MORE ON THE CLIMATOLOGY OF DERECHOS IN THE UNITED STATES

In the United States, the climatology of derechos has been documented by researchers Mace Bentley, Michael Coniglio, Thomas Mote, David Stensrud, and others in recent years. Some of their publications on the subject are available here.

The definition of a derecho, as defined by Johns and Hirt in their 1987 paper that re-introduced the term "derecho" to the meteorological community, requires that the thunderstorm outflow winds reach 75 mph or greater at several points along the damage path. This wind criteria is most closely matched by that used by Coniglio and Stensrud in their 2004 paper describing "moderate and high intensity" (MH) derechos in the United States. Figure 1 shows part of the grid that Coniglio and Stensrud used to create their climatological maps.



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



To create their climatology of derecho occurrence, Coniglio and Stensrud (2004) established a grid across the continental United States (Fig. 1). Then they determined the number of times each grid square was at least partly affected by a derecho event during the period 1980-2001. The grid values were then contoured, yielding a map that approximated the number of times any given point on the map was affected by a derecho. The results of their effort for "moderate and high intensity" derechos are presented in Figs. 2-4.

A FEW WORDS OF CAUTION: In Conglio and Stensrud's study, derechos were defined by reported occurrence of severe wind gusts and severe wind damage. This can lead to two problems when interpreting the data shown below. First, the number of events depicted near the Canadian and Mexican borders, as well as those near the coasts (including all of Florida), more than likely are lower than the actual numbers that occurred. This reflects the fact that only United States data were used in the tabulations, and the fact that few severe weather reports typically are received from the coastal waters. Second, severe weather observations are strongly population-dependent. The likelihood of a report being made is directly proportional to population density. This is especially true of the High Plains, where not only is population sparse, but there exist fewer structures and trees to be affected by severe winds relative to points farther east. For these reasons, it is likely that more derechos have occurred in some areas, particularly over western and northern parts of Great Plains, than is indicated by the maps.



Figure 2. 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 along the "Corn Belt," from Minnesota and Iowa into western Pennsylvania, and in the south central states, from eastern parts of the southern Plains into the lower Mississippi Valley. However, the frequencies vary by season. During the warm season (May through August), MH derecho events are most frequent in the western part of the Corn Belt (Fig. 3). During the remainder of the year (September through April), the maximum frequencies shift south into the lower Mississippi Valley (Fig. 4).

Given that the maps depict the number of events that occurred over a 22-year period, dividing the values by 22 allows one to estimate the annual or seasonal frequency of derechos for any point. The frequency of MH derecho occurrence 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 Fig. 2, but for "warm season" months from May through August (modified from Coniglio and Stensrud 2004).

 

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



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