Mesocyclone Formation from a Gust Front

All images and text © copyright Gene Moore
unless otherwise indicated

This case is a good example of wall cloud and mesocyclone formation from an average looking thunderstorm. Early observations were not encouraging that a tornado would occur on this day. The events leading up to the development of the mesocyclone present a good case for beginning storm chasers to recognize one set of precursors to wall cloud development. During hot summer months the convective cloud base is quite high. The outflow from the gust front provides a boundary for low level high dew point air to be lifted and cooled forming a wall cloud. This air is then lifted above the condensation base and into the storm. This is the difference in the convective condensation level of the thunderstorm base and the new lifted condensation level of the wall cloud due to cooler outflow air.

Synoptic Situation: During the mid-summer months the jet stream pulls north to Canada and the northern plains of the US. In this situation the southern extent of the jet flowed along the Kansas - Oklahoma border where a stalled front was located. During the early afternoon a 1004 millibar low formed near Gage, Oklahoma and a stationary front draped along the northern Oklahoma counties. Cooler air was behind the front and it was a typical hot muggy July day to the south of the boundary. The winds aloft were good for midsummer months with generally 30 knots from the southwest at midlevels and 40 knots from the west at upper levels. The first set of images provides an overview of the environment during storm formation.

building storm NNW - along front Developing storm distant west Developing storm ENE (of I-35)
The view looking NNW from northern Oklahoma. Sufficient vertical wind shear exists and may be noted by the character of this building storm. Each consecutive tower down stream is larger. Telephoto shot of an isolated storm developing in northwest Oklahoma. This cell was along the front and northeast of the surface low pressure system. Chasers on this storm reported no tornadoes. Storm developing to the NNE along the stationary front. Convective cloud bases are approximately 8,000 ft above ground level (AGL). This is this cell that will produce the only tornado of the day.

Outflow boundary begins to consolidate Gust front stalls
Large rain or core area of storm generates a cool outflow boundary or gust front. Note the curling of the cloud on the south (right) extent of the outflow boundary. This overturning is often seen in gust fronts. We are looking east down the length of the outflow boundary.

Gust front stalls and leading edge becomes convective. An updraft is beginning on the south end of the outflow boundary.

Wall cloud beginning three small funnels rotating around common center

During this image the gust front consolidated into a wall cloud as up motion began on the north or left side of the lowering. Ragged inflow clouds streamed into the wall cloud from the south (right). This visually obvious change will produce a dramatic physical difference in the intensity of the storm; very fast. An updraft such as this is probably not showing on a surface or low tilt radar image.

Strong inflow was lifting into the left or north side of the wall cloud during this shot. A separate cloud was taking shape on the back side of the wall cloud. During this time multiple funnels began rotating just below cloud base and may be seen if you enlarge the image. This image was shot from a moving vehicle because conditions were changing rapidly. From the time the wall cloud formed to this rotation started was less than 5 minutes. The spotter or chaser able to anticipates this set of events will be a jump ahead of the warning.

Scud rising into cloud from trees mesocyclone lifting rotating scud

In the last image a section of the large lowering had broken away from the main wall cloud and began to rotate. This region becomes totally separate from the main wall cloud and the motion was much stronger. Scud was lifting and rotating into the developing mesocyclone.

The strong updraft remained rain free as more cloud tags rotated into the base of the mesocyclone. The separate rotating cloud became totally concentric and spin independently of the larger lowering.

  tornado with helix circulation around small funnel

A helical shaped tornado formed under the mesocyclone. The funnel is shown on the back wall of the updraft and a thin tube extends to ground. Numerous shots were taken at this time, but the funnel generally remains about this size. It is assumed that only F-0 to F-1 tree damage resulted from this tornado. A small inflow tail is beginning to form on the south side of the mesocyclone but never became more organized. The tornado was near Red Rock, Oklahoma which would be location of a large tornado years later.

This is a good time to take note of the extent of the new updraft area. It is about 1 1/2 miles across and has lowered about ninety percent further to the ground that the original thunderstorm base. Look at the top left of the image. That's the original storm base that formed off of the convective lifting produced by convergence along the front and afternoon heating. This new base associated with the wall cloud is about 1000 feet or less AGL.

tornado in progress southeast rotating mesocyclone

Good size funnel remains along the back of the mesocyclone but only a thin condensation funnel extends into the trees. The mesocyclone continues to tighten it's spin and become smaller. The tornado dissipated a couple minutes after this shot.

To the west a new mesocyclone rapidly formed. This second and more menacing wall cloud amazingly never produced a tornado.

  gust front forming wall cloud.

This is another example of a wall cloud forming out of a gust front. Note the linear appearance especially on the left side of the image. This was the original outflow cloud. When this image was taken the top of the gust front was lifting into the base of the storm. Wind from the left and right are streaming into the center of the updraft. At this point the storm is going from being outflow dominated to inflow dominated. The best tornadic storms are mainly inflow dominated; although, a balance must be present. In this case the storm later formed a wall cloud, but the transitional phase is the subject shown here. We hope this series helps you recognize the early signs of a gust front storm developing a wall cloud. These observational techniques allow chasers and spotters to gain valuable position on the correct storm. Tornadoes that form quickly may be missed by waiting for the warning to be issued.

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