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HISTORICAL FACTS Probability of Occurrence The section of the common operational picture has been
largely developed from: Of the 42 tornados that have struck Cook County in the past 55 years seven (7) have crossed into the City of Chicago. Although the number of tornadoes to cross into Chicago seems relatively low, it is consistent with the number of tornadoes that occur per square mile in Illinois. The ratio of tornado intensities that have occurred in Chicago is consistent with FEMA Region V averages. Due to Chicago?s heavy population density and extensive infrastructure even a moderate tornado can cause significant causalities and large-scale damage. HISTORICAL TORNADOES The "Tri-state" tornado of 18 March 1925 killed 695 people as it raced along at 60-73 mph in a 219-mile long track across parts of Missouri, Illinois and Indiana, producing F5 damage. The death toll is an estimate based on the work of Grazulis (1993); older references have different counts. This event also holds the known record for most tornado fatalities in a single city or town: at least 234 at Murphysboro IL. On 3 April 1974, the main day of the two-day "Super Outbreak," tornadoes killed 308 people. The next deadliest day for tornadoes was 11 April 1965, the original "Palm Sunday Outbreak," where 260 perished. 147 tornadoes touched down in 13 U.S. states on 3 and 4 April 1974. The outbreak killed 310 in the U.S., 8 in Canada, with 5454 U.S. injuries and 23 hurt in Canada. 48 of the tornadoes were killers. Seven tornados produced damage rated F5 - the maximum possible - and 23 more were rated F4. This was one of only two outbreaks with over 100 confirmed tornadoes, the other being with Hurricane Beulah in 1967 (115 tornadoes The Hallam, Nebraska F4 tornado of 22 May
2004 is the newest record-holder for peak width, at nearly two and a half
miles, as surveyed by Brian Smith of NWS
Omaha. This is probably close to the maximum size for tornadoes; but it
is possible that larger, unrecorded ones have occurred. The record for most tornadoes in any month (since modern tornado record keeping began in 1950) was set in May 2003, with 543 tornadoes confirmed in the final numbers. This easily broke the old mark of 399, set in June 1992. Nobody knows. Tornado wind speeds have only been
directly recorded in the weaker ones, because strong and violent tornadoes
destroy weather instruments. Mobile Doppler radars such as the OU Doppler on Wheels have remotely sensed
tornado wind speeds above ground level as high as 318 mph (3 May 1999 near Bridge Creek OK) - the
highest winds ever found near earth's surface by any means. This tornado caused
F5 damage. But ground-level wind
speeds in the most violent tornadoes have never been directly measured. A tornado in central
and northern Georgia, on 31 March 1973, is listed in Storm Data as
having produced $1,250,000,000.00 in actual damage and $5,175,000,000.00 when
inflation-adjusted -- both record amounts. The Bridge Creek-Moore-Oklahoma
City-Midwest City, OK, tornado of 3 May 1999 currently ranks second in actual
dollars but fourth when inflation adjusted. Tornado season usually means the peak period for historical tornado reports in an area, when averaged over the history of reports. There is a general northward shift in "tornado season" in the U.S. from late winter through mid summer. The peak period for tornadoes in the southern plains, for example, is during May into early June. On the Gulf coast, it is earlier during the spring; in the northern plains and upper Midwest, it is June or July. Remember, tornadoes can happen any time of year if the conditions are right.
Historical Occurrences: Chicago Regional |