An Investigation of "Surprise" Tornado Events Associated with Remnant Mesoscale Convective Vortices

The “Surprise Tornado Outbreak” of August 24th, 2016 dealt over 40 tornado reports to the Midwest, despite a rather innocuous morning assessment made by the National Weather Service’s Storm Prediction Center (SPC). Why this happened has already been scrutinized in detail; however, a look at the past 15 years has revealed that events with similar setups (and morning assessments) have occurred by the regularity of at least 2 per year on average, meaning this will likely happen again. This study examines the similarities between 38 such events and 135 associated tornado reports, and finds that a Mesoscale Convective Vortex (MCV), or mid-level low pressure system leftover from robust overnight convection was present in each. By a wide margin, not all remnant MCVs are associated with tornadoes; however, this study reveals a specific region that is particularly prone.

Unfortunately, our sparse network of morning upper-air observations fails to resolve the full scope of most MCVs. The result? Both human and numerical analyses have little sense of the true thermodynamic and kinematic environment in its vicinity; if not for satellite and sometimes radar observations, we would be rendered nearly blind to one’s existence. Thus, remnant MCV tornadoes often come as a “surprise” to a local area, with the attendant SPC 1300z outlooks examined carrying a less than satisfactory detection rate. Using operational knowledge of the conventional supercell parameter space for deep-layer shear and instability have proven unsuccessful for these, but upon closer examination, a near-surface environment rich in both shear and instability does exist. This study will explore what we can observe about MCVs, including satellite presentations, and the positions relative to them that tornadoes tend to occur. It will also explore the climatological patterns and mesoscale precursors leading up to these events that may be the early warning we need.