When the National Weather Service issues Tornado and Severe Thunderstorm Warnings, they include how fast and the direction of travel that the warned storm is taking. The featured chart presents this combination for Tornado Warnings issued by NWS Des Moines. Values for the severe storms this past Saturday are highlighted in red. The lines on the plot represent a kernel density estimate with the centroid located near 30 MPH from the west-southwest. This dominate storm motion reflects the prevailing westerly motion of the middle part of the troposphere, which generally guide storms for west to east. The preference for a more southwesterly component reflects a flow pattern that is more favorable for warm air transport from the Gulf of Mexico north and cooler air transport of Canadian air south. Tornado producing storms feed off these contrasts.

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