Monday, November 12, 2012

Henderson Mini Supercell

Sunday afternoon a small but intense thunderstorm developed across Cherokee and Rusk Counties that on radar showed what appeared to be weak but persistent rotation for over one hour as it moved northeast paralleling highway 79 south of Henderson then highway 43 north of Henderson. After looking at the radar data it appears we had a mini supercell develop across East Texas. The storm was never more than 5 miles across and stayed for the most part under 30,000’ in elevation. This is typically not the type of storm that would produce severe weather but in Sunday’s environment, this little storm was able to ingest a lot of low level shear and spin like a top as it moved across the landscape.

At 3:10PM Sunday afternoon the thunderstorm in question was located near New Summerfield in eastern Cherokee County. At this time the storm’s top was just under 20,000’ in elevation and had a reflectivity core of with a maximum of 54.5dbz under 10,000’. This indicated basically a heavy rain shower, more than likely not even producing any lightning. But at the same time the velocity data did show rotation up to 8000’ in elevation.

Now the rotation was by no means strong but was the beginning of a long lasting rotation that would last in this storm for over 1 hour.

At 3:44PM the storm in question showed an increase in the inflow velocity a sign that it could be attempting to produce a tornado. Again the rotation was not very strong but at this point it had lasted for over 30 minutes and was showing an increase in strength. At this time the core was 62dbz at 10,000’ but the top of the storm was just barely reaching 30,000’. It was probably producing a few lightning strikes but again it was nothing you would normally consider severe.

At 3:48PM is when the storm finally peaked showing its mini supercell characteristics. At this time there was rotation, again not very strong, but it was the tightest it has been the entire life span. It is also around this time when a very suspicious photo was sent to us showing what appears to be a well organized mini mesocyclone with a funnel cloud lowering towards the ground. As of this writing it is unknown if the funnel ever touched down to produce a tornado due to no damage reports being submitted.

As mentioned before, the rotation on radar was not particularly strong and thus really not considered to be a tornado threat. But when you look at the strength of the rotation based on the diameter of the mesocyclone, it is not out of the question that a mini supercellular tornado occurred. A study by the National Weather Service Shreveport in 1997 developed a chart for warning tornadoes during mini supercellular events. A nomogram was developed in 1997 as a guideline for issuing tornado warning based on the rotational velocity (the absolute values of the inbound and outbound velocities divided by two) in a mesocyclone. Based on the velocity data on Sunday the storm in questioned rotational velocity was between 15 and 19, well below the threshold guideline for issuing a tornado warning.

But what the study found in the case of mini supercells was the rotational shear (rotational velocity times 2 divided by the diameter of the mesocyclone) was a better indication of tornado development. When using the rotational shear we come up with a value of 12.3 near the time of the suspicious photo based on a mesocyclone diameter of only 1.59 nautical miles on radar at 3:48PM. This places the rotational shear in the tornado possible area of the rotational shear guideline chart.

Since the only photo we have has the bottom of the funnel block by trees, and we have had no damage reports from the area, we will never know if a tornado was produced. Hopefully a few more photos will surface that are not obscured by trees to give us a better view of what occurred.

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