Long-lived storm in Illinois

A long-lived, periodically tornadic storm churned through Illinois yesterday. The storm formed ahead of an approaching cold front and produced large hail and severe straight-line wind gusts along with tornadoes. Tornadoes touched down west and east of Springfield, Illinois.

ProbTor v3 (PTv3) demonstrated a better handle on the tornadic threat than ProbTor v2 (PTv2). PTv3 uses gradient-boosted decision trees to predict the probability of a tornado in the near-term for a given storm. It incorporates more NWP, MRMS, and GOES-16 fields than PTv2. 

A predictor analysis at 21:40 UTC showed that the MRMS 0-2 km and 3-6 km azimuthal shears were highly contributing to the probability, which was 34% at this time (PTv2 was 6%). The significant tornado parameter [effective layer] (STP) was also helping, with a value of 0.8. The effective storm-relative helicity (ESRH) was about 140 J/kg, whereas the 0-1 km SRH was only 85 J/kg, which could be one reason PTv2 was rather low, as PTv2 does not use the ESRH as a predictor. 

At the HWT in 2021, forecasters will also be able to see any given storm's ProbSevere history with the a double-click of the storm object in AWIPSII. We hope this will improve users' situational awareness and help with warning-decision making. 

Figure 1: ProbSevere (colored contours) and MRMS MergedRef for a storm in central Illinois. Inset shows the history of the storm's ProbSevere probabilities. 

Typically, PTv3 is lower than PTv2 values, as a result of improved probability calibration in PTv3 (e.g., PTv2 was sometimes "too hot" and over-predicted the threat). But in this case, PTv3 better highlighted the tornadic threat with greater probabilities than v2.

Figure 2: Time series of ProbSevere v3 models. 

Tornadic thunderstorms menace Mississippi

Figure 1: SPC 1630Z outlook with 06Z verification.

A deep shortwave trough and strong 850mb jet brought ample low-level moisture and instability to a well-sheared environment in the Southeast U.S. yesterday. The NOAA SPC issued "Enhanced" outlook noting the potential for strong tornadoes.

ProbTor captured the tornadic threats in Mississippi during the afternoon and then during a second round of storms in the overnight hours. The animations in Figures 2 and 3 show outer contours colored by the ProbTor value (inner contours are colored by probability of any severe), which were configured to only appear when ProbTor ≥ 15%. NWS forecasters can configure this threshold using these instructions.

In the first bout of storms, tornadoes were observed for storms in Simpson and Smith counties (see storm time series of predictors), as well as Yazoo and Holmes counties (see storm time series), and Leake county.

Figure 2: ProbSevere/ProbTor contours with MRMS MergedReflectivity and NWS severe weather warnings for 19Z -- 23Z.

During the second round of storms, a potent thunderstorm dropped tornadoes in Jasper, Clarke, and Lauderdale counties (storm time series).

Figure 3: ProbSevere/ProbTor contours with MRMS MergedReflectivity and NWS severe weather warnings for 02Z -- 05:30Z.

Strong tornadoes in the Deep South

A seasonally strong shortwave trough tapped into abundant Gulf of Mexico moisture forcing severe and tornadic storms across Louisiana, Mississippi, and Alabama. The Storm Prediction Center issued a Moderate Risk outlook with tornadoes and strong wind gusts being the primary threats (see Figure 1).

Fig. 1: SPC outlook with 06Z 12/17 verification (dots).

ProbSevere/ProbTor models show the evolution of storms throughout the afternoon (Figure 2). The outer contours represent the ProbTor probability and are only present if ProbTor is ≥ 15%, here.

This environment was characterized by 1000 - 2500 J/kg of MLCAPE, 45-60 kts of effective bulk shear, and 1-3km AGL mean wind of 40-55 kts. The ProbSevere models generally track and discern the most dangerous threats well (see the NWS warning polygons). However, there are several storms with erroneously high ProbTor values (outer polygons with high probabilities) that quickly appear and disappear during the animation. Most of these false alarms are due to spurious MRMS azimuthal shear values which are produced by noisy Doppler velocity data. Work is ongoing to mitigate these errors in ProbTor.

Fig. 2: ProbSevere and ProbTor contours, MRMS MergedReflectivity, and NWS severe weather warnings.

A storm that formed in east Texas/west Louisiana spawned numerous tornadoes and prompted a tornado emergency for Alexandria, LA at 18:41 UTC. This storm was in a primed thermodynamic and kinematic environment, with 250-300 J/kg of 0-1km storm-relative helicity. The low-level and mid-level MRMS azimuthal shear values increased the ProbTor probabilities from 30% to 91% in about 20 minutes. You can see the saved time series of attributes for this storm here. Figure 3 shows how the ProbSevere products evolved for this storm in comparison to NWS severe weather warnings and local storm reports.

Fig. 3: Time series of ProbSevere probabilities for tornadic storm in Louisiana. The bottom axis plots durations for NWS severe weather warnings and times of preliminary LSRs.

At least one this this storm's tornadoes was expected to be rated as significant (EF3+).