CLIMATOLOGY OF DERECHOS IN THE UNITED STATES

In the United States the climatology of windstorms known as derechos has been studied by scientists Mace Bentley, Michael Coniglio, and others. A listing of their scientific publications on the subject can be observed by clicking here.

The definition of a derecho as defined by Robert Johns and William Hirt in their 1987 publication requires that the thunderstorm outflow winds reach 75 mph or greater at several points along the damage path. Of all the climatological studies that have been done, the "moderate and high intensity" (MH) derecho as described by Mike Coniglio and Dave Stensrud in their 2004 climatological publication most closely fits Johns and Hirts' "wind speed" definition of a derecho. Figure 1 shows a portion of the grid that Coniglio and Stensrud used to create the climatological maps and Figures 2-4 show the climatological maps of "moderate and high intensity" derecho occurrence in the United States.

Figure 1. A portion of the national grid (red lines) that Coniglio and Stensrud (2004) used to create a climatology of derecho occurrences in the United States. The grid squares are 200 km by 200 km (about 125 miles by 125 miles) in size.

To create a climatology of derecho occurrence in the United States Coniglio and Stensrud (2004) established a grid across the United States (see Fig. 1). Then they counted each time a grid square (the red squares on Fig. 1) was at least partially affected by a derecho storm event. Once they had the total number of derecho events affecting a square, they placed the value in the center of the square. Then they drew contours of constant values on the map. These contours represented approximately how many times a particular point on the map was affected by a derecho during the time period indicated. The climatological maps with these contours for "moderate and high intensity" derechos are shown in Figs. 2-4.

SOME WORDS OF CAUTION: In this study, derechos are defined by using reports of severe wind gusts and severe wind damage. This leads to two problems when attempting to interpret these maps. First, the values near the Canadian and Mexican borders and near the ocean coasts including all of Florida are probably lower than the number of events that actually occurred. This is because this study only used severe wind reports that were obtained within the United States and, therefore, derechos that partially occurred in Mexico, Canada, or over Oceanic waters were not completely identified. Second, the likelihood of severe winds being observed and reported is significantly lower in regions with relatively sparse human population and regions that don't have a lot of structures and vegetation to be affected by the severe winds. Therefore, it is likely that more derechos have occurred in some areas, particularly in the sparcely populated northern Great Plains region, than is indicated by the maps.

Figure 2. The approximate number of times "moderate and high intensity" (MH) derechos affected points in the United States during the years 1980 through 2001. Areas affected by 3 or more derecho events are shaded in yellow, orange, and red. (modified from Coniglio and Stensrud 2004).

The climatology of MH derecho occurrence in the United States is shown in Figure 2. The highest annual frequencies of occurrence appear to be along the "Corn Belt" from Minnesota and Iowa into western Pennsylvania and in the south central states from the eastern portion of the southern Plains into the lower Mississippi Valley. However, the frequencies do vary depending on the season. During the warm season (May through August) MH derecho events are most frequent in the western portion of the Corn Belt (see Fig. 3). And during the remainder of the year (September through April) the maximum frequencies shift southward into the lower Mississippi Valley (see Fig. 4).

Given that the maps shows how many events occurred within a 22 year period, dividing the values by 22 allows one to know the frequency of derecho events per year (or season) for any point. The frequency of MH derecho occurrences at any one point on the maps is related to the event numbers as follows:

6 events.............1 derecho occurrence every four years

11 events............1 derecho occurrence every two years

22 events....................1 derecho occurrence every year

30 events..............4 derecho occurrences every 3 years

 

Figure 3. Same as in Fig. 2 but only for the "warm season" months from May through August. (modified from Coniglio and Stensrud 2004).

 

Figure 4. Same as in Fig. 2 but only for the "cool season" months from September through April. (modified from Coniglio and Stensrud 2004).

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