A look back at last week's incredible Derecho

Extreme weather was a popular topic over the weekend. Extreme heat through much of the nation toppled records across the country over the past several days. The heat isn’t going anywhere though and will continue to be a big story as we go through the first week of July. The other big weather story was the derecho that barreled through the Ohio River Valley into the Mid-Atlantic last Friday. It wreaked havoc in these areas doing widespread wind damage. The Washington D.C. area was one of the areas hit hardest by damaging winds with trees down. Several people were killed by the heat and the storms. Millions were without power and many people will still be out of power through this week.

Before we go into what happened on the 29th first let’s talk about what a derecho is exactly? Derecho is a Spanish name that means either “straight” or “right”. They get that name from the straight-line winds they produce. The derecho is different from a normal squall line or bow echo because they last such a long time and can travel so far. The line of storms is classified as a derecho if it produces damaging winds of 58 mph or more over a path of 240 miles or more. The main threat from derechos is straight-lined winds that can gust as high as 100 mph. Spin ups can occur on the leading edge that can produce isolated tornadoes. There were two reports of tornadoes that day on the 29th in Ohio.

The derecho from the 29th began as a cluster of storms that developed around Chicago Friday in the early afternoon hours. The cluster of storms developed into a bow echo in Indiana then really took off in strength as it moved through Ohio. The line of storms would continue to the Mid-Atlantic as shown by this layering of radar images of the derecho through Friday afternoon into Friday night. As you can see by the image below, the line of storms made it to the coast around Midnight EST traveling a staggering 600 miles in just about 10 hours. It left a wide path of destruction and millions without power. Wind gusts ranged as high as 80 to 100 mph.

Over one thousand wind damage reports were recorded mainly from the derecho on the 29th. Hardest hit by the strongest winds was western Ohio along with the Washington D.C. area. You can see how the derecho expanded as it moved generally southeast. There is a little clear space in West Virginia. That’s mainly due to a lack of population to report wind damage. The Appalachian Mountains run through that part of the state.

You need the right kind of ingredients to come together to get one of these things. Derechos usually occur just on the outside of a very large and hot area of high pressure, which we defiantly had that day and was producing record breaking heat. The hot and humid weather made for a very unstable environment in the Ohio River Valley. A derecho composite parameter is shown as of 1pm on the 29th is shown below. It helps to show the likelihood for a derecho to occur and takes into account the dynamics and instability in when making this parameter. Usually a 4 is pretty good. However, values were as a high as a 12. That was more than enough to get a derecho.

After a line of storms has formed we use this next parameter, known as MCS maintenance, to see if it will persist and become a derecho. The numbers reflect the percent chance the complex of storms will hold together. Values out ahead of the June 29th derecho were 90% at 1pm that day. It’s no wonder why this storm system was so destructive and traveled so far.

Derechos aren’t incredibly common because you need such intense instability and the right conditions to come together. Below is a look at the climatology across the country for seeing a derecho. They are most common down south into the Ohio River Valley where we normally see one about once every year. In Western Wisconsin, we typically see a derecho about every two years or so.

Because you need such extreme instability and heat to power these wind machines, they are most common in the United States in late spring and summer months. The bar graph above shows May, June, and July the main months to see Derecho. We still have plenty of time in “derecho” season. This big ridge of high pressure will persist this week. That means more derechos could be possible in the northern tier of states through this week.


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