Figure 1: Picture of a shelf cloud taken by an amateur photographer and Ham Radio Operator Rob Fugina. Great Job! This was taken in St. Charles County, Missouri.
Figure 2: Another shelf cloud picture taken from another location in St. Charles County. This one was captured by Jon Carney, WFO St. Louis forecaster.
Figure 3: Doppler Radar 88D reflectivity image captured at approximately the same time (757 PM CDT) as the shelf cloud pictures above.
Figure 4: A very distinct outflow boundary stretching across eight counties, stretching from Missouri into Illinois (858 PM CDT).
Figure 5: A radar mosaic at 11 PM. Note that the outflow boundary is still very much intact. Given the right conditions, storms will later redevelop along these boundaries.
Figure 6: SPC Storm Report Map for May 31, 2004

NWS
St. Louis, Missouri

Memorial Day
May 31st, 2004

During the afternoon hours of Memorial Day, isolated showers and thunderstorms formed across northern Missouri.  They rapidly developed into an arc shaped broken line of thunderstorms that moved southeastward across northeast and central Missouri, the Greater St. Louis Metropolitan Area, and southwest Illinois during the evening hours. 

The combination of strong winds aloft and a relatively dry low-level atmosphere allowed the showers and thunderstorms to easily produce brief periods of damaging winds.  The severe wind gusts caused numerous trees and power lines to topple, minor roof damage to a few structures, and the destruction of a few sheds/small barns across the county warning area. 

Isolated reports of large hail were also received that evening in conjunction with the strongest thunderstorms, but the number of severe wind related reports far outweighed the number of large hail reports received that evening.

Shelf clouds were common with the thunderstorms that evening and this led to some amazing photography captured by one of our own meteorologists and a HAM radio operator in St. Charles County.  A shelf cloud is a low-level horizontal arcus-type cloud that appears to be wedge-shaped as it approaches.  It is usually attached to the thunderstorm base and forms along the gust front.  The leading edge of the shelf is often smooth and at times layered or terraced.  It is most often seen along the leading edge of an approaching line of thunderstorms, accompanied by gusty straight line winds as it passes overhead and followed by precipitation.  It is an extension of the main cloud and the underside is concaved upward, turbulent, boiling, or wind-torn.  Shelf clouds are formed when the rain cooled air beneath a thunderstorm sinks, hits the ground, and fans out in all directions.  Some of this rain cooled air surges east, lifting and condensing the moist air ahead of the storm, and producing a shelf cloud.  Our office received many reports of wall clouds and funnel clouds that evening.  While shelf clouds look very menacing, they are not wall clouds or funnel clouds and rarely produce tornadoes.

Shelf cloud
Another picture of shelf cloud
757 PM Radar Reflectivity Image

858 PM Radar Reflectivity Image

 11 PM Radar Mosaic

 SPC Storm Report Map for May 31, 2004

   

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