Meteorology: Understanding the Atmosphere Ackerman and Knox
Cyclostrophic balance occurs when the horizontal pressure gradient and centrifugal forces push equally in opposite directions.
What forces are important in a small, short-lived, sharply curved, windy and spinning low-pressure system such as a tornado? The Coriolis force is not, but the centrifugal and pressure gradient forces are. The balance of the horizontal pressure gradient and centrifugal forces is called cyclostrophic balance - cyclo meaning "cyclone" or low-pressure system and strophic meaning "turning." In other words, this balance describes situations in which the turning of the wind, not the Earth, is the dominant effect.
On June 8, 1995 a network of weather instruments captured the passage of a destructive, mile-wide tornado near Allison, Texas. One of the instruments measured the local atmospheric pressure at the edge of the tornado within 660 meters (0.4 miles) of its center (See Figure). Pressure observations (Figure) show rapid and extreme pressure changes of 60 mb within just a few minutes, and 15 mb in only 3 seconds! Using these observations and cyclostrophic balance, researchers were able to estimate the winds inside the tornado at between 52 and 130 m/s (116 and 291 mph), a range that is broadly consistent with the winds required to cause the damage observed along this tornado's path.
Because the Coriolis force is not involved in cyclostrophic balance, there is no preference for winds to rotate counterclockwise around a small, short-lived low such as a tornado. This explains why some tornadoes in the United States have winds that blow in a clockwise direction. Again, force-balances can help explain real-world observations.
The above figure is from http://www.nssl.noaa.gov/projects/vortex/events/damage/allisonTrack.gif
From Winn et al., Journal of Geophysical Research-D, Sept. 27, 1999, Figure 13, page 22077.