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4/19 IL Tornadoes
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How It Happened
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> wind shear
  shear profile

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Wind Shear
a key ingredient to tornado outbreak

Another key ingredient supporting the development of tornadoes is what is known as Wind Shear. Wind shear is basically the change of wind speed or direction over a given distance. For tornadoes to develop, significant speed and directional wind shear with height is very important.

[Image: 2100 4/19 surface winds and pressure (31K)]

With the low pressure center located in southwestern Iowa, and winds flowing counterclockwise around it, surface winds at 21Z, or 4:00 PM CDT, were generally from the southeast throughout much of east-central Illinois (shown by the arrows). As analyzed a short time later, mid level winds near 700 mb have veered around and were blowing from the southwest (see panel below).

[Image: 0000 4/20 700 mb winds and heights (28K)]

This indicates strong directional wind shear over the lower atmosphere, a vital component for tornado development. The vertical wind profile was also evident in the sounding from nearly the same time.

Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

shear profile