Tornadoes depend on three preconditions:
1 at least 2000 J/kg of convective available potential energy CAPE (Figure 7.6), which is possible when the planetary boundary layer (PBL) is warm and humid, and the air aloft is dry and cool,
2 a thin stable layer above the PBL (Figure 7.9), sufficient to prevent the instability being already released by small thunderstorms, and
This combination arises in the American Midwest when warm, moist, low-level winds blow northwards from the Gulf of Mexico and undercut a strong westerly flow which has lost most of its moisture over the Rockies. Once the stable layer between is breached, there is a sudden updraught of the buoyant lower air, creating a massive thunderstorm which can spawn a tornado.
Some tornadoes occur in association with small thunderstorms ahead of a cold front or within a tropical cyclone (Chapter 13). Many tornadoes in Australia are triggered by convection due to vigorous cold fronts.
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