Meteo 3: Chapter 14

1. Meteo 3: Chapter 14 Spawning severe weather Synoptically-forced storms Read Chapter 14

2. Thunderstorms arise from “deep convection” • INGREDIENTS FOR SEVERE WX • 1) Warm, moist air in lower troposphere (sunshine usually precedes severe wx) • 2) Instability (aided by cold air aloft) • 3) Strong triggering mechanism (low pressure, fronts, upper-level trough)

3. Severe Thunderstorm Watch: Issued by Storm Prediction Center when conditions are favorable for severe weather Tornado Watch: Issued by SPC if conditions favor tornadoes Warnings issued by local NWS offices if severe wx/tornadoes are observed or indicated by radar Watches- http://www.spc.noaa.gov/products/watch/

4. Vertical Wind Shear • Vertical Wind Shear: A change in the speed and/or direction of the wind with height • If wind in direction of t-storm motion increases with height, the updraft tilts forward • Updraft and downdraft separate, prolonging t-storm life & allowing it to strengthen

6. Narrow, linear line of storms that develop to east of cold front in warm sector Look for moisture convergence (fuel)…upper-level trough is the “spark” Lasts 6-8 hours…weaken as they move in more stable environment Squall Line

8. “State College Effect”

11. Derecho – damaging wind storm • Clusters of downbursts that produce a widespread, damaging wind storm • Associated with squall lines • Much larger area of wind damage than with microbursts • Bow echo on radar…intense wind at “surge region”

12. Derecho - St Louis July 19th, 2006 • 90 mph wind gusts knocked out power to half a million St. Louis metro area residents • Radar loop showing gust front • Heat indices of 100-110 degrees next day (what temperature feels like when moisture content is accounted for)

13. More on derechos • What makes a squall line w/ bow echoes a derecho? • 1) Many reports of severe wind damage over an area whose major axis is >= 250 mi • 2) Some damage as bad as seen with a weak tornado or reported gusts > 75 mph • Most triggered by mid-latitude cyclones in warm season…usually form along stationary front • Fast moving

14. Meteo 3: Chapter 15 Tornadoes Some material pulled from Zack Byko’s website

15. Tornadoes! • Tornado: A violently rotating column of air extending from a cloud to the ground.  • Sometimes there’s a classic condensation funnel, other times only rotating dirt/debris on ground • Appearance depends on moisture & ground surface • Powerful tornadoes usually have condensation funnel

16. Importance of capping inversion

17. Synoptic setup for tornadic thunderstorms Tornado Alley example

20. Tornado formation • Typically form from Supercells - large, solitary, long-lasting thunderstorms with rotating updrafts (mesocyclone)

21. Tornado formation • Strong vertical wind shear creates ‘horizontal rolls’ • Strong updrafts at rear of storm cell tilt horizontal roll vertically • Roll is tilted into mesocyclone, a rotating column of air sometimes found in severe storms

22. Tornado formation • Roll stretches vertically, spinning faster and faster (think conservation of angular momentum again!)

23. Tornado formation • If rotating column of air reaches ground, a tornado is born • Photos courtesy of the National Severe Storms Center

24. Cycloid damage path caused by suction vortices:how a tornado can destroy one house and spare the neighbors altogether

25. Suction Vortices

26. Fujita Scale • A scale measuring intensity of tornadoes • F0 – weakest -> F5 - strongest • http://en.wikipedia.org/wiki/Fujita_scale

28. OK, REAL Tornado Safety • http://www.srh.noaa.gov/oun/severewx/safety.html#tornado • Final note: Always take heed of watches and warning, even though severe weather usually impacts a very small fraction of a watch area.

29. Whirlwinds that are not tornadoes • Dust Devils ▪ Waterspouts

30. Now, some video • http://www.youtube.com/watch?v=otZR3VTh4Vo • http://www.youtube.com/watch?v=s7uW4SbtHEM • http://www.youtube.com/watch?v=2rK-ctpFBz8 • http://www.youtube.com/watch?v=2veLsNZrlSw