Deadly tornadoes in Dixie

Alabama and Georgia were hit particularly hard by a series of tornadoes on March 3rd. A subtle yet energetic shortwave trough with a strong 850mb jet zoomed through the deep south spawning the tornadic storms from the early afternoon through the evening hours. NOAA's Storm Prediction Center (SPC) issued a 10% hatched convective outlook for strong tornadoes (this is 10% probability of a tornado within 25 miles). SPC later issued a mesoscale discussion (MD) for one supercell that formed in central Alabama around 1pm CST (19 UTC). The MD cited pressure falls ahead of the surface low, very strong 0-1km helicity (~500 J/kg), and ample buoyancy and environmental shear for the textual warning of "tornadogensis will likely occur within the next 30-60 minutes with the possibility of a strong tornado occurring."

According to preliminary reports, that was an excellent prediction, as this storm first produced a tornado at about 2pm (at 20:03 UTC). ProbTor, a product of NOAA's ProbSevere*, had increasing probabilities for this storm from 18:36 to 18:50 UTC, jumping from 20% to over 70% in those fourteen minutes (Fig. 1 and Fig. 2), largely due to increasing MRMS azimuthal shear. Strong total lightning density later played a role in increasing the probability of tornado to over 90%. Unfortunately for Lee county, AL, a second tornadic storm followed close behind. The NWS issued a rare tornado emergency at about 20:20 UTC for Lee County, AL and Harris and Muscogee Counties, GA (for the first supercell). These two storms combined for at least 23 deaths in communities between Beauregard and Smith's Station, AL. Many other tornadic storms were reported elsewhere in Alabama, Georgia, and Florida.

Fig. 1: The probability of any severe (contours), with NWS severe weather warnings and MRMS MergedReflectivity composite. The thicker tornado warning polygons (red) denote tornado emergencies.

In AWIPS2, when the probability of any severe product is loaded, a second contour will appear around the radar-identified object when ProbTor exceeds a given value. In Figure 1, we set that threshold to 20%. Users can change this threshold through their ProbSevere USER bundle files. We created this feature so that forecasters can visually see changes in the probability of tornado as well as the probability of any severe. Setting a higher threshold (~10-25%) may be prudent in a strong kinematic environment, whereas a lower threshold (3-5%) may help users visually pick out potential tornadic threats in more mundane environment.

Fig. 2: ProbHail, ProbWind, and ProbTor (red) for the life cycle of this storm. Durations of NWS warnings and times of preliminary LSRs are plotted on the bottom axes.

* The NOAA/CIMSS ProbSevere model (v2.0) will be an operational subsystem within MRMS as early as August 2019.