Observations of Hail–Wind Ratios from Convective Storm Reports across the Continental United States

Abstract

The objective of this study is to provide guidance on when hail and/or wind is climatologically most likely (temporally and spatially) based on the ratio of severe hail reports to severe wind reports, which can be used by National Weather Forecast (NWS) forecasters when issuing severe convective warnings. Accordingly, a climatology of reported hail-to-wind ratios (i.e., number of hail reports divided by the number of wind reports) for observed severe convective storms was derived using U.S. storm reports from 1955 to 2017. Owing to several temporal changes in reporting and warning procedures, the 1996–2017 period was chosen for spatiotemporal analyses, yielding 265 691 hail and 294 449 wind reports. The most notable changes in hail–wind ratios occurred around 1996 as the NWS modernized and deployed new radars (leading to more hail reports relative to wind) and in 2010 when the severe hail criterion increased nationwide (leading to more wind reports relative to hail). One key finding is that hail–wind ratios are maximized (i.e., relatively more hail than wind) during the late morning through midafternoon and in the spring (March–May), with geographical maxima over the central United States and complex/elevated terrain. Otherwise, minimum ratios occur overnight, during the late summer (July–August) as well as November–December, and over the eastern United States. While the results reflect reporting biases (e.g., fewer wind than hail reports in low-population areas but more wind reports where mesonets are available), meteorological factors such as convective mode and cool spring versus warm summer environments also appear associated with the hail–wind ratio climatology.

Radar-Based Comparison of Thunderstorm Outflow Boundary Speeds versus Peak Wind Gusts from Automated Stations

“…The ground-relative speed of a thunderstorm's outflow boundary (i.e., the denominator of the WGR) is the sum of the parent storm motion and the intrinsic gust front speed. The former is largely dictated by the mean environmental wind in single cell or multicell thunderstorms; in supercells, pressure perturbations caused by environmental vertical wind shear also play a considerable role in controlling storm motion (e.g., Klemp 1987;Bunkers et al 2000). The latter (the gust front's intrinsic speed) depends on the density difference between the outflow air and ambient environment [along with low-to midlevel vertical wind shear, e.g., Markowski and Richardson (2010)]; in other words, the speed of the gust front depends on the buoyancy of the outflow (e.g., Benjamin 1968).…”