Lab 5: Nashville 2020 Tornado Outbreak

 Lab 5: Severe Weather Signatures on Doppler Radar in Nashville 2020

By: Brittany Kehrer

Figure 1: The picture was taken in Nashville, TN on March 3, 2020, after several violent EF3 and EF4 tornadoes passed through.  The image was taken from Wikipedia.  

On March 2-3, 2020, several supercell thunderstorms made their way across southeast Missouri, southern Kentucky, Tenessee, and central Alabama.  These thunderstorms brought - in some regions - baseball size hail and damaging EF3 to EF4 tornadoes.  For the purposes of this blog, I will be focusing on the tornado outbreaks that occurred near Nashville, TN in the early morning hours of March 3, 2020.  These damaging tornadoes seen in this region caused several injuries and 25 fatalities. The first tornado was categorized as an EF3 and stretched up to 60 miles - I will refer to this tornado as 'Tornado 1' during this blog.  The second tornado was categorized as an EF0, and was short-lived - I will refer to this tornado as 'Tornado 2' during this blog. Before we take a look at the radar data associated with these Tornadoes 1 and 2, I want to dive deeper into the background associated with each Tornado.  I also want to note that there was a severe EF4 tornado that occurred after Tornado 2, but the length of my data only covers Tornadoes 1 and 2.  

Tornado 1 touched down east of Nashville, TN at 12:32 AM CST (0632 Z) and was last seen at 01:35 AM CST (0735 Z).  This EF3 tornado stretched a total of 60.13 miles and had a maximum speed of 165 mph.  Tornado 1 caused over 200 injuries and 5 deaths.  Over $90 million worth of damage was reported from Tornado 1 alone.  Tornado 2 touched down north of Nashville, TN at 1:38 AM CST (0738 Z) and was last observed at 1:42 AM CST  (0742 Z).  This EF0 tornado stretched 3.32 miles and had a maximum speed of 75 mph.  For most of the figures provided - and for most of this blog discussion - I will be focusing on Tornado 1, however at the very end of every GIF provided, you can catch a slight glimpse of Tornado 2, so it is worth noting.

I first want to start with a zoomed-out radar reflectivity loop of Nashville, TN on March 3, 2020, from 0630Z to 0730 Z - Figure 2.  This GIF alone can allow us to see some interesting radar signatures.  It can first be clearly seen that this severe thunderstorm system is made up of several multi-cell thunderstorms.  With these multi-cell thunderstorms, we can see several radar reflectivity cores - or several regions of high reflectivity within the bands.  Because these multi-cell thunderstorms are very strongly broken up, it looks as though several multi-call lines are also present.  This is most clearly seen northwest of the radar.  Once we zoom into a more localized region, we will see some more radar signatures.     

Figure 2: Animated GIF of Radar Reflectivity (0.5 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, between 0630Z and 0730Z in Nashville, TN (BNA).  The hot pink line shows the region where RHI will be observed in Figure 3. 

Before looking at the zoomed-in radar data, I also want to examine the Range Height Indicator (RHI) of this severe weather event.  From Figure 2, the hot pink line shows the region of the RHI in Figure 3.  The most beautiful thing about Figure 3 is the way we can clearly see the storm approach the radar and then pass the radar.  One thing that I was looking for was regions of strong updraft for indications of Hail. This can be very briefly observed on the right-hand side of the radar directly before the storm approaches.  This updraft only occurs for 1 or 2 frames but is an indication that some hail was occurring ahead of the tornado, then followed by significant amounts of rainfall.  The height of this system is also not as high as I would have predicted, but because this is a multi-cell thunderstorm, the vertical height makes sense.  If I were to have taken an RHI scan north of the radar, where the multi cell thunderstorm pattern is not as present, I would expect the heights to be closer to 20,000 m.     

Figure 3: Animated GIF of Range Height Indicator (RHI) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, between 0630Z and 0730Z in Nashville, TN (BNA).  The scale used matches the scale from Figure 2. 

Now, let's zoom in and focus on Tornado 1. From a first glance (or from the first few glances) it might not be clear where Tornado 1 is occurring in Figure 4 (radar reflectivity at 0.5 degrees) and Figure 5 (radar reflectivity at 1.3 degrees).  This is where Figures 6 and 7 come into play.  In Figure 6, I have indicated where Tornado 1 is first seen on the radar screenshot.  In Figure 7, I have indicated where Tornado 1 occurs at a later time in its path on the radar screenshot. Figure 6 was taken at 0636 AM - 6 minutes after this EF3 tornado touched down.  Figure 7 was taken at 0708Z, approximately 20 minutes before the disappearance of Tornado 1 (and at a radar reflectivity height of 1.3 degrees). 

