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Severe and Unusual Weather ESAS 1115

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  1. Severe and Unusual WeatherESAS 1115 Spotter Training and Radar Meteorology Part 3 – Thunderstorm Varieties ESAS 1115 Severe and Unusual Weather

  2. Thunderstorm Ingredients • A source of lift – i.e. a front, sea-breeze, orography, outflows of previous thunderstorms, etc… • Moisture – low-level moisture provides the fuel needed for thunderstorms (dew point temperatures are the easiest way to assess the threat of strong to severe thunderstorms) • Instability – the ability of air to rise on its own (i.e. buoyancy) ESAS 1115 Severe and Unusual Weather

  3. Launching a Radiosonde ESAS 1115 Severe and Unusual Weather

  4. Receiving Data ESAS 1115 Severe and Unusual Weather

  5. Thunderstorm Development Cumulus Humilis Cumulus Congestus Towering Cumulus ESAS 1115 Severe and Unusual Weather


  6. Cumulonimbus (Cb) ESAS 1115 Severe and Unusual Weather

  7. Texture • A “rock hard” texture implies that most of the cloud is still in liquid phase • A growing updraft, or one that is strong, will exhibit this rock hard texture • It is marked by a “cauliflower”-type look with strong shadows ESAS 1115 Severe and Unusual Weather

  8. Rock Hard Growth with Anvil ESAS 1115 Severe and Unusual Weather

  9. Texture • When the updraft weakens or reaches high in the troposphere, the liquid freezes, giving the cloud a “glaciated” texture • When the updraft exhibits mostly a glaciated texture, the updraft is considered fairly weak • The glaciated texture refers to the updraft, not the outflow features of the storm nor falling rain ESAS 1115 Severe and Unusual Weather

  10. Texture Soft and fuzzy; glaciated Rock hard updraft growth ESAS 1115 Severe and Unusual Weather

  11. Glaciated AnvilCrisp vs. Fuzzy Crisp delineation between anvil and sky ESAS 1115 Severe and Unusual Weather

  12. Vertical Shear • Increases longevity and organization • Ultimately, strong shear results in storm-scale rotation by tilting horizontal vorticity into vertical vorticity ESAS 1115 Severe and Unusual Weather

  13. Supercell and Flanking Line Tower • Main tower is vertically erect in a sheared environment due to its very strong updraft • Flanking line tower to the main cell’s southwest is tilted due to the environmental shear • Notice the rock-hard overshooting top, another clue to the vigorous nature of the updraft ESAS 1115 Severe and Unusual Weather

  14. Extremely Sheared Updraft • Surprisingly, this tower maintained itself for some time before dying out • Strong localized point convergence helped maintain the inflow source of unstable air • If CAPE is too weak and shear too strong, thunderstorms tend to be sheared apart, resulting in a short-lived thunderstorm with minimal threat ESAS 1115 Severe and Unusual Weather

  15. Updraft vs. Downdraft • Updraft strength is primarily a function of instability • Downdraft strength is primarily a result of dry air • The top diagram would be a fairly weak shower or thunderstorm • The bottom diagram would result in primarily strong down drafts or microbursts ESAS 1115 Severe and Unusual Weather

  16. Updraft vs. Downdraft • High instability would increase the threat of severe hail (although most likely not very large hail) but would have a lesser threat for damaging winds • When dry air and high instability are both present, the likelihood of severe weather increases • Dry air increases the likelihood of large hail as well as strong winds • Supercells are the result of strong vertical shear where buoyancy and shear are well-balanced ESAS 1115 Severe and Unusual Weather

  17. Low Base • The Lifting Condensation Level (LCL) helps indicate the relative humidity of the sub-cloud layer • High LCL’s indicate lower RH and more chance for microbursts • Low LCL’s indicate more moisture in the sub-cloud layer • Lower LCL storms have a higher tornado potential ESAS 1115 Severe and Unusual Weather

  18. Weak Storm Still Dangerous • High base indicates threat for damaging winds • Forest fires are also a great threat due to lightning, strong winds and dry conditions ESAS 1115 Severe and Unusual Weather

  19. High-based Storm Low LCL’s correlate well to tornado potential. High LCL’s like in the storm shown here will struggle to produce tornadoes ESAS 1115 Severe and Unusual Weather

  20. Storm Evolution Movie ESAS 1115 Severe and Unusual Weather

  21. Thunderstorm Types • There are three basic types of thunderstorms: single cells, multicells and supercells. • Theses are loosely defined, as no thunderstorm is completely a “single” cell • Multicell storms can be either clusters or lines • The most important distinction in thunderstorm types is the presence or absence of storm-scale rotation ESAS 1115 Severe and Unusual Weather

  22. Buoyancy, Gust Fronts and Dynamics • Single cell thunderstorms are dominated by buoyancy processes • Multicell thunderstorms are dominated by gust front processes • Supercell thunderstorms are dominated by dynamic processes Instability Vertical Wind Shear ESAS 1115 Severe and Unusual Weather

  23. Single Cell Thunderstorms • Sometimes called “air mass” t-storms, these storms are poorly organized and pose relatively little threat to the public (lightning and hail) • Typical of afternoon thunderstorms • Updrafts form in relatively random locations • The dominant forcing feature is instability since they form in a low-shear environment ESAS 1115 Severe and Unusual Weather

  24. Convective Initiation on Radar ESAS 1115 Severe and Unusual Weather

  25. Convective Initiation on Radar ESAS 1115 Severe and Unusual Weather

  26. Single Cell Thunderstorms Movie • Buoyancy dominates single cell processes • Without significant vertical wind shear, the cold pool will undercut the updrafts • Lack of vertical shear means that new updrafts will not be able to be generated along the outflow boundaries • Cells will go through their cycle in about 30-60 minutes • The severe threat is minimal except in cases of strong instability and then even then the threat is relatively short-lived • Any severe weather occurrence in difficult to predict and even then, lead time will be minimal • Tornadoes are unlikely but can occur as landspouts ESAS 1115 Severe and Unusual Weather

  27. Pulse Severe Storm • A single cell storm may still produce large hail or damaging winds in which case the storm is called a “pulse” severe • A pulse storm produces severe weather, usually in the form of large hail, although the severe event will be relatively short-lived • Identification of a pulse severe storm is difficult and gives little lead time for warnings • “Popcorn severe” ESAS 1115 Severe and Unusual Weather

  28. Using VIL for Discriminating Between Ordinary and Pulse Storms Warn when the VIL exceeds the VIL of the day ESAS 1115 Severe and Unusual Weather

  29. Land Spouts • These are non-supercell tornadoes • Spouts form on boundaries where pre-existing vertical vorticity is already high • The updraft stretches the vertical vorticity into a vortex that results in tornado • Conservation of Angular Momentum helps explain their existence ESAS 1115 Severe and Unusual Weather

  30. Land Spouts ESAS 1115 Severe and Unusual Weather

  31. Waterspouts Multiple Waterspouts over Lake Michigan ESAS 1115 Severe and Unusual Weather