One week ago, a very interesting and rare weather phenomenon occurred in a small, but populated part of the Miami Valley.
On July 11th, severe storms erupted across Indiana and began moving eastward very slowly. The storms were responsible for numerous severe weather reports around Indiana including funnel cloud sightings. However, as the storms moved closer to the Miami Valley, they began to weaken, and weaken fast. By the time the storms reached the Ohio state line around 1:30 p.m., they were producing little if any lightning strikes.
As the storms fell apart, a wake low was formed producing wind gusts over 60 mph. So, what is a wake low? Well – first, let’s go back to what was happening at the time.
Early in the afternoon, a band of light to moderate precipitation pushing into the Miami Valley. At first glance on radar, nothing looked to ominous – but then – as the area of rain was pushing into Wayne County, Ind. – both meteorologist McCall Vrydaghs and I started to notice the wind speed being measured on doppler radar started to ramp up quickly. The wind speed about 2,000 feet off the ground was blasting through at nearly 80 mph. While typically wind speeds at this level of the atmosphere don’t usually make it to the ground, one way they can is with falling rain.
In a small geographical area about 40 miles wide and 80 miles long, the line of decaying storms started to bow outward in advance of the approaching line of rain, which helped bring down the higher winds being generated at higher altitudes. When this occurred, wind damage reports and power outages began to be reported. By the end of the wind event, over 50,000 people lost power across the Miami Valley. Dayton Power and Light said this was the worst storm related outage since the derecho windstorm of 2012.
So, if the storms were quickly weakening, what caused the severe winds? This is where the meteorology gets a bit more complicated, but let me see if I can explain what happens. Likely in school you remember learning about high pressure and low pressure on weather maps. You likely know that high pressure generally means good weather and low pressure typically means bad weather. You may or may not also remember that air typically moves from high pressure to low pressure. It turns out that these pressure differences can occur on a much smaller scale.
These big clusters of thunderstorms that form can generate mini- low and high-pressure systems. Both falling rain and the evaporation of rain create sinking air. This sinking air creates high pressure. Since air is being forced downward where it is raining, the area immediately behind the falling rain rises to “replace” the air moving into the mini-high pressure system. This becomes a cycle. This process begins to create a low-pressure system where the rain was expected. Sometimes the pressure gradient can increase rapidly which in turn, increases the wind speed. The higher the pressure gradient, the higher the wind speed .
It is believed that this is what happened in the Miami Valley last Tuesday. According to meteorologists from the National Weather Service, this rare weather event typically happens once every few years but is more common in the Plains. The legendary Ted Fujita, who is credited for coming up with the scale to measure tornadoes, is also credited for first describing wake lows back in 1955. For those who spent much of last week without power, I imagine they are hoping it is several more years (or longer) before we experience this event again.
Eric Elwell is WHIO StormCenter 7 Chief Meteorologist. Contact him at [email protected] or follow him on Facebook and Twitter.