Capturing strong downbursts from a supercell in Kansas City

Capturing strong downbursts from a supercell in Kansas City

July 29, 2015 1:07 pm Published by

Understory’s RTi weather station network has captured evidence of a downburst in Kansas City! A downburst is a powerful weather phenomenon that can result in straight-line winds reaching 150 mph with the damage potential of a tornado. Although the dangers of tornados are notorious, there are approximately 10 downburst damage reports for every one tornado report in the U.S., encouraging us to take note of any downbursts we observe.

The downburst we observed occurred during a thunderstorm on July 15, 2015, which entered the southwest side of the city first, moving east, and then began to dissipate while moving toward the northeast. The thunderstorm triggered the downburst when dry air entered the thunderstorm. Rain evaporated, cooling the dry air, which increased the density, causing the air mass to plunge toward the ground. When the cool, dry air hit the surface, a gust front expanded in all directions, with the strongest winds travelling in the direction of storm movement (northeast in this case).

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When dry air enters a thunderstorm, the rain can evaporate and cool the dry air mass, which then sinks. When it impacts the ground, it spreads out in all directions, creating strong straight-line winds.

We know when the gust front arrived at each RTi weather station by the rise in wind speed, and drop in both temperature and humidity caused by the fast-moving, cold, dry air. Examining our wind data reveals the sharp arrival of a gust front that progressed from southwest to northeast across Kansas City. The station that measured the strongest gust front recorded a 3-second sustained gust of 57mph, a temperature drop of 78°F to 71°F (unusual for 9 AM), and a humidity drop of 91.75% to 80.85%. We measure relative humidity, which is the percentage of water vapor in the air currently, compared to the amount needed for saturation at the current temperature. This implies a negative relationship, because colder air can hold less water than warmer air. Usually, when temperature decreases, relative humidity will increase, but in the case of the downburst, both temperature and humidity fall as the stations register the cold, dry gust front.

The first stations to be hit by the gust front (in the SW of Kansas City) were the closest to the origin of the downburst, so they recorded higher wind speeds, and larger temperature and humidity drops. As the gust front expanded towards the northeast and dissipated, the stations documented a weaker rise in wind speed and drop in temperature, although the dry air still caused a clear decrease in pressure.

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Stations’ time series are stacked for easy visibility, ordered by time gust front felt, approximately southwesternmost on the top to northeasternmost on the bottom. The RTi stations in the southwest of the city felt the gust front first around 8:50AM on July 15. The stations to the northeast registered the gust front later, with the latest station at 9:55AM. The time the gust front hit each station is determined by the sharp rise in wind speed, followed by the unusual case of a concurrent drop in temperature and relative humidity caused by the cold, dry gust front.

Our station density within Kansas City allows us to track the movement of the storm across the city. The time delay of the gust front between our stations as well as wind direction gives us information about how the storm progressed and dissipated. Only 21 of the 25 stations in Kansas City felt the gust front, revealing the gust front did not expand to the north of the city. By mapping the times the gust front arrived at each station, we can visualize the path of the storm.

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The storm entered the southwest area of Kansas City from the northwest, then progressed through the middle of the city to the northeast as it dissipated. We can estimate this path by analyzing the delay of the gust front between stations and the wind direction at the time of the gust front. This approximate path is corroborated by radar data.

We are producing an impressive quantity of data that can be used to track storms across cities and assess property damage. Our RTi weather station network allows us to see exactly where damage occurred and what properties a storm affected. Our property-level damage analytics portal gives a summary of storm damage and gives you the tools to understand how to react in a fast and efficient manner. Below is a page from our platform detailing the events of the downburst in Lenexa, Kansas.

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Overview of the damaging wind event in Lenexa, KS. We saw a maximum wind gust of 57 mph which lead to vegetation damage and minor property damage. You can view this storm by creating an account on our platform!