1. A Convective Wind Event over Southeastern Alberta on 15 July 2008 Stephen Knott and Chris Wielki Prairie and Arctic Storm Prediction Centre - Edmonton

2. Outline Synoptic Setup Radar Evolution Mesoscale Convective Vortex? Post Storm Investigation Summary

3. 00z 16 July 2008 250 MB500 MB 700 MB 850 MB

4. Surface Conditions

5. Adjusted 12z Olds Sounding UNSTABLE 2008 (25/12 0 C)

6. Radar Evolution (1 Hr)

7. SUDDS 21:00z

8. SUDDS 22:00z

9. SUDDS 23:00z

10. SUDDS 00:00z

11. SUDDS 01:00z

12. SUDDS 02:00z

13. SUDDS 03:00z

14. SUDDS 04:00z

15. Vulcan Storm 2240z 15 July 08 20 minutes prior to report of tornado

16. Vulcan Tornado Rated F1 (Sills) Path Length 100m; Path Width 30m, motion 330 Lat/Long50.4303/ -113.189 Two empty grain silos (approx. weight 5000lbs) were torn from their foundations (bolted about 8 cm into cement pads) and thrown through the air approx. 70m. No sign of being rolled or dragged. Adjacent silos had some damage but did not leave their foundation (even ones with wood foundations). A few trees in the path were damaged or destroyed. Straw bales (approx. weight 1000lbs) were tossed around. Evidence of convergence in the flattened grass along the path.

17. Mesoscale Convective Vortex At 0120z strong persistent rotation found at all three levels. Vortex persists 90 minutes, diameter of 15-20 km, tracks 80km from 260 and moves north of Cypress Hills. Radar echoes initially show hook but disintegrate into no definable pattern and yet wind damage to homes reported at Irvine.

18. Meso-Vortex

25. 7:31 pm

26. 7:32 pm RFD Dry Slot? Flanking Line associated with RFD gust front

27. 7:33pm

28. 7:48pm

29. Medicine Hat Funnel cloud picture is from Seven Persons looking towards 290 degrees ~ 0120 UTC 0120 UTC 3.5 km CAPPI 0120 UTC Damage Survey Route

30. Main damage swath was from an area just south of Seven Persons Eastwards. 0120 UTC 0130 UTC 0140 UTC 0150 UTC 215 kmh -1 measured 80 m above ground

31. Peak Winds and damage swath Estimated damage path (F1 or greater) was approx 250 km long and up to 31 km wide 2100 UTC 0200 UTC Speeds in kmh -1

32. Summary During the event operations believed the mesoscale vortex had a high likelihood of generating a tornado EPWS triggered 3 times during storm lifetime, first as a dangerous supercell then along with a tornado warning Damage survey results suggest this large-scale persistent rotation may not have equated to a tornado –apparent predominant straight line wind damage on the order of F1 and low F2 categories in the RFD region of the supercell More work required with respect to origin of large-scale meso-vortex but authors speculate a low-level jet from near Alberta US border interacted with storm enhancing the RFD and spinning up vortex. Was some speculation from damage path that the event may reach derecho criteria though this not the case Weakening radar echoes did not correlate with diminishing of severe winds

33. Thank You