Tulsa-Claremore, OK Tornado

Most likely the strongest tornado of the day occurred just northeast of downtown Tulsa, Oklahoma, and moved east-northeastward toward Claremore and Justice, OK.

A strong GOES-derived normalized satellite growth rate, along with an increase in total flash rate from 22 to 35 flashes/min, increased the probability of severe to 84% at 22:42Z, in this high MUCAPE, high effective bulk shear environment, despite MRMS MESH only being 0.61 in. Fourteen minutes later (22:56Z), the probability increased to 95%, while MESH decreased to 0.56 in., and the flash rate increased to 84 fl/min. The probability generally stayed over 90% until the storm's demise. The first severe thunderstorm warning was issued at 23:35Z, about 40 min after the storm attained a >90% value from the ProbSevere model.

A tornadic debris ball signature was evident from KINX radar at 00:17Z, manifest in the base radar fields: a 'hook' signature in the reflectivity, strong velocity couplet, low correlation coefficient, and near zero or varying differential reflectivity; all coincident features are an indication that a tornado is creating and lifting debris. The storm also produced golfball-sized hail.

Future development of the ProbSevere model will investigate radar velocity products and dual-pol fields to give more accurate and hazard specific probabilities of severe (e.g., hail, wind, tornado).

KINX low-level scan ~00:17Z March 31, 2016. Images are from social media. The coincident signatures indicate a tornadic debris ball. Top-left: hook signature in horizontal reflectivity; top-right: strong inbound-outbound velocity couplet; bottom-left: a 'hole' of low correlation coefficient; bottom-right: near-zero differential reflectivity.

Fast moving storms and MRMS MESH

A fast moving storm in west-central Illinois yesterday produced hail much larger than the expected maximum size, depicted by the MRMS MESH, part of the ProbSevere model. Though it might not look like it from the animation below, the storm was pegged moving NE at 50 mph.

In cases like these, a fast-moving storm that is also perhaps shallow can result in an underestimate of MESH, since the storm may move a non-negligible amount in between consecutive radar elevation scans, resulting in a storm that appears to have a 'tilted' core, but is really just a result of storm motion. A 'tilted' or legitimately tilted core can result in a lower than normal MESH since MESH is based off of vertical integration of reflectivity.

Despite a strong satellite growth rate, the weak MESH, weak total lightning flash rate, and weak-moderate shear only resulted in a probability of severe of 9% when a 1" hail report was received (18:35Z, 4 min before the warning was issued), and maximum probability of 27%. In this case, ProbSevere would not have been useful to highlight the hail potential, thus forecasters should bear in mind many other environmental factors as well as the limitations of observations.

Late March warm frontal convection in Iowa

A strong system brought severe storms to many parts of the U.S. yesterday, including a few riding along a warm front in Iowa.

The NOAA/CIMSS ProbSevere model was recently updated to include total lightning data. The total flash rate (flashes/min) is coupled with the RAP effective bulk shear to form a two-dimensional predictor. The RAP MUCAPE, MRMS MESH, and GOES-derived satellite growth rates are also still predictors in ProbSevere (see how the MUCAPE and effective bulk shear are used in ProbSevere) .

The highlighted storm in central IA developed in an environment with decreasing MUCAPE (1000 J/kg --> 600 J/kg) but excellent shear (~45-50 kts). The strong total flash rate (~35-70 fl/min) with the strong shear, as well as an observed moderate normalized growth rate, helped elevate the probability despite modest MESH (0.45-0.75 in). The probabilities ranged from 60% to 85% (mainly > 70%) before the first severe thunderstorm warning was issued, at 23:58Z, which was 5 min before the first 1" hail report.

In this case, the total lightning / effective shear predictor helped contribute to over 60 min of lead-time from the 60% threshold.

As this storm was diminishing, a conglomerate of convection erupted just to its east, producing wind and hail reports in the Des Moines metro.