Matthew J. Bunkers

Predicting Supercell Motion Using a New Hodograph Technique

A physically based, shear-relative, and Galilean invariant method for predicting supercell motion using a hodograph is presented. It is founded on numerous observational and modeling studies since the 1940s, which suggest a consistent pattern to supercell motion exists. Two components are assumed to be largely responsible for supercell motion: (i) advection of the storm by a representative mean wind, and (ii) propagation away from the mean wind either toward the right or toward the left of the vertical wind shear—due to internal supercell dynamics. Using 290 supercell hodographs, this new method is shown to be statistically superior to existing methods in predicting supercell motion for both right- and left-moving storms. Other external factors such as interaction with atmospheric boundaries and orography can have a pronounced effect on supercell motion, but these are difficult to quantify prior to storm development using only a hodograph.

An Observational Examination of Long-Lived Supercells. Part II: Environmental Conditions and Forecasting

Abstract The local and larger-scale environments of 184 long-lived supercell events (containing one or more supercells with lifetimes ≥4 h; see Part I of this paper) are investigated and subsequently compared with those from 137 moderate-lived events (average supercell lifetime 2–4 h) and 119 short-lived events (average supercell lifetime ≤2 h) to better anticipate supercell longevity in the operational setting. Consistent with many previous studies, long-lived supercells occur in environments with much stronger 0–8-km bulk wind shear than what is observed for short-lived supercells; this strong shear leads to significant storm-relative winds in the mid- to upper levels for the longest-lived supercells. Additionally, the bulk Richardson number falls into a relatively narrow range for the longest-lived supercells—ranging mostly from 5 to 45. The mesoscale to synoptic-scale environment can also predispose a supercell to be long or short lived, somewhat independent of the local environment. For example, long...

An Observational Examination of Long-Lived Supercells. Part I: Characteristics, Evolution, and Demise

Observations of supercells and their longevity across the central and eastern United States are examined, with the primary focus on understanding the properties of long-lived supercells (defined as supercells lasting 4 h). A total of 224 long-lived supercells, occurring in 184 separate events, are investigated. These properties are compared with those of short-lived supercells (lifetimes 2 h) to determine the salient differences between the two classifications. A key finding is that long-lived supercells are considerably more isolated and discrete than short-lived supercells; as a result, the demise of a long-lived supercell (i.e., the end of the supercell phase) is often signaled by a weakening of the storm’s circulation and/or a rapid dissipation of the thunderstorm. In contrast, short-lived supercells commonly experience a demise linked to storm mergers and convective transitions (e.g., evolution to a bow echo). Also noteworthy, 36% of the long-lived supercell events were associated with strong or violent tornadoes (F2–F5), compared with only 8% for the short-lived supercell events. Evolutionary characteristics of long-lived supercells vary geographically across the United States, with the largest contrasts between the north-central United States and the Southeast. For example, 86% of the long-lived supercells across the north-central United States were isolated for most of their lifetime, whereas only 35% of those in the Southeast displayed this characteristic. Not surprisingly, the convective mode was discrete for 70% of the long-lived supercell events across the north-central United States, compared with 39% for the Southeast.

Vertical Wind Shear Associated with Left-Moving Supercells

Vertical wind shear parameters are presented for 60 left-moving supercells across the United States, 53 of which produced severe hail (

Severe Convective Windstorms over the Northern High Plains of the United States

1.9 cm). Hodographs corresponding to environments of left-moving supercells have a tendency to be more linear than those of their right-moving supercell counterparts. When curvature is present in the hodographs of the left-moving supercells, it is typically confined to the lowest 0.5‐1 km. Values of 0‐6-km wind shear for left-moving supercells—both bulk and cumulative—are within the ranges commonly found in right-moving supercell environments, but the shear values do occur toward the lower end of the spectrum. Conversely, the absolute values of storm-relative helicity (SRH) for left-moving supercells are much smaller, on average, than what occur for right-moving supercells (although SRH values for many right-moving supercells also fall well below general guidelines for mesocyclone development). A significant fraction of the 0‐3-km SRH (25%) and 0‐1-km SRH (65%) for left-moving supercells is positive, owing to the shallow clockwise curvature of the hodographs. However, nearly all of the 1‐3-km SRH for left-moving supercells is negative, with absolute values comparable in magnitude to those for right-moving supercells. A limited climatological analysis of vertical wind shear associated with convective environments across parts of the central United States suggests that clockwise curvature of the low-level shear vector is most common in the central/southern plains, partially explaining the preeminence of right-moving supercells in that area. In contrast, hodographs are more linear over the northern high plains, suggesting left-moving supercells may be relatively more common there. It would be beneficial to implement operational radar algorithms that can detect mesoanticyclones across the United States.

