Tornado! Hurricane! Derecho?

Econintersect: Most are familiar with the tornados which wreak devastation hundreds of times in the spring every year in the U.S.  According to Wikipedia the U.S. experiences more than 1,200 tornados annually and they can occur any time of the year, but most commonly in the spring. There is also an average of nearly two hurricanes a year to make landfall in the continental U.S. Although tornados come three orders of magnitude more often and can carry winds more than twice as strong as the largest hurricanes, the impact of hurricanes is similar because each one covers massive land areas compared to a tornado and also is accompanied by rainfall deluges that can lead to widespread flooding.  The recent weekend storm damage in the area around Washington, DC has brought attention to a third major weather feature, the derecho.  Full sized image of storm damage in Pikeville MD available from the Baltimore Sun – click on picture.

From Wikipedia:

A derecho (pronounced [de̞ˈɾe̞tʃo̞]^[1]) is a widespread and long-lived convection-induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo. Derechos blow in the direction of movement of their associated storms, similar to a gust front, except that the wind is sustained and generally increases in strength behind the “gust” front. A warm weather phenomenon, derechos occur mostly in summer, especially June and July in the Northern Hemisphere. They can occur at any time of the year and occur as frequently at night as in the daylight hours.

Of course there are thousands of squall lines each summer that produce excessive winds, some as strong as small tornados and Category 1 hurricanes (74-75 mph).  The distinguishing character of a derecho is the size of area covered:  A derecho covers a swath width at least 240 miles, covering a length of path up to 800 miles, and has winds exceeding 58 mph.

The massive storm damage over the weekend in Virginia, Maryland and Washington DC was from a derecho which left more than three million without power and killed at least 12 people from the Mississippi across the eastern half of the country from Illinois and India, through Ohio and West Virginia before exiting the mainland through Maryland and Virginia.  ABC News reported that winds in excess of 90 mph were recorded in Chicago.

The picture below shows storm damage in Minnesota in 1999 that displays the characteristic signature of a derecho:  all the trees are broken off and felled in nearly the same direction. (From Wikipedia.)

ABC News called the weekend derecho a “rare event” but it occurs more often than the word “rare” implies.   The following map from The Baltimore Sun shows the historical record.

Derechos are actually about as common as hurricanes, though they are often less damaging.  Hurricanes affect primarily the Atlantic and Gulf coasts, but sometimes have impact far inland as Ilene did last summer.  Hurricanes occur almost exclusively in the summer and autumn:  Hurricane season runs from June 1 to November 30.  There are typically 20-30 tropical storms each year but only some of them develop hurricane strength and some of those spend their entire lives at sea without making contact with major land masses.  The average number of hurricanes striking the U.S. coast per year is less than 2

There have been a total of 285 hurricanes making landfall on the U.S. coast since 1851.  Below is a map from NOAA (National Oceanic and Atmospheric Administration) displaying the 109 hurricanes making landfall 1950 through 2007.

Click on map for larger image.

There are many more tornados each year (more than 1,200, as mentioned above) but those are localized events compared to derechos and hurricanes.  The extended area and amount of damage from last week’s derecho is likely equal to hundreds of tornados.  Of course the fairly rare F5 and higher tornados, such as the one which destroyed Joplin, MO last year, can create devastation of the same magnitude as a derecho.

The areas of the country that have experienced a tornado are more widespread than those that have experienced derechos.  See maps below.  The first map is from NOAA and the other two from Wikipedia.

John Lounsbury

Sources: Follow links in article.