1

2

3

4 THUNDERSTORMS

5 Ingredients for a thunderstorm Lift from Lift from Cold/warm front Cold/warm front Gust front Gust front Daytime heating Daytime heating Low-level moisture Low-level moisture Instability Instability Wind shear Wind shear

6 Severe thunderstorms One or more of the following: One or more of the following: Winds stronger than 58 mph Winds stronger than 58 mph Hailstones larger than 0.75 in Hailstones larger than 0.75 in Tornado Tornado

7 Air mass thunderstorms

8 Single Cell or “Pulse” Single-Cell Storms Single-Cell Storms Moves with mean environmental wind over 5-7 km. Moves with mean environmental wind over 5-7 km minute duration minute duration. Rainfall can be moderate to heavy, small hail is possible. Rainfall can be moderate to heavy, small hail is possible. Vertical wind shear is small. Vertical wind shear is small. Form in very weak shear environments with new storms forming in an unorganized manner Form in very weak shear environments with new storms forming in an unorganized manner Can be severe at dissipation Can be severe at dissipation

9 Multi Cell Clusters Cluster of 2-4 storms Cluster of 2-4 storms Feed off gust front Feed off gust front Moderate unidirectional shear Moderate unidirectional shear Moderate instability Moderate instability Follow mean 700 – 500 mb flow Follow mean 700 – 500 mb flow

10 Mesoscale Convective Complex May – Aug (86%) May – Aug (86%) Multi celled storms “gone wild” Multi celled storms “gone wild” Oval shaped Oval shaped Cloud shield Cloud shield < -32°C = 100,000km2 < -32°C = 100,000km2 < -52°C = 50,000 km2 < -52°C = 50,000 km2 Last at least 6 hours Last at least 6 hours Usually night events Usually night events Flash floods Flash floods Hail Hail

11 Squall Lines Linear multi cell storms Linear multi cell storms Frontal or pre-frontal Frontal or pre-frontal Vertical tilt increases severity Vertical tilt increases severity Moist low level S or SE inflow Moist low level S or SE inflow Mid level flow >25 knots and ┴ to low level flow Mid level flow >25 knots and ┴ to low level flow

12 Squall Line and gust front

13 Squall Line Lifespan

14Supercell Balanced updraft and downdraft Balanced updraft and downdraft Low level mesocyclogenesis more likely Low level mesocyclogenesis more likely Moderate hook echo Moderate hook echo Tornadoes are common Tornadoes are common

15 Supercell

16 Hook Echo of Supercell

17 Supercell Photo by: Steven Maciejewski

18 Supercell reflectivity

19

20 Lightning and thunder Lightning = flashes of light caused by enormous electrical discharges Lightning = flashes of light caused by enormous electrical discharges Superheats the air abruptly resulting in enormous shock waves Superheats the air abruptly resulting in enormous shock waves thunder thunder

21 Source: Environment Canada Lightning process

22 Types of lightning Within cloud Within cloud Cloud-to-cloud Cloud-to-cloud Cloud-to-air Cloud-to-air Cloud-to-ground Cloud-to-ground Negative lightning Negative lightning Positive lightning Positive lightning

23 Positive lightning (~10%)

24 Lightning strike density Flashes Days with flashes

25 Hail Ice pellets Ice pellets Only occur within cumulonimbus clouds Only occur within cumulonimbus clouds The longer a frozen pellet stays aloft in a cloud, the larger the hail can be The longer a frozen pellet stays aloft in a cloud, the larger the hail can be “Wet-bulb zero” height important “Wet-bulb zero” height important Courtesy of:

26 Hail frequency in the US

27 Tornadoes As cumulonimbus clouds rise high into the troposphere, the bottom winds move slow due to surface friction, but the upper winds move fast As cumulonimbus clouds rise high into the troposphere, the bottom winds move slow due to surface friction, but the upper winds move fast Results in rotation of air parallel to the ground Results in rotation of air parallel to the ground Updrafts or downdrafts can cause the vortex to become vertical Updrafts or downdrafts can cause the vortex to become vertical

28 Tornadoes

29

30

31 Some keys to tornado formation

32 Fujita scale F0-F1: 69% of tornadoes; 5% of deaths F4-F5: 3% of tornadoes; 70% of deaths

33 Tornado frequency in the US

34

35

36

37 Severe wind frequency in the US