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Howard B. Bluestein and Gary R. Woodall

Abstract

This is a detailed study of the structure and evolution of a convective storm that formed along the dryline in Oklahoma on 26 April 1984 and developed into a supercell. Dual-Doppler wind analyses, analyses of surface-mesonetwork and instrumented-tower data, storm-intercept photographs, analyses of raingage data from a dense network, and storm reports are presented and discussed. The storm exhibited the following low-precipitation (LP) characteristics: the absence of any strong, evaporatively cooled, storm-scale downdraft near the surface; a paucity of rain reaching the ground; large hail; long-lived rotating updrafts; and a strong, relatively small in areal extent, unicellular radar echo. The storm split, and its right-moving member developed into an isolated, high-precipitation supercell.

It is suggested that the region under cloud base was associated with strong radar echo caused by widely spaced, large water droplets and hailstones. Separate cyclonically and anticyclonically rotating updrafts were found on the southern and northern flanks of the storm, respectively. The high correlation between vorticity and vertical motion is circumstantial evidence that they are dynamically related. The storm split as the updrafts on the northern and southern flanks of the LP storm developed precipitation cores, and each propagated away from the original echo.

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Alan R. Moller, Charles A. Doswell III, Michael P. Foster, and Gary R. Woodall

Abstract

Supercell thunderstorm forecasting and detection is discussed, in light of the disastrous weather events that often accompany supercells. The emphasis is placed on using a scientific approach to evaluate supercell potential and to recognize their presence rather than the more empirical methodologies (e.g., “rules of thumb”) that have been used in the past. Operational forecasters in the National Weather Service (NWS) can employ conceptual models of the supercell, and of the meteorological environments that produce supercells, to make operational decisions scientifically.

The presence of a mesocyclone is common to all supercells, but operational recognition of supercells is clouded by the various radar and visual characteristics they exhibit. The notion of a supercell spectrum is introduced in an effort to guide improved operational detection of supercells. An important part of recognition is the anticipation of what potential exists for supercells in the prestorm environment. Current scientific understanding suggests that cyclonic updraft rotation originates from streamwise vorticity (in the storm's reference frame) within its environment. A discussion of how storm-relative helicity can be used to evaluate supercell potential is given. An actual supercell event is employed to illustrate the usefulness of conceptual model visualization when issuing statements and warnings for supercell storms. Finally, supercell detection strategies using the advanced datasets from the modernized and restructured NWS are described.

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Howard B. Bluestein, Eugene W. McCaul Jr., Gregory P. Byrd, and Gary R. Woodall

Abstract

On 7 May 1986 thunderstorms formed during the afternoon near a dryline in the Texas Panhandle under weak synoptic-scale forcing. Five tornadoes and large hail were produced by one storm near Canadian, Texas. The focus of the paper is the analysis of soundings obtained by a storm-intercept crew. A sounding launched into the wall cloud of the storm just after the fourth tornado indicated a nearly pseudomoist adiabatic lapse rate, a temperature excess of 10°C over the environment at 500 mb, and an updraft speed of almost 50 m s−1 near 6 km AGL, in reasonable agreement with parcel theory.

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Christopher J. Nowotarski, Justin Spotts, Roger Edwards, Scott Overpeck, and Gary R. Woodall

Abstract

Tropical cyclone tornadoes pose a unique challenge to warning forecasters given their often marginal environments and radar attributes. In late August 2017 Hurricane Harvey made landfall on the Texas coast and produced 52 tornadoes over a record-breaking seven consecutive days. To improve warning efforts, this case study of Harvey’s tornadoes includes an event overview as well as a comparison of near-cell environments and radar attributes between tornadic and nontornadic warned cells. Our results suggest that significant differences existed in both the near-cell environments and radar attributes, particularly rotational velocity, between tornadic cells and false alarms. For many environmental variables and radar attributes, differences were enhanced when only tornadoes associated with a tornado debris signature were considered. Our results highlight the potential of improving warning skill further and reducing false alarms by increasing rotational velocity warning thresholds, refining the use of near-storm environment information, and focusing warning efforts on cells likely to produce the most impactful tornadoes.

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Howard B. Bluestein, Eugene W. McCaul Jr., Gregory P. Byrd, and Gary R. Woodall

Abstract

No abstract available.

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Howard B. Bluestein, Eugene W. McCaul Jr., Gregory P. Byrd, Gary R. Woodall, Greg Martin, Stephen Keighton, and Lester C. Showell

Abstract

During the afternoon of 25 May 1987, thunderstorms, some of which were tornadic and produced large hail, formed near a dryline in the Texas Panhandle. This note discusses a sounding released into the updraft of a developing cumulonimbus south of Gruver, Texas by a University of Oklahoma storn-intercept team. The sounding indicated a nearly pseudo-moist-adiabatic lapse rate, a temperature excess of 4°–6°C over the environment at 500 mb, and updraft speeds of 35–40 m s−1between 6 and 7 km AGL, in good agreement with parcel theory. A radar-observed fine line appeared along the dryline and retreated westward, as was also noted in two earlier case studies.

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