Two Years Ago: a morning sky turned into night
In the early morning hours of Monday, April 27, a squall line developed in Central Texas, producing 1” diameter (severe) hail as far northwest as Jasper, Texas. Between 6am and 12pm, the squall line raced across the Gulf Coast leaving behind a long path of wind damage. Some unforgettable video was captured including a train being blown off of the Huey P. Long Bridge in New Orleans. The WBRZ Weather Team made the case that this system was what is known as a derecho.
Somewhat uncommon, a derecho (pronounced deh-REY-cho) is a long-lived, widespread wind storm associated with a fast-moving line of thunderstorms. Although damage produced by a derecho can appear similar to that of a tornado, it is often observed in a straight path, hence the term “straight-line” wind damage. According to experts with the National Weather Service Storm Prediction Center (SPC), for a storm complex to be classified as a derecho it must:
- Produce a swath of wind damage exceeding 240 miles
- Include wind gusts of 58mph along most of its length
- Include well separated wind gusts of 75mph or higher
Wind gusts eclipsed 57mph in numerous locations from Southwestern to Southeastern Louisiana with a few isolated gusts in excess of 75mph. The first confirmed reports of wind damage came in Cameron, Louisiana with the farthest eastern reports of wind damage coming from Shell Beach, Louisiana—a distance of approximately 230 miles. While this falls just shy of official criteria, gusts over 58mph were reported by oil rigs the Gulf of Mexico, extending the distance over 270 miles.
With a few confirmed tornadoes, it is worth noting that an isolated spin up within the squall line is not uncommon, nor does it eliminate the system from being classified as a derecho.
Perhaps the only strong case to be made against the classification is that damage was not widespread or significant enough.
Derechos are most common during the late spring and early summer months with more than 75% of them recorded between April and August. Geographically, derechos are most likely to strike from the Upper Midwest to the Lower Ohio Valley and from Northern Texas to the Mid-Mississippi River Valley. For those regions, a derecho occurs almost every year. Just outside of those areas, they happen every 1-2 years. Along the immediate Gulf Coast, derechos are increasingly rare, taking place only about once every 4 years.
SPC researchers only note two “noteworthy” derechos in Louisiana history dating back to 1969—the “April 4-5 2011 Southeast Derecho” and the “Texas Derecho of 1989.”