Question: What is a derecho?

What are some of the most damaging derechoes to ever occur?

Answer:

Never heard of a derecho before? If not, there's good reason. In the United States, these violent and fast-moving windstorms typically only occur once every 2-4 years.

 

Most common during the late spring and summer, a derecho is a widespread and long-lived wind event that's associated with a band of fast moving thunderstorms appearing in a curved shape.

These curved storms are known as bow echoes, and are the type of storms that cause derechoes.
Derechoes can occur with a single or multiple bow echoes. (Bow echoes vary in size, and can diminish, then re-develop over the course of a derecho's evolution. Also, the winds associated with a derecho can be enhanced on a smaller scale by supercell thunderstorms that may be embedded within the storm system.

Derecho Types

There are two main types of derechoes, serial and progressive.
 

• Serial derechoes: These occur with multiple bow echoes embedded in large squall lines that may extend for hundreds of miles. This type is generally associated with a strong low pressure system.
• Progressive derechoes: These are coupled with a shorter line of storms that may occasionally take the appearance of a single bow echo. The width of these severe weather features may remain narrow or may grow over time, reaching nearly 250 miles in width. They may also start out as a single bow echo, but later develop into multiple echoes.

 

Hybrid derechoes can also form, and will take on characteristics that are both serial and progressive in nature.

The winds of a derecho must meet the criteria of the NOAA National Weather Service (NWS) for a severe wind gust (58+ mph) at the majority of points along the storm's path and stronger derechoes may have wind gusts exceeding 100 mph. For example, a derecho event in eastern Wisconsin on May 31, 1998 resulted in a wind gust of 128 mph.

As may be evident, the winds associated with a derecho event are also not relatively constant, but rather, can vary greatly along its path. The areas of stronger wind gusts within its path are known as downbursts. These often occur in groups, in which case they are referred to as a "family of downburst clusters" which span a length of at least 240 miles long.

The tornado threat during these events is greater with storms associated with strong low pressure system-producing derechoes--or serial derechoes.

Historic Derecho Windstorms in the United States

In July 2006, two separate derechoes hit the St. Louis metro area over the span of a few days. The first one, which occurred on July 19, packed wind gusts up to 92 mph. Around thirty people who were attending a baseball game at Busch stadium were injured, and as many as half a million residents lost power.

Power had been restored to roughly half of these residents when the second derecho hit just two days later. On July 22, 2003 a long-lived derecho struck a swath from near Memphis, Tenn. to east of Atlanta, Ga. Over 338,000 lost power in the Memphis area. Some people were without power for up to 15 days. A swath from far northern Michigan to Southern New England was struck by a derecho on July 15, 1995. This derecho moved at speeds of up to 75 mph. Wind gusts up to 92 mph were measured along its path. The massive damaging windstorm resulted in 7 fatalities. The total damage estimate was placed at nearly half a billion dollars.