Severe and Unusual Weather ESAS 1115

1. Severe and Unusual WeatherESAS 1115 Spotter Training and Radar Meteorology Part 4 – Multicell Thunderstorms ESAS 1115 Severe and Unusual Weather

2. Multicell Thunderstorms • Several distinct updraft/downdraft couplets exist • Can form as a cluster or a squall line • A vertically sheared environment results in tilting of the updrafts and a longer lasting system ESAS 1115 Severe and Unusual Weather

3. Multicell StormsHow Many Cells are There? ESAS 1115 Severe and Unusual Weather

4. Multicell Cluster • Series of evolving cells where new updraft growth is on a preferred flank • Child cells kill their parents with cold outflow air • New updraft growth is generated along outflow from the parent cells preferentially based upon low-level convergence and in conjunction with low-level wind shear ESAS 1115 Severe and Unusual Weather

5. Multicell Evolution Southwest side of multicell cluster (looking to the northeast) ESAS 1115 Severe and Unusual Weather

6. Overshooting Tops ESAS 1115 Severe and Unusual Weather

7. Multiple Updraft Growth in Close Proximity to Each Other In moderate wind shear environments, multicell storms may take a few hours to evolve into a supercell storm ESAS 1115 Severe and Unusual Weather

8. Multicell (Squall) Line • A squall line is a line of thunderstorms with a continuous, well developed gust front at its leading edge • Discrete cells are difficult to distinguish • New cell growth is generally along the whole line rather than on just one preferred flank • Large hail is possible in the highest reflectivity cores • Damaging winds are possible at the leading edge but also further behind the front edge ESAS 1115 Severe and Unusual Weather

9. Line Echo Wave Pattern (LEWP) Refers to a series of bow echo segments; a wavy appearing squall line; can result in an enhanced severe threat at the leading edge of the bow or in the “crest” of the waves Crest ESAS 1115 Severe and Unusual Weather

10. Squall Line Cross Section • Compare with supercell schematic • Updrafts are on the leading edge of the cold pool, hence tornado threat is small after passage of gust front although hail will now occur • Squall lines have well-developed cold pools ESAS 1115 Severe and Unusual Weather

11. Cross Section of Multicell ESAS 1115 Severe and Unusual Weather

12. Outflow Boundary (OFB) ESAS 1115 Severe and Unusual Weather

13. OFB on Composite Radar ESAS 1115 Severe and Unusual Weather

14. Shelf Cloud Along the OFB ESAS 1115 Severe and Unusual Weather

15. Shelf Cloud ESAS 1115 Severe and Unusual Weather

16. Underneath the Shelf • The texture underneath the shelf cloud is considered boiling • Although turbulent and swirling, the underside of a shelf cloud is a region of warm and cold air mixing and thus not tornadic ESAS 1115 Severe and Unusual Weather

17. Boiling Texture ESAS 1115 Severe and Unusual Weather

18. Typical Look Movie • Look for ragged appearance, even if it is not extremely turbulent • Think about where the rain is falling and whether or not cool air is pushing away from the storm • Be aware that sheared motion along the outflow is often mistaken for rotation • Sometimes, the only significant feature readily identifiable is the shelf cloud ESAS 1115 Severe and Unusual Weather

19. Intense Skies ESAS 1115 Severe and Unusual Weather

20. On Top of the Shelf • The top of a shelf is usually quite laminar (smooth) • Laminar cloud texture is usually the result of forced ascent, not convective in nature • Laminar banding is not uncommon in a stacked shelf cloud ESAS 1115 Severe and Unusual Weather

21. Laminar Bandings in a Stacked Shelf Cloud ESAS 1115 Severe and Unusual Weather

22. Forced Ascent along Squall Line Cold Pool Definitely dark and scary! ESAS 1115 Severe and Unusual Weather

23. Outflow Dominant Line of Storms ESAS 1115 Severe and Unusual Weather

24. Outflow Features of a Squall Line ESAS 1115 Severe and Unusual Weather

25. Haboob (Dust Storm) ESAS 1115 Severe and Unusual Weather

26. Gustnadoes ESAS 1115 Severe and Unusual Weather

27. Gustnadoes • Gustnadoes are not really tornadoes as they are not pendant to a Cb • Although these vortices are relatively weak, they still pose some threat to those in their paths • Note the boiling texture underneath the shelf cloud ESAS 1115 Severe and Unusual Weather

28. Broken Squall Line Threats • Tornadoes are most likely within supercells and can form where storms are more isolated • These storms are most likely at the southern most end (“Tail End Charlie”) or just north of a break • Isolated storms ahead of a squall line also pose significant risk for severe events and/or tornadoes ESAS 1115 Severe and Unusual Weather

29. Bow Echoes ESAS 1115 Severe and Unusual Weather

30. Development of a Bow Echo Movie ESAS 1115 Severe and Unusual Weather

31. Bow Echo with Trailing Stratiform Rain ESAS 1115 Severe and Unusual Weather

32. Underneath a Bow ESAS 1115 Severe and Unusual Weather

33. MCS and MCC • A large organization of thunderstorms is called an MCS (Mesoscale Convective System) • A squall line is considered a linear MCS • When the IR satellite presents a more circular perspective of a large cluster of thunderstorms and it persists for more than 6 hours, we refer to this MCS as an MCC (Mesoscale Convective Complex) ESAS 1115 Severe and Unusual Weather

34. Linear vs. Circular Eccentricity: Ratio of the minor axis to the major axis Eccentricity much less than 1 Eccentricity close to 1 ESAS 1115 Severe and Unusual Weather

35. Mesoscale Convective Vortex (MCV) ESAS 1115 Severe and Unusual Weather

36. Multicell Thunderstorms • Gust front processes dominate multicell processes • The addition of vertical wind shear allows for new updrafts to reach maturity in some organized fashion • Even though each cell moves with the mean wind of the cloud-bearing layer, new updrafts form where low-level convergence is strongest and is enhanced by the vertical wind shear • When updrafts initiate at a preferred location, multicell clusters form • Updrafts that initiate along the leading edge of the cold pool will result in a squall line • The severe threat increases as instability and shear increase and where strong cold pools do not overwhelm new storm growth • Severe weather events are more predictable ahead of strong cells and with features that are easily identifiable on radar (e.g. bow echoes) • Tornadoes are possible with storms that are supercellular or isolated (such as a head of a line) and with storms north of a bow echo ESAS 1115 Severe and Unusual Weather