June 9, 2011

...Severe Thunderstorms Rattle The Region With Large Hail, Damaging Winds and Flash Flooding...

ROUND 1: Destructive Hail

Severe thunderstorms quickly developed during the afternoon and evening hours of Thursday, June 10th 2011, just north of a warm front that stretched from near St. Joseph to Macon. As series of supercell thunderstorms produced large swaths of ping-pong to baseball sized hail, with a few stones around 3" in diameter observed. Numerous storms impacted the same corridor of north central Missouri for several hours, resulting in nearly 4 inches of rainfall in spots with flash flooding observed from Jamesport to Trenton and surrounding areas.

As bad as the hail was, the event could have very easily unfolded in a far worse scenario. North and northeast storm motions Thursday evening carried storms away from the warm front into much cooler air present over northern Missouri. Had storm motions been to the east, long-lived supercells would have instead ridden along the warm front, likely producing softball size hail and significant tornadoes.

Radar Loop (click to enlarge):

Storm Reports (click to enlarge):

Local Storm Reports 

Click Here For A List of Specific Storm Reports


Wind damage in La Plata, MO    

Photos (Top Left/Below) courtesy of Michael Shively. Taken from La Plata, MO.
Photo top right courtesy of Kim Heaton via facebook, near Tenmile, MO (Macon County).

Large hail nean Tenmile, MO (northeast of Macon)



ROUND 2: The Heat Burst

Maybe even more impressive than the large hail, was a heat burst that impacted portions of Johnson, Miami and Linn counties in Kansas around midnight Friday. This event last several hours from Emporia to Olathe between 10PM and 1AM, less than 24 hours after such a phenomenon struck the Wichita area, making national headlines (ref: http://www.kansascity.com/2011/06/09/2939426/heat-burst-sends-temperature-in.html) as temperatures spiked above 100 degrees at 1 AM Friday. At both Olathe airports, temperatures rose into the upper 80s at midnight, while the dewpoint (a measure of moisture in the air) crashed from muggy values in the upper 60s to the mid 40s. This heat burst was accompanied by measured winds of 60 to 65 mph and visibilities of 1 mile or less in blowing dust and dirt stirred up by the hot downburst. Little to no rainfall was observed, nor was any lightning, confusing residents as the winds knocked down trees and caused other minor damage across east central Kansas.

What Is a Heat Burst?

A heat burst is a rarely experienced phenomenon, owing to its typical overnight occurrence when most people are asleep. They most commonly occur across Nebraska, Kansas and Oklahoma during the late Spring and early Summer season as the active severe weather season yields to increasingly hot air aloft that limits widespread thunderstorm development.

Heat bursts are caused by collapsing thunderstorms which mave moved into an airmass that is too warm and too dry and the mid levels of the atmosphere to sustain them. Rain falling out of the rapidly decaying thunderstorm first evaporates completely. At roughly 10,000 feet above ground level, the air is now cool and wet, owing to the evaporated rainfall. This rain-cooled air weighs much more than the surrounding air and thus accelerates toward the ground. As air sinks toward Earth from above, it undergoes both a rapid warming and drying. Because the air came from such great altitiudes, the warming and drying can be significant, in some cases spiking thermometers to over 100 degrees! Heat bursts can last from several minutes up to several hours (when more organized thunderstorm complexes are continually fed into the collapsing stage), before temperatures return to normal.

A person experiencing a heat burst will notice an abrupt change in temperature (sometimes climbing more than 20 degrees in several minutes), as well as a sharp decrease in humidity. The mild/humid evening airmass is often replaced by one that is compared to feeling like the person was transported to Pheonix or another desert location. Very strong winds will always accompany a heat burst, with speeds typically peaking between 60 and 80 mph. These winds are the equivalent of a severe thunderstorm and are certainly capable of producing significant damage, especially when the heat burst is long lived. Visibilities may be significantly reduced as the winds slamming into the ground will often stir up loose dirt, dust and pollen.

Meteogram from Olathe New Century Airport                    Olathe Jo-Co Executive Meteogram

The vertical black lines show interesting break points in the meteograms (weather graphs) for both Olathe airports. Interesting features to note include temperature rises (spiking at 92 degrees at 120 AM at Johnson County Executive), a sharp decrease in moisture (dewpoint), a sharp spike in pressure, and strong winds which gusted over 60 mph at New Century Airport (IXD) and to 58 mph at Johnson County Executive (OJC).

Graphs courtesy of Daryl Herzmann at Iowa State

 Photos of heat-burst induced tree damage in Gardner KS, taken by Danedri Thompson

Tree Damage in Gardner KS Photo 1   Tree Damage in Gardner KS Photo 2

Story by Evan Bookbinder
Senior Meteorologist, National Weather Service Pleasant Hill, MO

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