Tornadic storm near Des Moines

As a surface low pressure system tracked northeast through central Iowa last night, one storm near the center of the low spawned at least one tornado southeast of Des Moines, IA. The first report came at 00:38 UTC, about 13 minutes after the first tornado warning. The ProbTor product from ProbSevere version2 showed elevated probability of tornado values (≥ 20%) as early as 23:48 UTC.

Figure 1: ProbTornado contours, MRMS MergedReflectivityComposite, and NWS severe weather warnings.

From the time series below, we see that increased rotation (0-2km MRMS AzShear; solid orange line) and increased total lightning density (solid green line) helped jump the probability of tornado from single digits to 40% at 00:00 UTC, and again from 20% to 50% between 00:26 and 00:38 UTC. The effective bulk shear and 0-1 km storm-relative helicity were clearly adequate, but not outrageous (~40 kts and ~150 m2/s2, respectively).

Figure 2: Time series of ProbTor and predictor values. The bottom axes contain NWS warning durations and LSRs.

One thing forecasters should look for when using ProbTor is when a storm's probability of tornado "sticks out" amongst the values of neighboring storms. This storm is a good example of that, where neighboring storms in a 4-county radius had probabilities in the single digits.

Forecasters should also be aware that the probability will fluctuate more with ProbTor than the probability of severe. This storm exhibited some fluctuation, whereby probabilities rose from 7% to 43%, dipped to 20%, and rose again to 55% during the tornado(es). ProbSevere developers are working on incorporating a visual time series function in AWIPS2 for ProbSevere storm objects (similar to Figure 2), so that forecasters can quickly see the history of a storm, which may help in warning decision making.

Lastly, there is no "magic" threshold of ProbTor for when tornadogenesis is imminent. Very potent environments characterized by SPC's "moderate" or "high" risk for tornado might see ProbTor values in the 70-90% range for tornadic storms, whereas for a more common tornadic environment like this, 20-50% can be expected for tornadic storms. In environments where tornadoes are not expected, ProbTor values of even 10% might be significant.

Users of the ProbTor product should keep these points in mind, but ultimately, forecasters should become more comfortable with the product as they gain experience with it in different regimes.

Please see our training module for more details on ProbHail, ProbWind, and ProbTor, and this fact sheet regarding the differences between ProbSevere version1 and version2.