....High Winds, Not Thunderstorms, Cause Damage Across Portions of the Area Monday...


While numerous severe thunderstorms roared across portions of southern Missouri on Monday Mar 31st, their secondary effects reached out and caused significant high winds and damage across the southern half of the Pleasant Hill forecast area.  Non thunderstorm wind gusts were measured officially in several locations across the Kansas City metro at 55 to 65 mph between 7 and 8 AM, with speeds up to near hurricane force estimated by spotters near Moberly, Missouri a few hours later. Many reports of shingle, tree, and sign damage were received between 7 am and 11 am this morning.  The images below represent local storm reports from this high wind warning event. 



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Overview of March 31, 2008 High Wind Reports

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Kansas City High Wind Damage Summary
Central Missouri High Wind Damage Summary



So how did these strong winds develop? Earlier in the morning a large complex of thunderstorms developed across southeastern Kansas and southern Missouri.  At the same time, around 4 AM, a strong warm front raced northward across northern Missouri, allowing winds to change from the east to the south.  As a large complex of thunderstorms pushed across the Ozarks to our south, the large area of rain-cooled air created a local area of high pressure, known as a "bubble" or "mesoscale" high pressure system, often seen under large thunderstorm complexes. This further increased the southerly winds in our region, measured generally at 15 to 25 mph at most reporting stations prior to daybreak. The main culprit for our damaging winds, however, was the result of light rain along the northern edge of this thunderstorm complex. 

Why would light rain cause so much damage? The precipitation that was forced by the thunderstorms well to our south was trying to fall through a layer of very dry air that was present along and just south of the Missouri river. Due to the large amount of dry air at the lower levels of the atmosphere, most of this precipitation fell in the form of "virga", meaning it mostly evaporated as it fell to the ground. While this occurrence is fairly common and of no significance, the long duration of evaporating precipitation was coincident with an area of strong southerly winds 2000 to 3000 feet above the ground, known as a low level jet. Common in the Spring season ahead of developing storm systems, these low level rivers of air can routinely have speeds of 40 to 60 mph during the overnight and early daytime hours. Thus, the evaporating light rain, which caused the air to cool and accelerate toward the ground, helped transfer these low level jet winds to the surface. The result was a 30 to 45 minute period of sustained strong to damaging winds along and south of the Missouri River. The combined effect of the already breezy south winds *and* the low level jet allowed for speeds of 50 to 70 mph to buffet the area, causing widespread minor damage.

The images below represent a "trace" in time of the weather conditions at Lees Summit Airport (KLXT) as well as at the New Century Airport (KIXD) in Olathe, Kansas where winds peaked right around 60 mph!  In this trace, the time highest peak of wind gusts (marked by a solid white line), occurred between 7 am and 8 am in the morning near Kansas City, and between 9 and 11 am in areas near Warrensburg and Moberly.  One very interesting feature you can see by these plots is the transfer of very warm, but also much drier air to the ground.  If you follow the "dew point" trace (yellow line at the top of the chart), you can see that as the wind speeds picked up, the temperature (tan line) also rapidly warmed and the dew points actually fell rapidly.   This is often the case as winds transfer parcels of air from above the ground to the surface, and demonstrates the layer of dry air aloft being brought to the surface.

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Olathe New Century Observation Plot
Lees Summit Airport Observation Plot





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