A strong, upper-level trough ejected from the Southern Plains toward the Mississippi Valley on Saturday night, carrying with it severe storms along and ahead of a cold front in Arkansas, fueled by low-level moisture and a very strong mid-level jet streak (75 - 90 kt). A portion of the squall line quickly increased in the probability of tornado, indicated by the ProbTor model (Figure 1).
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| Fig. 1: ProbTor contour north of Little Rock, AR, 66% probability of tornado; MRMS MergedReflectivity (shaded). |
Soon after the timestamp in Figure 1, this storm dropped an EF-1 tornado in the small town of Romance, AR, which destroyed or damaged numerous homes and resulted in at least 4 injuries. The NWS in Little Rock, AR noted that there was a brief but concentrated area of rotation as well as a debris signature.
Figure 3 demonstrates the large increase in 0-2 km azimuthal shear at around 06:40 UTC, coupled with a very conducive environment for tornadoes (eff. shear ≥ 50 kt, 1-3 km AGL mean wind ≥ 60 kts, 0-1 km storm-relative helicity ~ 400 J/kg) led to the rapid increase in ProbTor. There was very little lightning activity evident with this storm.
With MRMS v12 now in operations, NWS forecasters can receive ProbSevere output in their offices. There was also an update to the azimuthal shear products in MRMS v12, improving their accuracy and reducing false alarms. This should help reduce the false alarms of algorithms dependent on MRMS azimuthal shear, such as ProbTor.
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| Fig. 2: Time series of the ProbSevere models as well as local severe storm reports and NWS severe weather warnings. |
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| Fig. 3: Time series of the ProbTor model, select constituent predictors, as well as local severe storm reports and NWS severe weather warnings. |