1. Severe and Unusual Weather ESAS 1115
Spotter Training and Radar Meteorology
Part 3 – Thunderstorm Varieties
Prof. Paul Sirvatka
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)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

3. Launching a Radiosonde
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

4. ESAS 1115 Severe and Unusual Weather
Receiving Data
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

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

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

7. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

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

9. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

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

11. Glaciated Anvil Crisp vs. Fuzzy
Crisp delineation between anvil and sky
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

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

15. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

16. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

17. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

19. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

20. ESAS 1115 Severe and Unusual Weather
Storm Evolution
Movie
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

21. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
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
Prof. Paul Sirvatka
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

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

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

27. ESAS 1115 Severe and Unusual Weather
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”
Prof. Paul Sirvatka
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

29. ESAS 1115 Severe and Unusual Weather
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
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

30. ESAS 1115 Severe and Unusual Weather
Land Spouts
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather

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