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.
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| 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.
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| Figure 2: Time series of ProbSevere v3 models. |