It was a wild weather night over much of the Midwest, Great Lakes, and Mid-Atlantic on Wednesday, as tornadoes and an organized complex of severe thunderstorms known as a bow echo brought damaging winds to a large swath of the country. NOAA’s Storm Prediction Center (SPC) logged twelve preliminary reports of tornadoes in Iowa, Illinois, and Ohio, but no injuries or major damage were reported with the twisters. A large area of severe thunderstorms organized into a curved band known as a “bow echo” over Indiana during the evening. The bow echo raced east-southeastwards at 50 mph overnight, spawning severe thunderstorm warnings along its entire track, and arrived in Washington D.C. and Baltimore, Maryland near 9 am EDT Thursday morning. SPC logged 159 reports of high thunderstorm wind gusts of 58 mph or greater in the 26 hours ending at 10 am EDT Thursday morning, and three of these gusts were 74 mph or greater. SPC did not classify this event as a “derecho”, since the winds were not strong enough to qualify. Last year’s June 29, 2012 derecho had 675 reports of wind gusts of 58 mph or greater, with 35 of these gusts 74 mph or greater. Thirteen people died in the winds, mostly from falling trees; 34 more people died from heat-related causes in the areas where 4 million people lost power in the wake of the great storm.

Another round of severe weather is expected over the Mid-Atlantic states Thursday afternoon and evening, and SPC has placed portions of this region in their “Moderate Risk” area for severe weather.

Figure 1. Lightning strikes the Willis Tower (formerly Sears Tower) in downtown on June 12, 2013 in Chicago, Illinois. (Photo by Scott Olson/Getty Images)

Figure 2. An organized line of severe thunderstorms took the shape of a “bow echo” over Indiana last night, triggering severe thunderstorm warnings along the entire front of the bow.

Figure 3. Severe weather reports for the 24 hours ending at 8 am EDT June 13, 2013, from SPC.

Big wind in the Windy City
I watched with some trepidation Wednesday evening as a large tornado vortex signature on radar developed west of Chicago, heading right for one of the most densely populated areas in the country. Fortunately, the storm pulled its punch, and Chicago was spared a direct hit by a violent tornado. But 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, eclipsing the Galveston Hurricane (8,000 fatalities.)

Figure 4. Tornadoes to affect the Chicago area, 1950-2005. Background image credit: Google Earth. Tornado paths: Dr. Perry Samson.

Huge tornado death tolls are very rare
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 only been one tornado death toll over 100 (the 158 killed in the Joplin, Missouri tornado in 2011) 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.

Figure 5. 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 6. Aerial photographs from Google Earth of densely populated area of Chicago (insets x, y, and z from Figure 5) 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.

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 5), 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. Indeed, the May 31, 2013 EF5 tornado that swept through El Reno, Oklahoma, killing four storm chasers, could have easily killed 1,000 people had it held together and plowed into Oklahoma City, hitting freeways jammed with people who unwisely decided to flee the storm in their cars. 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.

from:    http://www.wunderground.com/blog/JeffMasters/show.html