The Greensburg Tornado
A series of destructive tornadoes swept across the Central US Plains from May 4th-6th, 2007 with the Greensburg super cell most powerful.
Above is a satellite image of the super cell formation courtesy of http://rsd.gsfc.nasa.gov/goes/
Map of the Flint Hills of Kansas and surrounding geographic regions courtesy of a partnership between the Kansas Geological Survey and Symphony in the Flint Hills
My climate region of choice is the Flint Hills of East-Central Kansas. The Flint Hills lie within the Central US Plains and major cities in the region include Wichita and Manhattan. This region is subject to a humid-continental climate with high seasonal variance and contains the largest remaining section of native tall-grass prairie in North America. The Greensburg tornadoes serve as a powerful example of the severe weather events this region is prone to seasonally.
A days long chain of intense tornadoes move swiftly across the Great Plains
A photo of the rubble in town via KAKE news.
The Greensburg tornado that destroyed 95% of Greensburg, KS on May 4th, 2007 was the strongest of a 3 day long outbreak of tornadoes from South Dakota down to Oklahoma. That tornado alone caused 11 fatalities and numerous injuries, but 25 other twisters were confirmed on the same day. The funnel cloud was 1.5 miles wide, touched down for 22 miles, and was rated as an F-5 storm with top winds at 205 mph. The town was evacuated completely out of safety concerns and some residents remained in temporary homes for the entire year following the incident.
The tornado occurred around 9:30pm with little nighttime visibility, but a storm chaser captured this image of the formation.
A NOAA diagram of the tornado risk severity across the Central US Plains. Greensburg lies in the high risk zone labeled in pink.
How do tornadoes form and why are they prominent in the Central US Plains?
A funnel cloud forms over farmland via time.com.
Tornadoes form alongside thunderstorms when the typical updraft winds associated with storms are knocked off-kilter by winds from various directions and speeds. This is illustrated in the figure below with blue cyclic arrows. The strongest forms of thunderstorms (super cells) do have horizontal spiral features in upper cloud layers, but tornadoes are unique in that the cyclic air movement is near the Earth's surface. Tornadoes are destructive at ground level as the wind speed increases and air is drawn in towards the axis of rotation. The process is alike to a figure skater pulling their arms in to their chest while spinning which increases their speed of rotation.
A diagram via Paul Markowski of Penn State University details the typical formation characteristics of tornadoes in the Northern Hemisphere.
Why is the Tornado Alley so prone to these twisters?
Multiple air masses of varying temperatures and moisture contents converge in tornado alley. A dry line where sever weather often occurs is indicated by the brown dotted line between the cool (blue triangles) and warm (red semi-circles) fronts.
Across Tornado Alley cool, dry air from the northwest Rockies descends southeast and warm, moist air from the Gulf of Mexico rises North. The diagram above illustrates the interactions between these air masses and respective warm and cool fronts. A continental dry line (illustrated in brown dots) lies between the cool and warm front and thunderstorms tend to form on the moist side of this line. The variations in these air mass characteristics bring about winds from numerous directions that can set thunderstorms in horizontal motion to form funnel clouds near the Earth's surface.
A vertical profile of tornado formation.
Tornadoes typically form in this region in the spring and summer when the wind currents off the Gulf of Mexico are the strongest moving north towards the Great Plains. The warm air propels the formation of cyclonic storms in Tornado Alley and cool winds from the northwest converge leading to spiral motion perpendicular to the typical rainstorm uplift patterns.
Conclusion
The Greensburg tornado and sequence of events illustrates the severity of weather events possible in an flat area where numerous air masses of varying characteristics converge. Emergency action plans, tornado drills, and proper household shelters are effective means of preparing for safety during these sometimes catastrophic events.
