1. More Thunderstorms

2. Today Homework in Wind shear More multicellular storms

3. Importance of Wind Shear Single cell –Storm is vertically stacked –Outflow boundary may “outrun” the motion of the storm cell –New storms that develop may be too far from the original to be a part of it Multicell –Storm motion keeps up with outflow –New development forms adjacent to the older cells and connects with the old cell

4. Wind Shear Example of little or no vertical wind shear: 7 kts 6 kts 7 kts 6 kts There is very little change in the speed or direction of the wind with height. Height

5. Wind Shear Definition: –The change in the direction or speed of the wind over a distance. Vertical Wind Shear –The change in the direction or speed of the wind with height. Low wind shear indicates little change in direction or speed of the wind over a distance.

6. Multicellular Thunderstorms As each cell matures, the updraft holds the precipitation aloft. Eventually the precipitation “unloads” and a gush of heavy rain occurs at the ground.

7. Multicellular Thunderstorms At low levels: –The outflow produces the surface convergence to form new updrafts –New updrafts often form at the right or right-rear flank of the storm At high levels: –As the cell matures, the updraft may pass the equilibrium level –This forms a penetrating updraft of an overshooting top

8. Multicellular Thunderstorms As the next cell grows and develops, the top of the older cells tends to collapse. –This enhances the upper-level divergence which helps to expand the anvil –The collapsing air mixes with the downdraft air and enhances the downdraft.

9. Multicellular Thunderstorms Some multicellular storms show little or no motion –Very localized rainfall that can cause significant flooding Rapid cell development on the rear of the storm New cells move over the same land region as the previous cells The individual cells move with the mean wind but the storm is stationary

10. Hazards of Multicellular storms Same as single cells Greater hazard of flooding due to cell regeneration over the same area Greater hazard of strong winds when gusts combine

11. Supercells What are they? How do they form? What conditions are necessary? Severe weather

12. Thunderstorms Supercell Thunderstorms –Relatively long lived intense thunderstorm. –Consists of one large cell usually with exceptionally strong updrafts. –The structure of the storm is such that the updraft and downdraft are separated from each other. –Often the entire storm can begin to rotate. –May produce large tornadoes.

13. Thunderstorms Supercell Thunderstorms The updraft and downdraft are tilted and are thereby separated from each other.

14. Severe Thunderstorms Figure 15.5

15. Severe Thunderstorms Severe Thunderstorm or Tornado Watch –Conditions are favorable for the development of severe weather or tornadoes. –Does not mean that severe weather or tornadoes will occur at your location. –Severe weather or tornadoes are likely in and around the watch area.

16. Severe Thunderstorms Severe Thunderstorm or Tornado Warning –Severe weather or tornadoes are occurring or have been indicated by Doppler weather radar. –Take immediate action to protect life and property.

17. Classic Severe Weather Conditions Idealized “Classic” conditions for severe weather. Low level moisture Instability -- Cold air aloft associated with the upper level trough Inversion (Cap) -- Warm air near 800 mb Strong Wind Shear Lifting Mechanism -- “Cap Breaker”

18. Classic Severe Weather Conditions Severe Storm Setup

19. Tornado Breeding Supercell Storm Supercell thunderstorms may have many of the features illustrated here, including a mesocyclone of rotating winds formed when horizontal vorticity was tilted upwards. Figure 15.38

20. Radar Image of Supercell Figure 15.39 The area of precipitation and winds in the mesocyclone is known as the bounded weak echo region (BWER) which the radar is unable to detect and displays as a black core to this storm. The cyclonic flow of precipitation on the radar screen is often shaped like a hook echo.

21. Thunderstorms Criteria for Severe Weather –Winds  50 kts –Hail Diameters  ¾ inch (20mm) –Tornado

22. Flash Flood A sudden overflow of a river channel or other drainageway, often caused by torrential rains associated with thunderstorm activity. –Slowly moving thunderstorms. –“Training” thunderstorms –Snowfall runoff –Slowly moving tropical storms Example: Big Thompson Canyon –31 July 1976