Dueling Derechos: Anatomy of weekend "macrobursts"

Meteorologists may be studying last Friday and Saturday’s severe storm outbreaks for a long time to come. The storm complex blew up over Minnesota Friday afternoon and morphed into a massive Mesoscale Convective System (MCS) Friday night over Wisconsin.

This highly detailed 1-minute #GOES16 visible satellite loop with SPC Storm Reports overlaid shows the life cycle of Friday’s incredible system.

Two derechoes

A second storm complex blew up over South Dakota Saturday, then raced through southern Minnesota into Wisconsin. NOAA is still studying the back-to-back weekend derechoes.

Both systems appear to qualify as derechos according to NOAA.

A derecho (pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") is a widespread, long-lived wind storm. Derechos are associated with bands of rapidly moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi-linear convective systems.

Although a derecho can produce destruction similar to that of a tornado, the damage typically occurs in one direction along a relatively straight path. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the swath of wind damage extends for more than 250 miles (about 400 kilometers), includes wind gusts of at least 58 mph (93 km/h) along most of its length, and also includes several, well-separated 75 mph (121 km/h) or greater gusts, then the event may be classified as a derecho.

Record “CAPE”

This is why I and other meteorologists were sounding alarm bells Friday. One key measure we look at when assessing severe storm potential is called Convectively Available Potential Energy. (CAPE) A typical summer CAPE value in southern Minnesota might waver around 1,500 j/kg.

Friday’s value of 8,300 j/kg from the NWS balloon launch in Chanhassen is the highest ever recorded.

Widespread damage in Wisconsin

The storms caused scattered tree damage and produced hail up to 3 inches in diameter in Minnesota.

But damage is much more extensive in Wisconsin. Tens of thousands of trees were snapped off or uprooted across northern Wisconsin.

Bow echoes and “bookend vorticies”

There were a few tornadoes reported over the weekend.

But most of the the widespread wind damage associated with the weekend storms is called a downburst. These damaging wind events occur along the leading edge of rapidly advancing squall lines called bow echoes. Swirls along either edge of the bow are called bookend vorticies. You can clearly see the bowing signature evolve on Friday’s evening’s radar loop across northern Wisconsin.

Anatomy of a macroburst

Smaller scale downbursts are referred to as microbursts. The scale of last weekends damage lead the Green Bay NWS office to use the term macroburst. Damage was extensive and widespread.

The damage swath for one of these events was 15 miles wide and 25 miles long, with wind speeds over 100 mph. Here’s the event description from the Green Bay NWS Office.

A line of severe thunderstorms moved across much of central, north central, and eastern WI during the evening hours on July 19, 2019. Widespread tree and power line damage was reported from central Wisconsin into the Fox Valley and lake shore. WPS reported over 50,000 outages at the height of the storm.

The worst damage appears to have been associated with a "macroburst," a large downburst of straight-line winds that affected Langlade and Oconto counties. Many tens of thousands of trees were snapped or uprooted, resulting in damage to dozens of homes and cottages. The path of the most extreme damage may have been 15 miles wide, and more than 25 miles long. Winds were probably over 100 mph in this area.

More study will be done on these events. But it’s already clear this was one of the most intense and widespread severe storm outbreaks on record in the Upper Midwest.