(Photo source above Ben Yates FaceBook)
Sunday and Monday were very active weather days across the Heartland with severe weather and flooding. Monday afternoon parts of the Heartland were under consideration for a tornado watch but, one was never issued. Conditions were favorable for tornadoes if deep convection(strong thunderstorms) could occur. All afternoon small thunderstorms developed showing weak signs of rotation as the atmosphere was extremely sheared. Around 5PM a thunderstorm rapidly developed to the south of Union City, TN and would go on to produce a very strong EF2 tornado (top winds 130MPH) with out warning. This type of storms is called a mini supercell, a small cousin of the usual large severe storms we see produce tornadoes across the Heartland.
The image above is what the storm appeared like on Doppler Radar as the tornado was on the ground. At first glance it does not appear very strong at all. Our lightning detection was showing a few bolts to the north but at this time this storm was not very electrical, another sign of a weak storm.
The three dimensional view the tornadic storm shows a small core not even reach 20,000' into the atmosphere. The red colors which show heavy rain and hail are barely above 12,000' at this time. normally we see cores to 50,000' on storms that produce tornadoes. So what made this small storm produce such a strong tornado for its size? I had to know and decided to take a closer look.
The image above is a skew-t diagram from the 22:00Z (CDT +5 hours) hour RUC near Union City to get as close to the storm's environment as I could. In the above forecast sounding there is enough CAPE (Convective Available Potential Energy) for surface based thundershowers to develop. Overall it is pretty weak and this typically wouldn't throw out the "red flag" that something bad is about to happen. So we need to look a little deeper.
The image above is the hodograph from the 22:00Z (5PM CDT) hour RUC near Union City, TN. Here the large clockwise curve indicated an enormous amount of shear in the atmosphere, or spin to cause thunderstorms to rotate. Most of the shear is occurring in the same area of the atmosphere we see the positive CAPE. So any thundershower that could develop in this environment would more than likely become a supercell, or mini supercell in this case. Not shown is the 0-1KM bulk shear, or the difference between the surface wind speed and the 1KM wind speed. At the time of this storms the 0-1KM bulk shear was around 30 knots. Any value above 20 knots is favorable for tornado development.
We have already seen what the storm looked like on radar and in 3D. Now let's see what the storm's velocity data was showing, or the winds inside the storm. The image above shows an area of bright green next to an area of brighter red. The bright green indicates winds moving towards the radar at 35 knots. The red colors show winds moving away from the radar at 29 knots. This rotation is indicating a mesocyclone, a rotating thunderstorm. This storm showed a mesocyclone, although weak, for up to 15 minutes before the tornado was produced.
So we now there was a mesocyclone so why was no warning issued? Well to be honest, the mesocyclone was never that impressive on radar in terms of strength to produce a tornado. The chart above shows the comparison to rotational velocity [the absolute value of the inbound plus the absolute value of the outbound velocities divided by two or Vr= (|Vi|+|Vo|)/2] to the range from Doppler Radar. The red dots on the chart show the time of the sampled mesocyclone and its strength, with the time moving from right to left. Notice as the storm gets closer to the time it produces the tornado, the mesocyclone gets stronger but never more than a minimal mesocyclone strength. SO no warning was issued. But the above chart does not take into consideration the size of the mesocyclone, or the diameter. Since mini supercells are much smaller in size than classic supercells, the diameter of their mesocyclone is usually much smaller as well.
Research performed by Kenneth Falk and William Parker from the Nation Weather Service in Shreveport, LA in 1998 showed that looking at the Rotational Shear (Sr=2(Vr)/D where D is the diameter of the mesocyclone in meters) which takes into consideration the diameter of the mesocyclone may be a better determination in whether a storm will produce a tornado. The above chart shows the Sr*10^3 with values in orange showing a tornado is possible and in purple showing a tornado is probably. Notice the depth of the mesocyclone is lower than 10,000' for most of the observations. Normally we would see mesocylone depth greater than 10,000' but again with this being a mini supercell, everything is much smaller.
The above chart is from Falk and Parker's study in which they examined 50 mesocyclones to determine when a tornado is more likely. Again the time line of the Woodland Mills storm goes from right to left. With the rotational shear showing a tornado is probably and the near storm environment likely for tornado development, we will be looking to use this guideline more as we go through severe weather events. Right now we do not have a way this for this information to be generated automatically but it is mathematically possible for us to perform these calculations during sever weather events.
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