Characteristics
Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds may exceed 100 mph (160 km/h) (in some cases, sustained wind) in these clusters and straight-line wind gusts of up to 200 mph (320 km/h) are possible in the most extreme cases. Tornadoes sometimes form within derecho events, although such events are often difficult to confirm due to the additional damage caused by straight-line winds in the immediate area.
With the average tornado in the United States and Canada rating in the low end of the F/EF1 classification at 85 to 100 mph peak winds and most or all of the rest of the world even lower, derechos tend to deliver the vast majority of extreme wind conditions over much of the territory in which they occur. Data compiled by the United States National Weather Service and other organizations, shows that a large swath of the north-central United States and presumably at least the adjacent sections of Canada and much of the surface of the Great Lakes, can expect winds above 85 mph to as high as 120 mph (135 to 190 km/h) over a significant area at least once in any 50-year period, including both convective events and extra-tropical cyclones and other events deriving power from baroclinic sources. Only in 40 to 65 percent or so of the United States resting on the coast of the Atlantic basin, and a fraction of the Everglades, are derechos surpassed in this respect — by landfalling hurricanes, which at their worst have winds as severe as EF3 tornadoes.
Certain derecho situations are the most common instances of severe weather outbreaks which may become less favourable to tornado production as they become more violent; the height of the 30–31 May 1998 upper Middle West-Canada-New York State derecho and the latter stages of significant tornado and severe weather outbreaks in 2003 and 2004 are only three examples of this. Some upper-air measurements used for severe-weather forecasting may reflect this point of diminishing returns for tornado formation, and the mentioned three situations were instances during which the rare but quite possible particularly dangerous situation severe thunderstorm variety of severe weather watches were issued from the Storm Prediction Center of the U.S. National Oceanic & Atmospheric Administration.
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