Observations of Low Level Thermodynamic and Wind Shear Profiles on Significant Tornado Days

Recent research has contributed to a more robust understanding of the atmospheric conditions supportive of significant tornadoes and the storms that produce them (e.g. Markowski, et al, Rasmussen, et al, Evans and Doswell, Thompson, et al). This research suggests, rather strongly, that much of the tornado process seems to happen at very low levels in the atmosphere (i.e. at or below 1 kilometer above ground level). In particular, two forecast parameters have emerged as a primary focus in forecasting meteorological environments supportive of supercell thunderstorms that are capable of producing long-tracked, strong or violent tornadoes: strong low level wind shear and moderate to high low level relative humidity.

On many significant tornado days, a unique combination of thermodynamic and wind shear profiles in the lowest 1 kilometer of the atmosphere has been observed. These observations are in general agreement with the findings of the most recent research. However, when combined with other visual and radar observations of storms, the presence of these shear and thermodynamic profiles pose some intriguing, and perhaps troublesome questions regarding our use of the terms “elevated” and “surface-based” convection, and parcel theory in general. Since choosing the “correct” parcel to lift is often critical, this raises additional questions about the use of several popular near-storm environment significant tornado forecast parameters used in short term forecast and warning operations, including 1) lowest 100 millibar mean-layer CAPE (MLCAPE), 2) lowest 100 millibar mean-layer CIN (MLCIN), 3) surface to 1 km AGL wind shear, 4) surface to 1 km, and surface to 3 km energy-helicity index (EHI), and 5) surface to 3 km vorticity generation potential (VGP).

Numerous examples of observed soundings and hodographs exhibiting this unique superposition of low level thermodynamics and wind shear are presented, and radar and visual observations from some cases will also be shown as a basis for discussion of questions regarding parcel theory, and the use of the aforementioned severe weather indices in forecast and warning operations.