UNR - SVR issued off of ProbSevere. Not a great idea!!

2125Z: Strong/Severe storms continue to move from SE WY into the UNR CWA as expected so took after quickly glancing at GLM/ProbSevere after a blog post, decided to issue a SVR based on ProbSevere & MESH values. WHen I first noticed the storm it was nearing the CYS/UNR CWA boundary & had a ProbHail of 77%, ProbWind of 53%, & MESH of 1.68". Loopping through the last 10 minutes of data I noticed an increasing trend for all values so issuing a quick warning for golf-ball sized hail based on MESH & ProbHail values seemed like a no-brainier to me. Furthermore GLM total energy showed relatively high values compared to other storms in the area so far this afternoon.

Unfortunately... after spending time on a blog post I committed the cardinal sin of warning operations & lost my situational awareness before issuing the warning. The storm began to weaken almost immediately after I issued my warning ProbHail values quickly dropping to less than 10% & GLM lightning data showing a clear decreasing trend as well. SPC mesoanalysis clearly showed these storms moving into a poor environment in NE WY with less than 500 J/kg MLCAPE, less than 30 kts effective bulk shear, & less steep lapse rates as well. I looked at the all-sky CAPE post warning as well but it was not useful in depicting the CAPE gradient from south-north in eastern WY.

Obviously it goes without saying but situational awareness & a proper analysis of the near-storm environment is essential in a warning environment. ProbSevere cannot be used by itself in a vacuum for warning decision making but rather as part of a package with other storm interrogation techniques.

ProbSevere/GLM 4 panel animation:

2030Z all-sky CAPE:

Peter Sunday

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.