Widely scattered, but intense thunderstorms erupted across west central Missouri and far eastern Kansas on Wednesday (Aug 11 2010) as extreme instability developed amidst 100 degree temperatures and a very tropical airmass over the region. A small cluster of intense storms impacted the Kansas City suburb of Lee's Summit, producing widespread damage between 615 and 630 PM.
Eyewitness accounts, video, and a survey of the damage confirm that thunderstorm damage across southeast sections of Lee's Summit was caused by a series of wet microbursts. Numerous large trees were snapped in an area that encompassed portions of the Windsboro, Charleston Park and Cantebury subdivisions between Highway 50 to just north of Langsford Road, east of Todd George Rd. The fallen trees resulted in damage to fences, porches and several homes in the area. In addition, minor damage to roof shingles and home siding was also observed. Maximum wind speeds are estimated to have been between 70 and 80 mph.
So what is a microburst? A microburst is a very localized column of rain-cooled air that accelerates to the surface and speads out in all directions along the ground. This results in a divergent damage path (think rock in a pond), which is the opposite of a tornado which drawns extreme winds inward toward a common center. Wet microbursts are favored on very hot days, especially when very dry air exists below the cloud base. This existing dry air promotes the evaporation and drag of precipitation (rain/hail) falling out of the thunderstorm, which causes it to accelerate downward. Visit this Wikipedia article for a more indepth explanation.
The graphic above depicts data from a weather balloon launched from the Topeka Kansas National Weather Service at 6PM CDT Aug 11th. The right line is the temperature and the left line is the dewpoint (a measure of the amount of moisture in the air). Where these lines come together is where the air is nearly saturated, resulting in clouds -- in this case, the bases of rapidly developing thunderstorms. Notice that below cloud base the temperature (red arrow line) and dewpoint (green arrow line) quickly spread apart, indicating very dry low level air. This may come as a surprise to most who realize that it felt very humid on Wednesday. While this is true to the apparent feel of the air (which we indicate by the heat index), humidity is relative to the air temperature. At 100 degrees, even with a very muggy dewpoint of 75 degrees, the relative humidity is only 45%! Thus, an intense area of rain and hail falling from a thunderstorm will have ample chance to cool and evaporate as it exits the base of the cloud. Both drag (gravity) and evaporation cause the precipitation to accelerate to the ground, resulting in wind speeds capable of producing intense, localized damage. In fact, a National Weather Service employee weather station located in the heart of the microburst experienced a temperature drop from 97 to 77 degrees in just a couple minutes!
This image shows a cluster of intense thunderstorms affecting eastern portions of Lee's Summit toward Lake Lotawana around 622 PM CDT. This "reflectivity" image measures the intensity of the rainfall, and detects some small hail with the storm, but nothing further. This storm did produce some pea to nickel size hail (amazing considering the 99 degree temperatures just a few minutes prior!), and dumped just over 2 inches of rain in less than an hour!
This is a classic microburst signature from a Doppler radar, taken at 622 PM from the National Weather Service in Pleasant Hill. Doppler radar is able to measure winds along the radar beam. The circle indicates the impact point of this particular microburst about 3 miles east of Lee's Summit, in a heavily populated part of the city. The winds spread out in all directions, much like throwing a rock into a pond. The bright reds associated with the white arrow indicate southeast winds blowing away from the radar site. The bright greens and aqua associated with the other white arrow are associated with northwest winds blowing back toward the radar site. Because the radar can only measure winds along the radar beam, winds from the west and east show up as a "grey" color, indicating near zero wind speeds. Don't be fooled! The winds are equally intense in all directions. A trained operator can quickly identify this signature and the yellow arrows are overlaid to infer how this image is correctly interpreted. A severe thunderstorm warning was actually issued well in advance of this and subsequent microbursts that impacted the Lee's Summit area between 615 and 630 PM.
The following pictures are a before (late April 2010) and after (715PM August 11th) of a Bradford Pear that succombed to ~70 mph winds in Lee's Summit, taken by NWS meteorologist Evan Bookbinder.