Module 8 Analyzing The April 2011 Super Outbreak.docx

Module 8 Analyzing The April 2011 Super Outbreak.docx -...

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References: 1. Chapter 14 Topics Thunderstorm Types and Supercell Thunderstorms 2. Chapter 15 topics Tornado Outbreaks, Supercell Tornadoes, and Observing Tornadoes and Severe Weather , and the chapter 15 Focus Topic Forecasting Severe Thunderstorms and Tornadoes 3. Module 8 Mini-Lesson: Derechos and Squall Lines and Gust Fronts, Oh My! – Ordinary vs. Severe Thunderstorms 4. NWS Birmingham - Historic Outbreak of April 27, 2011 5. NWS Storm Prediction Center Archived Event: April 27, 2011 6. National Weather Service Sample Station Model plot, for decoding station model observations. Convective Available Potential Energy (CAPE) Carefully review the Chapter 14 Topic Thunderstorm Types and the Chapter 15 Focus Topic Forecasting Severe Thunderstorms and Tornadoes discussion of Convective Available Potential Energy (CAPE), which is a measure of the energy available within the surrounding environment to produce positive buoyancy in air parcels that reach saturation. The more CAPE that exists in the local atmosphere, the greater the potential for strong updrafts within any thunderstorm that develops in that environment. For high values of CAPE, the potential for strong convective activity is high, and thus supercell thunderstorm development becomes more likely. If thunderstorms do develop, this potential energy is converted into kinetic energy of the storm’s updraft, and the CAPE becomes a measure of the actual intensity of the storm. CAPE is measured using atmospheric soundings, as explained in the Chapter 15 Focus Topic. You will use Table 1 below in the case study analysis later in the activity. Atmospheric Conditions Required for Supercell Development As described in Chapter 14, a supercell thunderstorm is a large, long-lasting thunderstorm with a single violently rotating updraft core. The conceptualized cross-section showing the features of a tornadic supercell thunderstorm (from Chapter 14) is illustrated below.
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Supercell thunderstorms have the potential to produce large tornadoes, but not all supercell thunderstorms will generate tornadoes. Strong vertical wind shear (change in wind speed and/or direction with height) is required to generate supercell thunderstorms. As explained in Chapter 15 topic Tornado Formation , and illustrated below, strong vertical wind shear creates a horizontally-spinning air column is then tilted into the vertical by a strong updraft, causing it to then rotate about a vertical axis. The result is a mesocyclone - a violently rotating updraft that forms the core of the supercell thunderstorm. The mesocyclone is tilted due to the vertical wind shear, which prevents the inflow warm, moist air being drawn into the storm from being cut off by the outflow, allowing the storm to last for several hours .
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In addition to strong vertical wind shear, other atmospheric conditions at the surface and aloft must be present to enhance the convective instability necessary for the development of the mesocyclone and supercell thunderstorms. Carefully review the Chapter 14 topic Supercell Thunderstorms for an in-depth discussion. The necessary conditions are summarized on the following pages.
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  • Fall '17
  • Thunderstorm, Supercell Thunderstorms

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