Surprise Indiana tornadoes and total lightning in the ProbSevere model

A number of tornadoes spawned from storms in central and northern Indiana yesterday afternoon -- including strong ones -- in an area where tornadic activity was not expected. NOAA's Storm Prediction Center (SPC) forecasted a corridor of marginal to slight risk for general severe weather extending into Indiana, Ohio, and lower Michigan (Figure 1). However, the probability of a tornado within 25 miles of any given point was less than 2% in Indiana and Ohio (Figure 2).

Figure 1: SPC categorical outlook for August 24, 2016.

Figure 2: Tornado probability outlook for August 24, 2016.

Storms formed in the early afternoon in a juicy warm sector of an occluding system and put down multiple tornadoes, including an EF-3 tornado in Kokomo, Indiana, causing substantial damage to a shopping mall and leaving thousands without power.

Figure 3: SPC storm reports for August 25, 2016.

Though the NOAA/CIMSS ProbSevere model doesn't provide guidance to the type of severe weather, it could have given forecasters a heads up to some of the storms during this event. The animation below shows two storms developing in central Indiana (Figure 4). The first storm (near the Illinois border) had initially modest MESH (0.5-0.7"), but an excellent total lightning flash rate (50+ fl/min), before increasing to over 1" of MESH briefly. ProbSevere provided about 30 minutes of leadtime to the first severe thunderstorm warning from the 70% threshold. A tornado warning was issued at 18:37 UTC, and a damaging tornado reported at 18:55 UTC. One-inch hail was later reported in Indianapolis.

Further to the northeast, where MUCAPE was markedly less (~1600-1800 J/kg) but effective shear about the same (~45 kts), another storm formed west of the city of Kokomo, which produced the EF-3 tornado. This storm had low MESH (0.3-0.5"), but a rapid increase in flash rate (18->46->62 fl/min) in a well-sheared environment. This helped give the storm a probability of severe of 68% at 18:42 UTC, 8 minutes before a tornado warning was issued. The total lightning flash rate/effective shear predictor helped increase the probability of severe despite poor integrated radar reflectivity and satellite growth rates. A tornado emergency would later be issued for the community of Kokomo.

Figure 4: Two storms in central Indiana showcasing the utility of total lightning data in ProbSevere.

To show the effect of total lightning flash rate explicitly, ProbSevere was run with just radar, satellite and near-storm environment NWP data. As figure 5 shows, the probability of severe on the first storm only became elevated when the MESH neared or exceeded 1". In the Kokomo storm, the probability was 46% greater at the time of the initial tornado warning with lightning than without (70% vs 24%)! For both storms, the probability was generally 20-40% greater with the inclusion of total lightning data.

Figure 5: ProbSevere for the two storms in central Indiana, with total lightning data OMITTED from the probability computation.

The tornadic storm that traversed Kokomo traveled to the east side of Indiana, where it again re-intensified and became warned at 20:46 UTC. The probability of severe jumped up largely in response to an increasing flash rate. The storm would go on to produce numerous more tornado reports, beginning at 20:45 UTC.

This storm and another to it's north moved into Ohio, still producing tornadoes. However, the ProbSevere model only had low probabilities at this point for both storms, as the flash rates dropped into the single digits and MESH was largely below 0.33". So the flash rate did not contribute much with these two storms later on (see Figure 6). It is still uncertain what aspect of the environment in far northeastern Indiana modified the morphology of these storms.

Figure 6: Two storms in northeastern IN / northwestern OH that had poor ProbSevere values but produced tornadoes. These storms show that total lightning doesn't help every storm.

This event in Indiana and Ohio is interesting for a number reasons, including the unexpected number and severity of tornadoes, as well as the seemingly different morphologies of storms in reasonably close proximity. These examples highlight where total lightning flash rate improves ProbSevere probabilities, despite a meager reflectivity signature, as well as where total lightning doesn't contribute. I hope this case also shows the utility of an 'ingredients-based' approach to forecasting using observations (and near-storm environment data), where one data source may give insight to future storm severity when another is not, or when multiple observation sources may corroborate each other to enhance forecaster confidence and leadtime. This case also underscores NOAA/CIMSS's efforts to provide hazard specific guidance in future improvements to ProbSevere.

EDIT: This blog post by Jeff Frame gives a good post-mortem of the event. It appears the MCV in Illinois/Indiana played a key role, and that the environment itself was actually reasonably favorable for tornadoes, but that NWP guidance struggled depicting it as well as depicting the morphology of the storms.