Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 2:30 PM GMT on April 24, 2007
Tornadoes hit the Plains again last night, and several people in rural Nebraska were injured when a tornado ripped through farmland in the western part of the state. The Storm Prediction Center has put a large area of the Plains under their Moderate Risk region for severe weather today. Wunderphotographer Mike Theiss chased the tornado that caused severe damage in Tulia, Texas over the weekend, and will be out chasing storms in the Plains today. Mike will be chasing frequently over the coming month, so be sure to tune into his blog for the latest!
Big wind in the Windy City
What would happen if a violent, long-track EF4 or EF5 tornado ripped through a densely populated urban area like Chicago? That was the question posed by tornado researcher Josh Wurman of the Center for Severe Weather Research in Boulder and three co-authors in a paper published in the January 2007 issue of the Bulletin of the American Meteorological Society. Their astonishing answer: damage of $40 billion and 13,000-45,000 people killed--the deadliest natural disaster in American history.
A tornado death toll in the ten of thousands seems outlandish when one considers past history. After all, the deadliest tornado in U.S. history--the great Tri-state Tornado of March 18, 1925--killed 695 people in its deadly rampage across rural Missouri, Illinois, and Indiana. That was before the advent of Doppler radar and the National Weather Service's excellent tornado warning system. In fact, there has not been a tornado death toll over 100 since 1953, the year the NWS began issuing tornado warnings. Chicago has been hit by one violent tornado. On April 21, 1967 a 200-yard wide F4 tornado formed in Palos Hills in Cook County, and tore a 16-miles long trail of destruction through Oak Lawn and the south side of Chicago. Thirty-three people died, 500 more were injured, and damage was estimated at $50 million.
The paper by Wurman et al., "Low-level winds in tornadoes and the potential catastrophic tornado impacts in urban areas" opens with an analysis of the wind structure of two F5 tornadoes captured on mobile "Doppler on Wheels" radar systems--the May 3, 1999 Bridgecreek-Moore tornado, which hit the southern suburbs of Oklahoma City, and the Mulhall, Oklahoma tornado of the same day, which moved over sparsely populated rural regions. The Bridgecreek-Moore tornado had the highest winds ever measured in a tornado, 302 mph. Winds of EF4 to EF5 strength (greater than 170 mph) are capable of completely destroying a typical home, and occurred over a 350 meter (1150 foot) wide swath along this tornado's path. The Mulhall tornado had weaker winds topping out at 245-255 mph, but had EF4 to EF5 winds over a much wider swath--1600 meters (one mile).
The F4 to F5 winds of the Bridgecreek-Moore tornado killed 36 people. Given the population of the area hit, between 1% and 3% of the people exposed to these winds died. The authors thought that this number was unusually low, given the excellent warnings and high degree of tornado awareness in Oklahoma's population. They cited the death rate in the 1998 Spencer, South Dakota F4 tornado that destroyed 30 structures and caused six deaths, resulting in a death rate of 6% (assuming 3.3 people lived in each structure). There are no studies that relate the probability of death to the amount of damage a structure receives, and the authors estimated crudely that the death rate per totally destroyed structure is 10%. This number will go down sharply if there is a long warning time, as there was in the Oklahoma tornadoes. If one takes the Mulhall tornado's track and superimposes it on a densely populated region of Chicago (Figure 1), one sees that a much higher number of buildings are impacted due to the density of houses. Many of these are high-rise apartment buildings that would not be totally destroyed, and the authors assume a 1% death rate in these structures. Assuming a 1% death rate in the partially destroyed high-rise apartment buildings and a 10% death rate in the homes totally destroyed along the simulated tornado's path, one arrives at a figure of 13,000-45,000 killed in Chicago by a violent, long-track tornado. The math can applied to other cities, as well, resulting in deaths tolls as high as 14,000 in St. Louis, 22,000 in Dallas, 17,000 in Houston, 15,000 in Atlanta, and 8,000 in Oklahoma City.
Figure 1. Wind speed swaths for the 1999 F5 Mulhall, Oklahoma tornado if it were to traverse a densely populated area of Chicago. Units are in meters/sec (120 m/s = 269 mph, 102 m/s = 228 mph, and 76 m/s = 170 mph). Winds above 170 mph usually completely destroy an average house, with a crudely estimated fatality rate of 10%, according to Wurman et al.. Insets x, y, and z refer to satellite photo insets in Figure 2. Image credit: Bulletin of the American Meteorological Society.
Figure 2. Aerial photographs from Google Earth of densely populated area of Chicago (insets x, y, and z from Figure 1) These areas contain mainly single-family homes, with housing units densely packed on small lots. A mixture of three-story apartments and single-family homes is typical across the Chicago metropolitan area and many older cities such as New York City and Detroit. At lower right is a photo of Moore, OK, showing lower density housing like the 1999 Bridgecreek-Moore tornado passed through.
How realistic are these estimates? Could a violent tornado really eclipse the Galveston Hurricane (8,000 killed) as this nation's worst natural disaster? The authors admit that their method of estimating deaths is crude, and considered death rates from only two storms to arrive at their estimates. I took a look at the fatalities from some other F5 tornadoes since 1991, and the death rates are highly variable (3% for the 1000 buildings destroyed by the 1998 Birmingham, AL tornado; 66% for the 41 buildings destroyed by the 1997 Jarrell, Texas tornado; 0% for the 66 buildings destroyed in the 1996 Oakfield, Wisconsin tornado; and 5% for the 350 buildings destroyed in the 1991 Andover, Kansas tornado). Based on these numbers, a 5% death rate may be more typical than the 10% death rate assumed in the Wurman et al. study. Violent tornadoes have hit downtown areas in the past, but have not affected nearly the number of structures as considered in the Wurman et al. study. The authors emphasize that even if their death rate estimates are off by a factor ten, a violent tornado in Chicago could still kill 1,300-4,500 people. The authors don't give an expected frequency for such an event, but I speculate that a violent tornado capable of killing thousands will probably occur in a major U.S. city once every few hundred years--or perhaps as long as 1000 years, considering that there have not been any F5 tornadoes in the U.S. since 1999.
Figure 3. Tornadoes to affect the Chicago area, 1950-2005. Background image credit: Google Earth. Tornado paths: Dr. Perry Samson.
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