, pub- 

 lished in 1882, lists the observation 

 of thunder and lightning in 425 asso- 



138 



