What caused the perfect storm?

Photo by Rob Karner

By Linda Alice Dewey
Sun contributor

According to the National Weather Service (NWS) website, conditions on Aug. 2 were ripe for something big to happen. “Northern Michigan experienced a complex severe weather setup,” it reports, “which began with a warm front lifting northward from southern Michigan toward the Straits of Mackinac and into the eastern Upper Peninsula.”

At the same time, “we had an unusually strong upper level low pressure system across the area,” said NWS meteorologist Jeff Zoltoski. “Generally stronger than what we usually see here.” Measurements of air instability were two- to three-and-a-half times the level that would indicate the potential for severe thunderstorms. Asked whether something like this was new, he replied, “It was nothing that was unheard of.”

Mark Torregrossa, chief meteorologist at MLive.com and Farmerweather.com, exclaims, “this severe weather outbreak had four rounds of severe thunderstorms. That’s really amazing to me as a meteorologist. Most of the time, storms here in Michigan [have] one round of storms and then the atmosphere stabilizes and can’t brew another batch.”

Packing winds of 60-70 miles per hour across much of the state, the storms pounded an area north of West Branch with softball (4-inch) sized hail; trees fell by the thousands from just south of the Straits to Kalamazoo; and torrential rains drenched the Lower Peninsula, dropping 2.75 inches in a short period on Detroit. But nowhere was hit as hard as Glen Arbor, where the winds were the story.

Judging by the twisted trees in certain areas, some thought it was a tornado (including initial reports in this newspaper). But WOOD-TV Meteorologist Bill Steffen of Grand Rapids wrote that evening that a tornado would not have caused such widespread damage. Even so, the spottiness of the destruction within the damage area left some to speculate. Several days after the storm, officials from the NWS came in to investigate the damage, which they ultimatelly concluded was caused by “straight-line winds,” which they said reached as high as 110 mph.

This left some of us with many questions:

What confluence of meteorological conditions caused such a severe storm in the first place?

“We had fairly good evidence beginning the night before that there was an enhanced risk, which was pretty high for us — 15-30 times greater than the normal risk,” reported meteorologist Dave Lawrence of the NWS in Gaylord. “The chances were high that something weird would happen.

“Weather is interesting,” Lawrence continued, “in that, if the right amount of heat and humidity build, that fuels thunderstorms. Plus, we had a strong core of wind at 10,000-30,000 feet. It developed very strong thunderstorms and tapped into some of that wind.”

Torregrossa described it this way: a very strong updraft “hit the 100 mph [horizontal] jet stream and got re-directed toward the ground. Now, a heavy, colder, moisture laden swath of high winds is heading right toward the ground. That’s a textbook case of straight-line winds.”

What caused this to happen?

“Straight-line winds,” Torregrossa answered, “are caused by strong horizontal winds aloft getting transported down to the ground. Rain can help the increase in wind speed, as evaporating rain cools the air and makes it heavier. The heavier air rushes to the ground, increasing wind speeds.”

“Part of the reason it was so severe along the coast,” Lawrence explained to the Sun, “was that Lake Michigan is a frictionless surface, so there is nothing to slow that wind down. You guys were essentially the windbreak. There were incredibly strong winds coming onshore. That’s why we saw that swath from Glen Arbor to the Sleeping Bear Dunes, where it was really the worst.”

If these winds were “straight-line,” why were so many trees twisted at the break-off point?

Torregrossa, who hails from Saginaw, visited Glen Arbor the day after the storm. “I was at the damage scene,” he wrote afterward. “Even though trees look ‘chewed up’ and twisted at the top, this can still be straight-line wind damage, as the NWS declared. Even a line of storms generating ‘straight-line winds’ will have some rotation. There will be enough rotation in the wind at times to make some think it was a tornado.”

One would expect that “straight-line” winds would cause everything to fall in the same direction. Here, trees fell east, but in at least one spot, they fell southeast. In others, they’re a tangled mess. Why?

Lawrence said it’s because 100 mph winds at 30-40,000 feet were brought straight down to the earth. “It was classic straight-line wind damage. Picture the biggest swimming pool you can imagine, fill it with water, send it 40,000 feet up, then drop it down. What happens? It goes in all directions in a straight line. This one loaded up and hit the ground with such force that it did much damage.”

That explains the southeast-falling trees. But if the wind hit and went in all directions, why do we see so many trees falling primarily to the east (or southeast)?

“You’re just a single point,” said Lawrence. “Within the current you’ll get little currents of eddies.”

Torregrossa elucidated further. “Yes, the wind is directed down toward the ground, but with a west to east push in this case. The wind has vortices in it. That’s why you get swirls of damage and twisted trees even in a straight-line wind event. So folks see a twisted tree or two and want to proclaim it a tornado. If this had been a tornado, we would have had hundreds of pictures of the funnel. Have you seen even one picture of a funnel? I haven’t.”

“Maybe we should come up with another name so people don’t get hung up on ‘straight-line,’” Torregrossa suggested. “Blow a candle out, and does the smoke travel straight from your breath? No. And, at the edges of the smoke, there are vortices.”

Channel 7& 4 chief meteorologist Mark Watkins, added this, “The powerful winds are downbursts … rain-cooled air that comes down with no twisting in the debris. Downbursts knock things down in a line, or the damage may look like something flattened it from above.” Aerial after-the-storm photos of Alligator Hill come to mind. “Think of a downburst as a foot stomping down. Trees will fall in all directions.”

“Although I’ve not heard anyone else call it this,” commented Watkins, “I think what we experienced was a ‘derecho’ (duh-RAY-cho). Thunderstorms and high winds were forecast, but when these storms develop straight-line, tornadic speed winds, and it lasts for hundreds of miles, then you could call it a derecho. Derechos happen once or twice a year around the country.”

Torregrossa agreed. “Yes, it probably was. A derecho is a long-lived straight-line wind event causing damage along a path 250 miles long or more.”

Lawrence of the NWS doesn’t think so. “Derechos usually are longer lived, high wind events that typically span several states. The most notable one here was in 1995. It came all the way from North Dakota into Michigan and into the Ohio Valley. This wasn’t as long-lived.”

Those high-level winds seem to be the culprit causing the magnitude of the storms. What caused them to be so strong in the first place? Dare we ask? Could this storm’s magnitude be attributed to Climate Change?

“It’s not a change like global warming,” answers Torregrossa. “We get straight-line winds here in northern Michigan every 5-10 years. It’s just that it’s so isolated that any given spot has a good chance of missing it. Early August was just Glen Arbor’s turn to get hit. On the larger scale, the upper level winds were stronger than normal for this time of year and so produced more severe winds with the storm.”

“It is exceedingly difficult,” Zoltoski pointed out, “to blame any single event on global warming or climate change.”