The Use of Updraft Helicity as a Severe Weather Surrogate for Convective Systems

Forecast parameters/surrogates that are based on a foundation of physical understanding will tend to be the most generally useful. For example, the physical model on which updraft helicity (UH) is based is that of supercell storm, composed of a mid-level rotating updraft along with a low-low level rotating updraft that at times can be tornadic. As such, 2-5-km UH is found to be very helpful in highlighting mid-level supercell structures in 1-3-km convection allowing forecasts, with 0-1-km UH then adding further useful guidance as to the probability that the supercell may be tornadic. In potentially tornadic cases, narrow strips of high UH can be identified collocated at both low- and mid-storm levels.

High values of UH at both low and mid-levels are also often identified in association with larger convective systems, especially bow echoes, but in these cases, the regions of high UH typically appear as continuous wide swaths along the leading line of the convective system, and may or may not show colocation between low and mid-level UH features, as is apparent in an isolated supercell. Nonetheless, the same degree of potential for severe activity is often attributed to such wide swaths of UH, when in fact they can be representing differing physical phenomena with especially very different tornadic potential. Better understanding of how UH behaves within different convective modes is clearly needed to improve forecasts of specific convective hazards.

In this paper, we will present examples of model-produced UH swaths for both supercell and convective system modes, clarifying the differing structural features, such as leading-line or line-end mesoscale vortices, as well as the differing severe weather potential that the UH swaths within such convective systems may be representing.