Tuesday, March 3, 2020

March 3, 2020: Double-digit death toll from nighttime tornadoes in Tennessee

The first truly major tornado episode of 2020 took place last night in Tennessee where at least 24 people were killed by a tornadic supercell moving straight east through central Tennessee.  A large tornado moved through Nashville near the downtown (see above) after midnight, killing 5 people in Nashville and the area east of the city.

The most deaths (at least 18) occurred in and near Cookeville, 60-70 miles east of downtown Nashville, with a tornado from the same supercell.

The upper air setting associated with this tornado episode was a little unusual for March.  Most such events occur with a large midlevel (500 mb) trough moving east through the southern states of the U.S. and prominent southwest flow aloft over the Tennessee area.  But in this case, midlevel flow was west to east over Tennessee (see the 18-hour NAM model forecast below), with a positive tilt trough (thick dashed red line) approaching from the west-northwest (rather than the southwest) within a northern branch of the jet stream:

Another midlevel trough and closed low were way back to the west in northwest Mexico within a separate southern branch of the jet stream.  But the flow between these two jet stream branches was "spreading out" over the Tennessee area (see large white arrows in the graphic above), providing strong lift within an area where significant combinations of instability and low-level shear as indicated by the energy-helicity index (EHI) were located (see inset and the yellow oval shown above).

At midnight CST (0600 UTC, about 35 minutes before the tornado hit Nashville), the effective- layer significant tornado parameter (STP) from the SPC mesoanalysis was sizable (around 3.0) over central Tennesssee, indicating combinations of instability and wind shear that were favorable to support supercell tornadoes:

At around the same time, the surface map showed a warm frontal segment  moving east through central Tennessee, just ahead of the favorable STP environment shown above:

Areas near warm fronts where instability and wind shear are increasing (as indicated by the earlier STP graphic) are good tornado producers when the warm, moist air moves north and east, and this case was no exception.

The High-Resolution Rapid Refresh (HRRR) model forecast from mid-morning on March 2nd did a reasonable job suggesting the tornado potential for the coming nighttime hours going into March 3rd.  The model forecast supercell storms over western into central Tennessee (note the black rotation tracks on the radar forecast below), along with forecast 6-hour updraft helicity swaths from 0200 UTC to 0800 UTC (also below) suggested storm rotation and certainly possible tornadoes given the instability and shear environment discussed earlier.

Just prior to the tornado in Nashville, the RAP model analysis sounding at 0600 UTC showed a setting with large low-level and deep-layer shear (0-1 km SRH > 400 m2/s2, and 0-6 km shear > 60 kt) and adequate instability (MLCAPE around 800 J/kg):

Above, I've highlighted in yellow the most important parameters regarding support for supercell tornadoes.  The STP and EHI values, while supportive of tornadoes (around 2.0 to 3.0), weren't unusually impressive for a killer tornado with double-digit deaths, probably due to this being an early season event with large shear but relatively small CAPE, typical of cool season tornadoes in the southeast U.S.  But, notice the large amount of low-level CAPE below 3 km (125 J/kg of "3CAPE" at lower left on the graphic above).  Put that with the large low-level shear/helicity and deep-layer shear, and that probably facilitated strong low-level tilting and stretching of vorticity within the supercell storm updraft to generate a tornado stronger than one might expect with less than 1000 J/kg.

Here's another image of the tornado moving near downtown Nashville, as indicated by the power flash left of the skyline:

As I write this, preliminary survey information suggests at least EF3 intensity with the tornado in areas just east of downtown Nashville.  Given that this tornado occurred in a metro area after midnight, it is fortunate that the death toll in Nashville wasn't higher, possibly indicating some effectiveness of warnings there. Sadly, death tolls were larger farther east as the storm tracked toward east-central Tennessee in the middle of the night, a difficult time to make people aware of tornado warnings.

Jon Davies  - 3/3/20

***********  UPDATE 3/5/20 *************
The tornado that hit Putnam County and Cookeville around 70 miles east of Nashville has been rated EF4 (violent in intensity) with 18 deaths and 88 injuries. Please consider helping the tornado victims in Tennessee... here are a couple sites you can visit regarding donations:
CNN - How to help Tennessee storm victims
KIRO - How to help Tennessee tornado victims