ProbSevere in Illinois

The NOAA/CIMSS ProbSevere model picked up on some warm sector convection in Illinois (neighboring our operational domain) during the evening of May 12. Figure 1 shows that the storm in the center with the hovering read out had a strong satellite normalized growth rate and strong glaciation rate at 22:15 UTC. The strong growth rates, MESH of 0.92", and large MUCAPE ( > 3000 J/kg) contributed to the probability of severe of 92%. The NWS issued a severe thunderstorm warning (Figure 2) on the center cell at 22:53 UTC (no reports yet received).

Figure 1: Storms developing in northern Illinois exhibit strong satellite growth.

The storm to the west also had a high probability of severe. It too exhibited strong satellite growth rates (though not as strong as the center storm), and MESH over 1". It was never warned on and no reports have been received. There may be some model overforecasting bias in high CAPE / lower shear regimes (as noted in the training), so forecasters should consider this while using ProbSevere guidance during warning operations. One forecaster also noted that the MESH seemed to be overforecasting hail size, based on the reports coming in.

Figure 2: NWS thunderstorm warning issued at 22:53 UTC, shortly after ProbSevere forecasted over 90% probability on this storm.

EDIT

More storms have been warned in northern Illinois. The storm that traveled near Chicago has been highlighted via AWIPS-II readout (see animation below). It had strong satellite growth rates well before a warning was issued (it merged together with other satellite-identified storms objects), which contributed to a high probability of severe. The first probability of severe greater than 50% was forecasted 37 minutes before the first NWS warning. The last image also shows additional NWS severe thunderstorm warnings on storms to the southwest, which also had strong satellite growth rates and probabilities of severe.

-Cintineo