July is the peak month for numbers of damaging wind reports from thunderstorms, averaging about 90 reports per day in the United States. By contrast, May is the peak month for tornadoes and large hail. July ranks fourth in tornadoes and large hail (also trailing June and April). July has averaged 105 tornadoes per month over the past 10 years, and an average July day has about 51 reports of large hail (0.75 inch diameter or larger) in the United States.
Many of the July damaging winds reports come in big clusters, in windstorms called "derechoes." Derecho is a Spanish word for "straight", and these windstorms leave wide, long swaths of "straight-line" wind damage, from severe winds all in nearly the same direction. This can be seen in swaths of downed trees in a forest, for example. Derechoes are widespread damaging windstorms caused by one or more curved lines of thunderstorms, usually bow echoes. As in the first figure below, a bow echo is shaped like the kind of bow used to shoot arrows or like a backwards letter C. An ordinary thunderstorm produces a swath of damaging winds usually only a mile or two wide and a few miles long, but derechoes can produce damage swaths tens of miles wide and several hundred miles long.
One of those derechoes charged from near Milwaukee, Wisconsin across southern Michigan to near Toledo, Ohio on Wednesday, July 2nd. Wind gusts of 60 mph or higher were measured in several locations in the swath of winds across Michigan near or across Muskegon, Grand Rapids, Lansing, Ann Arbor, and Detroit. The radar image below shows the bow echo associated with this derecho at 3:51 PM, north of Lansing. The strongest winds are usually near the leading edge of the precipitation, east of my arrowhead. Often the strong winds begin aloft at about 10,000 feet near the tail of my arrow and descend to the ground northeast of the arrowhead. This descending core of fast winds is called a "rear inflow jet."
The fourth of July has had several well-documented derechoes over the years. One in 1977 caused a swath of downed trees up to 17 miles wide and 166 miles long across northern Wisconsin. Its winds were up to 135 mph, and it caused $24 million in damage, 35 injuries, and one death. Another in 1999 blasted from Fargo, North Dakota across northern Minnesota. It caused $85 million in damage in Fargo and $12-18 million in timber damage, including in the Boundary Waters Canoe Area Wilderness, where 20 campers were injured.
The figures below show examples of some notable recent derechoes. The first figure shows the swaths of damaging winds from a pair of derechoes that hit St. Louis, Missouri, two days apart on July 19 and 21, 2006. The first one moved southward, packing wind gusts up to 92 mph. Thirty people were injured at the baseball stadium where the Cardinals were preparing to play the Atlanta Braves. Downed power lines left about 590,000 customers without power.
Service had been restored to about 270,000 of the customers by the time that the second derecho rolled in from the west on July 21. This second derecho sent the number without power back up to 560,000!
The next figure shows the damaging wind swath from a derecho that blasted Memphis, Tennessee on July 22, 2003. It produced the largest blackout in the city's history, leaving 338,000 customers without power, some for up to 15 days. This long-lived derecho reached Atlanta, GA and beyond in a somewhat-weakened intensity.
Another infamous derecho raced at a speed up to 75 mph from northern Michigan across Ontario and into New York and southern New England on July 15, 1995, shown in the figure below. Wind gusts were measured of at least 92 mph. The derecho caused 7 deaths and nearly a half billion dollars of damage.
Because of the widespread nature of the winds in a derecho, the impact is somewhat like that of a landfalling hurricane, and affects a much greater area than most tornadoes. The extensive swath of downed trees and power lines causes a major cleanup and restoration effort that takes days to weeks and often requires relief workers to come in from other states to aid in these efforts.
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