Wednesday, April 1, 2020

The Jonesboro, Arkansas tornado on March 28, 2020: A subtle and difficult forecast setting.


As Covid-19 keeps expanding throughout the U.S. (we all need to keep following social distancing guidelines!), thoughts and well wishes go to people like fellow storm chaser Dr. Bill Hark in Virginia who has been sick with the virus.  We hope you recover soon, Bill.

With this going on and more than 4500 Covid-19 deaths now in the U.S., tornadoes may seem like "small potatoes".  But spring is upon us, along with the threat of tornadoes, no fooling intended on this April Fools Day.

Saturday's EF3 tornado that struck Jonesboro, Arkansas (AR) around 5:00 pm CDT (2200 UTC) is a reminder of that (see photos above).  With a tornado watch out a couple hours in advance and good warnings, no one died with 22 injuries reported, which is good news.

Most meteorologists on March 28 (including me) were focused on the potential for tornadoes over Iowa and Illinois, but the local environment really ramped up quickly in northeast AR during the afternoon, and was more subtle than one might expect prior to a large tornado.  So, I spent some time taking a closer look at this event.

Morning model forecasts did not suggest much low-level shear over AR compared to farther north near a warm front (see 0-1 storm-relative helicity / SRH forecast for mid-afternoon below):

But as the southern end of a large upper system approached (not shown), a low-level jet at around 5000 ft MSL (not shown) increased from 30 kt to 45 kt over northeast AR during the afternoon, helping to generate enough low-level shear to support tornadoes, which we'll discuss in a bit.  What might also be easy to miss is that a subtle boundary also appeared to play a role.

Below is a 3-panel composite radar image from late evening on March 27 through midday on March 28 showing this northeast-southwest boundary (see white arrows) drifting eastward across Arkansas during the 18 hours before the tornado:


Although it's not clear what originated this boundary (NWS-analyzed surface maps showed this as a cold front, which I don't think it was), it appeared to stop near Jonesboro and even back up a bit to the west during the afternoon.  Here's the surface map I analyzed at 2100 UTC (4:00 pm CDT) about 45 minutes before the tornado struck Jonesboro (notice the backed southeasterly surface wind at Jonesboro). This boundary is shown as a thick red-blue dashed line over AR:
And here's a composite radar image at 2125 UTC (4:25 pm CDT) with the boundary from the 4:00 pm CDT surface map above superimposed as a white dashed line:

Notice that the Jonesboro tornadic supercell (white arrow, producing its first tornado near Amagon AR at this time, southwest of Jonesboro) appeared to be moving northeast right along this boundary.  It's possible this boundary provided increased low-level wind shear and convergence to help support tornadoes.

Regarding storm environment parameters over northeast AR, the SPC mesoanalysis showed an area of enhanced low-level MLCAPE (0-3 km above ground, 1st panel below) and, although not particularly impressive, an area of somewhat enhanced 0-1 km SRH (around 150 m2/s2, 2nd panel below) near Jonesboro:


This "overlap" may have set the stage for enhanced tilting and stretching of low-level SRH with the supercell storm moving along the aforementioned boundary.  The 2100 UTC SRH over northeast AR (larger than forecast from the morning model runs) appeared to be the result of the low-level jet increase during the afternoon that I mentioned earlier.

An additional graphic I put together (1st panel below) shows areas of overlap of 0-3 km MLCAPE > 75 J/kg and 0-1 km SRH > 150 m2/s2 in purple from the 2100 UTC SPC graphics above:


Notice that the genesis region of the Jonesboro tornadic supercell was in the purple area over northeast AR, again suggesting localized potential for enhanced low-level tilting and stretching of SRH along the boundary discussed above.  The 2100 UTC effective-layer significant tornado parameter (STP, 2nd panel above) also suggested support for supercell tornadoes over northeast AR.

Here's the 1-hour forecast sounding from the RAP model for Jonesboro valid at 5:00 pm CDT (2200 UTC) at the time of the tornado, suggesting the localized support for supercell tornadoes near the boundary:


Although both STP and the energy-helicity index (EHI) weren't especially impressive (both parameters around 2.0) for a large EF3 tornado, they do indicate an environment supportive of tornadoes, one that may have been given a "boost" by the presence of the boundary in addition to the sounding environment shown above.

Here's a larger view of the forecast afternoon setting from the morning NAM model run, showing the large 500 mb trough moving through the central U.S. with the typical spreading jet stream pattern (thick white curved arrows) ahead of it, providing dynamic forcing and lift:


The insets on the graphic above show EHI and fixed-layer STP forecasts for mid-afternoon (again, not very impressive for northeast AR compared to areas farther north), and SPC tornado reports during the afternoon.

This case is a good illustration of how important it is to monitor features and parameters in real time, as the mid-afternoon SPC mesoanalysis and surface map indicated increasing low-level shear and support for supercell tornadoes near the boundary over AR, which was not forecast well by the morning model runs. 

Away from Arkansas, notice that over Iowa this was a "cold-core" tornado event  with a closed 500 mb low nearby and a boundary intersection (warm front and Pacific cold front) over central Iowa (see the surface map earlier above).  As is typical with cold-core settings, several tornadoes occurred near this boundary intersection over Iowa as it evolved northeastward during the afternoon, including this tornado northeast of Des Moines near Rhodes, Iowa after 4:00 pm CDT:

Back to Arkansas, the Jonesboro tornado setting and environment was more subtle than those accompanying recent large tornadoes such as the nighttime Nashville tornado back on March 3 and yesterday morning's tornado near Eufaula, Alabama (March 31).  In both those cases, the tornadic supercells were near a warm front or stationary front, and low-level shear was larger and more evident (0-1 km SRH 300-450 m2/s2) than near Jonesboro on March 28 (0-1 km SRH 150-190 m2/s2).

I hope everyone reading this navigates the Covid-19 outbreak carefully and in reasonable health over the next couple months!

- Jon Davies  4/1/20

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

Wednesday, January 22, 2020

ChaserCon 2020 is almost here... Tornado Forecasting class on Friday, January 31!



Hi everyone!

ChaserCon 2020 is a little over a week away.  I'm doing a Tornado Forecasting class on Friday evening January 31 from 7:00 to 9:30 pm, with a break in the middle.  You can sign up at:

              http://chasercon.com/node/18

It's $30.00 per person... Just scroll down to the bottom of the page at the link above to register for the class.

I'll do more examples and interactive exercises covering supercell tornadoes than last year at Wichita, including a cold-core tornado case.  So don't miss it if you're interested in forecasting.

There's room for around 100-110 people, and we're up to 83.  Sign up soon!

I'll also be doing a talk on Saturday afternoon (February 1) about the crazy and confusing tornadic HP supercell in northeast Kansas last May 28, as part of the conference.

This will be the last ChaserCon in Denver hosted by Roger and Caryn Hill, so come if you can.  Hope to see you there!

(Thanks to Bill Hark for the photo at top from my class last year in Wichita.)

- Jon Davies  1/22/20