Storms pummel the Midwest

An energetic short-wave and attendant surface low rapidly intensified on April 12th, bringing quickly developing storms to a number of regions in the Midwest U.S. Large hail, severe wind gusts, and potent tornadoes were reported from Wisconsin to Texas.

Figure 1: SPC categorical outlook at 06 verification.

In the middle of the afternoon, a lone elevated storm along a stationary boundary traversed the state of Wisconsin, causing a 67-mph wind gust in La Crosse, WI, and dropping hail ranging from 1" to 1.5". ProbSevere version 3 (PSv3) had a pretty good handle on it over the course of several hours. 

Figure 2: ProbSevere storm-based contours, MRMS MergedReflectivity, and NWS severe weather warnings for a storm in Wisconsin.

While the MRMS products contributed positively to the PSv3 probabilities (e.g., MESH, Reflectivity -10C, AzShear), the IntenseStormNet probability (a predictor in PSv3 models) also contributed in the models. IntenseStormNet uses images of visible and long-wave infrared channels from GOES-R ABI, as well as images of flash-extent density from GOES-R GLM to detect intense parts of storms. From the animation below, we see that IntenseStormNet "probability of intense convection" for the storm in Wisconsin largely stayed between 50% and 90%.

The storm of the day spawned in northeast Iowa, ahead of a cold front. From ProbSevere hover-output in AWIPS, we saw that it had a strong normalized satellite growth rate at 21:31Z. PSv3 was 32% when the NWS issued its first severe thunderstorm warning, at 22:00Z. The probability of severe then soon increased to 70% by 22:18Z. The storm produced its first 1"-diameter hail report at 22:25Z. 

Figure 3: An animation of ProbSevere contours, MRMS MergedReflectivity, and NWS severe weather warnings for a tornadic storm in Iowa

A tornado warning was issued at 22:59Z, coincident with ProbTor v3 rapidly increasing to 49%. ProbTor v2 was at 13%. There was an increase in the MRMS azimuthal shears at this time, along with an increase in the significant tornado parameter (a predictor in PTv3). v2 was likely underestimating the threat due to too much contribution from stout MLCIN (-86 J/kg), dampening the probability. The machine-learning model of PTv3 (gradient-boosted decision trees) appears to better incorporate the MLCIN information than it's predecessor, in this case. 

Figure 4: ProbSevere contours, MRMS MergedReflectivity, and NWS severe weather warnings for a tornadic storm in Iowa. The ProbSevere time series window can be activated by double-clicking inside a storm object.

The IntenseStormNet's probability also contributed to the higher probability of tornado. See in the animation below how probabilities ≥ 90% are well-correlated with the most vigorous portions of the convection.

 

Later, the cold front zipped down from Nebraska into Kansas. Very strong satellite growth rates were observed, as the PSv3 values regularly exceeded 80%. The cold front was essentially warned continuously from western Iowa to southern Kansas.

Figure 5: ProbSevere contours, MRMS MergedReflectivity, and NWS severe weather warnings for a cold front from Iowa to Kansas.

The intense convection probability from IntenseStormNet quickly went from < 10% to ≥ 90% for most of the line, which later produced numerous hail and wind reports.