Radar Observations of the Early Evolution of Bow Echoes

Abstract During the warm seasons (May–September) of 1996–99, Weather Surveillance Radar-1988 Doppler (WSR-88D) data and severe wind reports (either gusts >25 m s–1, or damage-related reports) over the northern High Plains (NHP) of the United States were analyzed in order to document the primary modes of convection responsible for severe winds. It was found that two-thirds of the convectively generated severe wind reports over the NHP were identified as being produced by organized convective structures rather than by isolated downburst or microburst activity. Specifically, at least 29% of all severe wind reports were produced by bow echoes, 20% by squall lines, 9% by supercell thunderstorms, and 7% by other convective systems not organized in a linear fashion. The occurrence of linear convective storm types that typically produce high winds (i.e., squall lines and bow echoes) were also documented over the NHP during the period of study. It was found that 51% of all squall lines and 86% of all bow echoes we...

Hailstorm Damage Observed from the GOES-8 Satellite: The 5–6 July 1996 Butte–Meade Storm

Abstract The evolution of 273 bow echoes that occurred over the United States from 1996 to 2002 was examined, especially with regard to the radar reflectivity characteristics during the prebowing stage. It was found that bow echoes develop from the following three primary initial modes: (i) weakly organized (initially noninteracting) cells, (ii) squall lines, and (iii) supercells. Forty-five percent of the observed bow echoes evolved from weakly organized cells, 40% from squall lines, while 15% of the bow echoes were observed to evolve from supercells. Thunderstorm mergers were associated with the formation of bow echoes 50%–55% of the time, with the development of the bow echo proceeding quite rapidly after the merger in these cases. Similarly, it was found that bow echoes formed near, and moved generally along, synoptic-scale or mesoscale boundaries in about half of the cases (where data were available). The observed bow-echo evolutions demonstrated considerable regional variability, with squall line-to...

Observations of the Relationship between 700-mb Temperatures and Severe Weather Reports across the Contiguous United States

Late in the evening of 5 July 1996, a supercell thunderstorm developed near the Montana–Wyoming–South Dakota border and moved to the southeast across western South Dakota. This storm was particularly notable for its persistent combination of large hail and extremely strong winds, which caused almost complete vegetative defoliation and destruction within a 120-km-long path. So extensive was the impact of the storm (especially in Butte and Meade counties of South Dakota) that the vegetation scar was visible for over a month on Geostationary Operational Environmental Satellite-8 (GOES-8) visible satellite imagery. This article briefly describes the nature of this extreme local storm event (hereafter referred to as the Butte–Meade storm) and the conditions responsible for creating the satellite-observed damage swath.

Observations of a Severe, Left-Moving Supercell on 4 May 2003

Abstract The relationship between 700-mb temperatures and convective severe storm reports is examined using data from 1993 to 2006 for the contiguous United States. Severe storm reports are used as a rough “proxy” for the occurrence of deep moist convection, and spatial and temporal distributions of 700-mb temperatures associated with these reports are analyzed. Secondarily, the distributions are assessed by individual severe storm report type, and convective inhibition also is evaluated. The motivation for this study is derived from the occasionally used 10°–12°C at 700 mb rule of thumb for estimating the extent and strength of the capping inversion. Whereas there is a semblance of merit for using this rule at times, its utility is shown to be strongly dependent on 1) geographic location, particularly with respect to surface elevation and the frequency of elevated mixed layers, and 2) the time of year. Calculation of convective inhibition, careful examination of the sounding, and assessment of lifting me...

Comments on “Satellite Observations of a Severe Supercell Thunderstorm on 24 July 2000 Made during the GOES-11 Science Test”

Abstract A case study of a left-moving supercell with a rapid motion is presented to (i) elucidate differences in anvil orientations between left- and right-moving supercells and (ii) highlight the interaction of the left mover with a tornadic right mover. It is shown how anvil orientations, as viewed from satellite, may be used to assist in the identification of thunderstorms with differing motions and how this applies to splitting supercells. Additionally, the movement of the left mover into the forward flank of the right mover may have temporarily affected its tornadic circulation, as tornadoes occurred both before and after the merger, despite the structure of the right mover being interrupted during the merging process. Given the dearth of literature on thunderstorm mergers in general, and how mergers affect tornadic supercells in particular, this is an area that demands further research.