Now that the tornado was pointed out in Figures 6 and 7, it can be more easily followed in Figures 4 and 5.  Because Figure 4 was taken at the base height of 0.5 degrees, there is a ton of ground clutter that is noticed in the loop.  In Figure 4, there is also a hail spike that occurs very briefly following the tornado, another indication that hail occurred.  However, because of the ground clutter, the hail spike is very faint. Figure 7 was taken at a height of 1.3 degrees, so any ground clutter has mostly disappeared and the tornado features are much more easily noticed.  It is also amazing to see the vertical depth of the tornado between Figure 4 at 0.5 degrees and Figure 5 at 1.3 degrees.  

Figure 4: Animated GIF of Radar Reflectivity (0.5 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, between 0630Z and 0730Z in Nashville, TN (BNA).  This GIF is similar to Figure 2 but is more focused on the region of observation.

Figure 5: Animated GIF of Radar Reflectivity (1.3 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, between 0630Z and 0730Z in Nashville, TN (BNA).  This GIF is of the same observation region as Figure 4.

Let's look at some radar signatures associated with Figure 6.  The first, and probably most noticeable, is the hook echo that is starting to form (indicated by the right white arrow).  As the storm progresses, this hook echo gets deeper.  This hook echo, with a bright center of reflectivity, indicates a region of a debris ball forming or a debris ball that has already started to form.  The next radar signature that I notice is a v-notch to the left of the hook echo (indicated by the left white arrow).  This v-notch is indicative of a very strong updraft, and I find it interesting how it is following the hook echo (to the left of the hook echo).   

Figure 6: Screenshot Image of Radar Reflectivity (0.5 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, at 0636 Z in Nashville, TN (BNA).  This screenshot is similar to Figure 2 but is more focused on the region of observation.  The two white arrows show regions of discussion.

The radar signatures seen in Figure 6 and also seen in Figure 7.  I want to note that I decided to use Figure 7 at a radar reflectivity at 1.3 degrees because the radar signatures are more clearly seen at this scan height (similar to Figure 5).  I also want to state again that Figure 7 is taken at a later time stamp and is now many miles away from the origin, and has grown in strength.  Similar to Figure 6, there is a noticeable hook echo in Figure 7 (indicated by the white arrow).  The maximum in reflectivity in the hook echo represents a debris ball formed by the tornado.         

Figure 7: Screenshot Image of Radar Reflectivity (1.3 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, at 0708Z in Nashville, TN (BNA).  This screenshot is similar to Figure 2 but is more focused on the region of observation.  The white arrow shows a region of discussion.

I wanted to include the following two Figures to show the radial velocity of this severe weather event.  Figure 8 is an animated GIF of Tornado 1 in the same region shown in Figure 7 - to the right of the radar.  As the GIF progresses, you can see a light blue velocity ball make its way across the screen.  This makes me believe that the blue velocity ball is infact associated with the Tornado 1.  

Figure 9 is a matching screenshot of Figure 7 showing the radial velocity.  As you can see, the blue ball in Figure 9, matches the location of the hook arrow in Figure 7.  I find it interesting how these radial velocity images show the blue velocity ball as an almost perfect sphere shape instead of a common Tornado Vortex Signature.  My theory is from the strength of the winds (165 mph).    

I also want to note that at the end of Figure 8 - where the blue velocity ball seems to die down - is where Tornado 2 picked up shortly after.  

Figure 8: Animated GIF of Radar Velocity (0.5 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, between 0630Z and 0730Z in Nashville, TN (BNA).  This GIF focuses on the right side of the radar scan (second tornado outbreak observed).  

Figure 9: Animated GIF of Radar Velocity (0.5 degrees) of the severe thunderstorm with EF3-EF4 tornadoes that occurred on March 03, 2020, at 0708Z in Nashville, TN (BNA).  This screenshot focuses on the right side of the radar scan (second tornado outbreak observed).  The white arrow indicates an area of discussion. 

Overall with this tornadic event, you had to zoom far into the radar in order to see the event occurring.  Because this severe thunderstorm was a multi cellur event, there are a lot of different sections of the thunderstorm to consider for tornados.  In fact, while some tornadoes were occurring in Tennesee, tornadoes were also occurring in Kentucky. 

Some questions I am left with is concerning the ground clutter seen in Figure 4.  There are many areas where the radar is picking up a "splatter" like pattern but does not seem to connect or correlate to the severe weather system.  It makes me wonder what the radar was picking up.  Some of this "splatter" pattern is seen in figure 5, but is significantly less frequent.  Could this be debris from high winds not associated with the tornado?  It makes me think about what a radar scan of BNA looks like on a clear day.  What ground clutter will be seen on that radar scan?    

The links below provides immense amounts of information of the tornado outbreak that occurred on March 3, 2020, if you want to find out more.  

https://www.weather.gov/ohx/20200303 

https://www.spc.noaa.gov/exper/archive/event.php?date=20200302